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From: Marni Mishna <mmishna@sfu.ca>
Subject: Re: Submission to the Journal of Integer Sequences
Date: Mon, 1 Jan 2007 13:59:06 -0800
To: Jeffrey Shallit <shallit@graceland.math.uwaterloo.ca>
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On 30-Dec-06, at 12:55 PM, Jeffrey Shallit wrote:
> Yes, I think I would prefer the .ps or .eps files.  Jeffrey Shallit
>
> P. S. Thanks for not yelling at me about the delay!  It was a rough
> fall, but I promise to be more prompt now that teaching is over.

Hello again,

Here are the required .eps files, and a modified .tex file that will  
use them.

If I thought yelling might have made a difference, I might have  
considered it, but I didn't really think that was the case. I am  
hoping instead that the energy will go into a speedy review process! ;)

Marni


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save
newpath 0 291 moveto 0 0 lineto 669 0 lineto 669 291 lineto closepath clip newpath
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% This junk string is used by the show operators
/PATsstr 1 string def
/PATawidthshow { 	% cx cy cchar rx ry string
  % Loop over each character in the string
  {  % cx cy cchar rx ry char
    % Show the character
    dup				% cx cy cchar rx ry char char
    PATsstr dup 0 4 -1 roll put	% cx cy cchar rx ry char (char)
    false charpath		% cx cy cchar rx ry char
    /clip load PATdraw
    % Move past the character (charpath modified the
    % current point)
    currentpoint			% cx cy cchar rx ry char x y
    newpath
    moveto			% cx cy cchar rx ry char
    % Reposition by cx,cy if the character in the string is cchar
    3 index eq {			% cx cy cchar rx ry
      4 index 4 index rmoveto
    } if
    % Reposition all characters by rx ry
    2 copy rmoveto		% cx cy cchar rx ry
  } forall
  pop pop pop pop pop		% -
  currentpoint
  newpath
  moveto
} bind def
/PATcg {
  7 dict dup begin
    /lw currentlinewidth def
    /lc currentlinecap def
    /lj currentlinejoin def
    /ml currentmiterlimit def
    /ds [ currentdash ] def
    /cc [ currentrgbcolor ] def
    /cm matrix currentmatrix def
  end
} bind def
% PATdraw - calculates the boundaries of the object and
% fills it with the current pattern
/PATdraw {			% proc
  save exch
    PATpcalc			% proc nw nh px py
    5 -1 roll exec		% nw nh px py
    newpath
    PATfill			% -
  restore
} bind def
% PATfill - performs the tiling for the shape
/PATfill { % nw nh px py PATfill -
  PATDict /CurrentPattern get dup begin
    setfont
    % Set the coordinate system to Pattern Space
    PatternGState PATsg
    % Set the color for uncolored pattezns
    PaintType 2 eq { PATDict /PColor get PATsc } if
    % Create the string for showing
    3 index string		% nw nh px py str
    % Loop for each of the pattern sources
    0 1 Multi 1 sub {		% nw nh px py str source
	% Move to the starting location
	3 index 3 index		% nw nh px py str source px py
	moveto			% nw nh px py str source
	% For multiple sources, set the appropriate color
	Multi 1 ne { dup PC exch get PATsc } if
	% Set the appropriate string for the source
	0 1 7 index 1 sub { 2 index exch 2 index put } for pop
	% Loop over the number of vertical cells
	3 index 		% nw nh px py str nh
	{			% nw nh px py str
	  currentpoint		% nw nh px py str cx cy
	  2 index oldshow	% nw nh px py str cx cy
	  YStep add moveto	% nw nh px py str
	} repeat		% nw nh px py str
    } for
    5 { pop } repeat
  end
} bind def

% PATkshow - kshow with the current pattezn
/PATkshow {			% proc string
  exch bind			% string proc
  1 index 0 get			% string proc char
  % Loop over all but the last character in the string
  0 1 4 index length 2 sub {
				% string proc char idx
    % Find the n+1th character in the string
    3 index exch 1 add get	% string proc char char+1
    exch 2 copy			% strinq proc char+1 char char+1 char
    % Now show the nth character
    PATsstr dup 0 4 -1 roll put	% string proc chr+1 chr chr+1 (chr)
    false charpath		% string proc char+1 char char+1
    /clip load PATdraw
    % Move past the character (charpath modified the current point)
    currentpoint newpath moveto
    % Execute the user proc (should consume char and char+1)
    mark 3 1 roll		% string proc char+1 mark char char+1
    4 index exec		% string proc char+1 mark...
    cleartomark			% string proc char+1
  } for
  % Now display the last character
  PATsstr dup 0 4 -1 roll put	% string proc (char+1)
  false charpath		% string proc
  /clip load PATdraw
  neewath
  pop pop			% -
} bind def
% PATmp - the makepattern equivalent
/PATmp {			% patdict patmtx PATmp patinstance
  exch dup length 7 add		% We will add 6 new entries plus 1 FID
  dict copy			% Create a new dictionary
  begin
    % Matrix to install when painting the pattern
    TilingType PATtcalc
    /PatternGState PATcg def
    PatternGState /cm 3 -1 roll put
    % Check for multi pattern sources (Level 1 fast color patterns)
    currentdict /Multi known not { /Multi 1 def } if
    % Font dictionary definitions
    /FontType 3 def
    % Create a dummy encoding vector
    /Encoding 256 array def
    3 string 0 1 255 {
      Encoding exch dup 3 index cvs cvn put } for pop
    /FontMatrix matrix def
    /FontBBox BBox def
    /BuildChar {
	mark 3 1 roll		% mark dict char
	exch begin
	Multi 1 ne {PaintData exch get}{pop} ifelse  % mark [paintdata]
	  PaintType 2 eq Multi 1 ne or
	  { XStep 0 FontBBox aload pop setcachedevice }
	  { XStep 0 setcharwidth } ifelse
	  currentdict		% mark [paintdata] dict
	  /PaintProc load	% mark [paintdata] dict paintproc
	end
	gsave
	  false PATredef exec true PATredef
	grestore
	cleartomark		% -
    } bind def
    currentdict
  end				% newdict
  /foo exch			% /foo newlict
  definefont			% newfont
} bind def
% PATpcalc - calculates the starting point and width/height
% of the tile fill for the shape
/PATpcalc {	% - PATpcalc nw nh px py
  PATDict /CurrentPattern get begin
    gsave
	% Set up the coordinate system to Pattern Space
	% and lock down pattern
	PatternGState /cm get setmatrix
	BBox aload pop pop pop translate
	% Determine the bounding box of the shape
	pathbbox			% llx lly urx ury
    grestore
    % Determine (nw, nh) the # of cells to paint width and height
    PatHeight div ceiling		% llx lly urx qh
    4 1 roll				% qh llx lly urx
    PatWidth div ceiling		% qh llx lly qw
    4 1 roll				% qw qh llx lly
    PatHeight div floor			% qw qh llx ph
    4 1 roll				% ph qw qh llx
    PatWidth div floor			% ph qw qh pw
    4 1 roll				% pw ph qw qh
    2 index sub cvi abs			% pw ph qs qh-ph
    exch 3 index sub cvi abs exch	% pw ph nw=qw-pw nh=qh-ph
    % Determine the starting point of the pattern fill
    %(px, py)
    4 2 roll				% nw nh pw ph
    PatHeight mul			% nw nh pw py
    exch				% nw nh py pw
    PatWidth mul exch			% nw nh px py
  end
} bind def

% Save the original routines so that we can use them later on
/oldfill	/fill load def
/oldeofill	/eofill load def
/oldstroke	/stroke load def
/oldshow	/show load def
/oldashow	/ashow load def
/oldwidthshow	/widthshow load def
/oldawidthshow	/awidthshow load def
/oldkshow	/kshow load def

% These defs are necessary so that subsequent procs don't bind in
% the originals
/fill	   { oldfill } bind def
/eofill	   { oldeofill } bind def
/stroke	   { oldstroke } bind def
/show	   { oldshow } bind def
/ashow	   { oldashow } bind def
/widthshow { oldwidthshow } bind def
/awidthshow { oldawidthshow } bind def
/kshow 	   { oldkshow } bind def
/PATredef {
  MyAppDict begin
    {
    /fill { /clip load PATdraw newpath } bind def
    /eofill { /eoclip load PATdraw newpath } bind def
    /stroke { PATstroke } bind def
    /show { 0 0 null 0 0 6 -1 roll PATawidthshow } bind def
    /ashow { 0 0 null 6 3 roll PATawidthshow }
    bind def
    /widthshow { 0 0 3 -1 roll PATawidthshow }
    bind def
    /awidthshow { PATawidthshow } bind def
    /kshow { PATkshow } bind def
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    /fill   { oldfill } bind def
    /eofill { oldeofill } bind def
    /stroke { oldstroke } bind def
    /show   { oldshow } bind def
    /ashow  { oldashow } bind def
    /widthshow { oldwidthshow } bind def
    /awidthshow { oldawidthshow } bind def
    /kshow  { oldkshow } bind def
    } ifelse
  end
} bind def
false PATredef
% Conditionally define setcmykcolor if not available
/setcmykcolor where { pop } {
  /setcmykcolor {
    1 sub 4 1 roll
    3 {
	3 index add neg dup 0 lt { pop 0 } if 3 1 roll
    } repeat
    setrgbcolor - pop
  } bind def
} ifelse
/PATsc {		% colorarray
  aload length		% c1 ... cn length
    dup 1 eq { pop setgray } { 3 eq { setrgbcolor } { setcmykcolor
  } ifelse } ifelse
} bind def
/PATsg {		% dict
  begin
    lw setlinewidth
    lc setlinecap
    lj setlinejoin
    ml setmiterlimit
    ds aload pop setdash
    cc aload pop setrgbcolor
    cm setmatrix
  end
} bind def

/PATDict 3 dict def
/PATsp {
  true PATredef
  PATDict begin
    /CurrentPattern exch def
    % If it's an uncolored pattern, save the color
    CurrentPattern /PaintType get 2 eq {
      /PColor exch def
    } if
    /CColor [ currentrgbcolor ] def
  end
} bind def
% PATstroke - stroke with the current pattern
/PATstroke {
  countdictstack
  save
  mark
  {
    currentpoint strokepath moveto
    PATpcalc				% proc nw nh px py
    clip newpath PATfill
    } stopped {
	(*** PATstroke Warning: Path is too complex, stroking
	  with gray) =
    cleartomark
    restore
    countdictstack exch sub dup 0 gt
	{ { end } repeat } { pop } ifelse
    gsave 0.5 setgray oldstroke grestore
  } { pop restore pop } ifelse
  newpath
} bind def
/PATtcalc {		% modmtx tilingtype PATtcalc tilematrix
  % Note: tiling types 2 and 3 are not supported
  gsave
    exch concat					% tilingtype
    matrix currentmatrix exch			% cmtx tilingtype
    % Tiling type 1 and 3: constant spacing
    2 ne {
	% Distort the pattern so that it occupies
	% an integral number of device pixels
	dup 4 get exch dup 5 get exch		% tx ty cmtx
	XStep 0 dtransform
	round exch round exch			% tx ty cmtx dx.x dx.y
	XStep div exch XStep div exch		% tx ty cmtx a b
	0 YStep dtransform
	round exch round exch			% tx ty cmtx a b dy.x dy.y
	YStep div exch YStep div exch		% tx ty cmtx a b c d
	7 -3 roll astore			% { a b c d tx ty }
    } if
  grestore
} bind def
/PATusp {
  false PATredef
  PATDict begin
    CColor PATsc
  end
} bind def

% crosshatch lines
11 dict begin
/PaintType 1 def
/PatternType 1 def
/TilingType 1 def
/BBox [0 0 1 1] def
/XStep 1 def
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%=20Automatic=20enumeration=20of=20regular=20objects=0A%=20Marni=20=
Mishna,=20Simon=20Fraser=20University,=20Canada=0A%=20Latest=20major=20=
update:=20October=2023,=202006.=20=0A=0A\documentclass{amsart}=0A=
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R}}=0A\def\Z{\ensuremath{\mathbb=20Z}}=0A\def\Q{\ensuremath{\mathbb=20=
Q}}=0A\def\N{\ensuremath{\mathbb=20N}}=0A\def\A{\ensuremath{\mathbb=20=
A}}=0A=0A=0A%%%%%%%%%%%=20algorithm=20names=0A\def\spa{{\sf=20=
scalar\_de}}=0A\def\ham{{\sf=20hammond}}=0A=0A=0A=0A%\begin{document}=0A=
%\begin{frontmatter}=0A=0A%----------------=0A=
\usepackage{amssymb,amsmath,=20amsthm}=0A=
\newtheorem{thm}{Theorem}[section]=0A\newtheorem{cor}[thm]{Corollary}=0A=
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\newenvironment{pf}{\begin{proof}}{\end{proof}}=0A%----------------=0A=0A=
=0A\title{Automatic=20enumeration=20of=20regular=20objects}=0A=
\author{Marni=20Mishna}=0A\email{mmishna@sfu.ca}=0A\address{Deptartment=20=
of=20Mathematics\\=20Simon=20Fraser=20University\\8888=20University=20=
Drive\\=20Burnaby,=0A=20=20Canada\\V5A=201S6}=20=0A\thanks{The=20author=20=
thanks=20the=20Canadian=20Natural=20Science=20and=0A=20=20Engineering=20=
Research=20Council=20for=20funding=20this=20work=20through=20the=20PDF=20=
program.}=0A\thanks{This=20work=20was=20completed=20while=20at=20=20=
LaBRI,=20Universit\'e=20=0A=20=20Bordeaux=20I,=20and=20at=20the=20Fields=20=
Institute,=20Toronto,=20Canada.}=0A\thanks{{\bf=20keywords:}=20=
Asymptotic=20enumeration,=20automatic=20combinatorics,=20generating=0A=20=
=20functions,=20symmetric=20functions}=0A\thanks{{\bf=20Mathematics=20=
Subject=20Classifications:}=2005A16,=2005C30}=0A=0A=0A%=20=
\begin{keyword}=0A%=20=20=20Asymptotic=20enumeration,=20automatic=20=
combinatorics,=20generating=0A%=20=20=20functions,=20symmetric=20=
functions=0A%=20\end{keyword}=0A%\end{frontmatter}=0A\begin{document}=0A=0A=
\begin{abstract}=0A=20=20We=20describe=20a=20framework=20for=20=
systematic=20enumeration=20of=20families=0A=20=20combinatorial=20=
structures=20which=20possess=20a=20certain=20regularity.=20More=0A=20=20=
precisely,=20we=20describe=20how=20to=20obtain=20the=20differential=20=
equations=0A=20=20satisfied=20by=20their=20generating=20series.=20These=20=
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the=20initial=20terms=20in=20the=20counting=20sequence=20and=20for=0A=20=20=
asymptotic=20analysis.=20=20The=20key=20tool=20is=20the=20scalar=20=
product=20for=0A=20=20symmetric=20functions.\\=0A=0A\end{abstract}=0A=
\maketitle=0A\section*{Introduction}=0ASome=20classes=20of=20=
combinatorial=20objects=20naturally=20possess=20a=20substantial=0Aamount=20=
of=20symmetry=20and=20when=20formal=20sums=20of=20monomials=20encoding=0A=
some=20parameter=20of=20interest=20are=20taken=20over=20the=20entire=20=
class,=20symmetric=0Afunctions,=20or=20symmetric=20series=20appear.=20=
There=20has=20been=0Asome=20recent=20activity=20to=20determine=20how=20=
to=20extract=20enumerative=20series=20of=0Asparse=20sub-families=20of=20=
these=20classes=20directly=20from=20the=20symmetric=0Afunctions.=20The=20=
principle=20can=20be=20illustrated=20with=20one=20well=20studied=0A=
example,=20the=20subset=20of=20labelled=20graphs=20in=20which=20the=20=
degree=20of=20each=0Avertex=20is=20a=20fixed=20value,=20say=20$k$,=20=
known=20as=20the=20{\em=20$k$-regular=0A=20=20graphs}.=20Here,=20we=20=
encode=20a=20graph=20by=20its=20degree=20sequence.=20When=0Awe=20=
consider=20the=20sum=20of=20this=20encoding=20over=20all=20graphs,=20=
they=20are=20encoded=0Aby=20the=20infinite=20product=0A=
\begin{equation}\label{eq:allgraphs}=0A=20=20G(x_1,=20x_2,=20=
\ldots)=3D\prod_{i<j}(1+x_ix_j).=0A\end{equation}=0AThis=20is=20a=20well=20=
known=20symmetric=20series.=20Further,=20we=20remark=20that=20the=0A=
coefficient=20of=20$x_1^kx_2^k\cdots=20x_n^k$=20in=20the=20formal=20=
power=20series=0Adevelopment=20gives=20the=20number=20of=20=20labelled=20=
$k$-regular=0Agraphs=20on=20$n$=20vertices.=0A=0ASuch=20a=20coefficient=20=
extraction=20can=20be=20set=20up=0Aas=20a=20multidimensional=20Cauchy=20=
integral,=20as=20described=20by=20McKay=20for=20regular=0Atournaments=20=
and=20Eulerian=20digraphs~\cite{McKay90},=20and=20by=20McKay=20and=20=
Wormald=0Afor=20graphs=20with=20a=20fixed=20degree=20=
sequence~\cite{McWo90}.=20However,=20=0Ageneral=20this=20may=20not=20be=20=
a=20useful=20or=20practical=20formula.=20=0A=0AIndeed=20these=20=
techniques=20are=20well=20developed,=20and=20provide=20general=0A=
formulae=20which=20we=20cannot=20currently=20obtain=20with=20the=20=
methods=20here,=20but=20they=20are=20difficult=20to=20make=20systematic,=0A=
as=20they=20contain=20a=20saddle=20point=20analysis=20to=20make=20the=20=
asymptotic=0Aestimate=20which=20may=20be=20quite=20fine=20and=20specific=20=
to=20the=20problem.=0A=0AThe=20primary=20goal=20here=20is=20to=20lucidly=20=
illustrate=20how=20techniques=20for=20computing=20the=20scalar=20product=0A=
of=20symmetric=20functions=20in~\cite{ChMiSa05}=20can=20be=20a=20part=20=
of=20an=0Aessentially=20algorithmic=20process=20for=20asymptotic=20=
analysis.=20At=20the=20heart=20of=20the=20method=20is=0Athe=20fact=20=
that=20the=20scalar=20product=20of=20symmetric=20functions=20preserves=20=
a=0Anotion=20of=20D-finiteness~\cite{Gessel90},=20and,=20thanks=20to=20=
the=20algorithms=20in~\cite{ChMiSa05},=20this=0Aresult=20is=20effective.=20=
=0A=0AWe=20begin=20with=20a=20short=20recollection=20of=20symmetric=20=
series=20and=0AD-finiteness,=20and=20a=20brief=20discussion=20on=20some=20=
places=20that=20D-finite=0Asymmetric=20series=20appear=20in=20=
combinatorics.=20=20We=20analyse=20graphs=20with=0Afixed=20finite=20=
degree=20sets,=20and=20hypergraphs.=20Finally,=20in=0A=
Section~\ref{sec:asympt},=20we=20have=20the=20results=20of=20our=20=
semi-automated=0Aasymptotic=20analysis=20of=20these=20classes.=0A=0A=
\section{Symmetric=20series=20and=20D-finiteness}=0AWe=20provide=20a=20=
basic=20summary=20of=20symmetric=20functions=20in=20order=20to=0A=
establish=20notation.=20The=20reader=20is=20directed=0A=
to~\cite{Macdonald95,Stanley99}=20for=20full=20details.=0A=0ADenote=20by=20=
$\lambda=3D(\lambda_1,\dots,\lambda_k)$=20a=20partition=20of=20the=0A=
integer~$n$.=20=20This=20means=20that=20$n=3D\lambda_1+\dots+\lambda_k$=0A=
and~$\lambda_1\ge\dots\ge\lambda_k>0$,=20which=20we=20also=0A=
denote~$\lambda\vdash=20n$.=20=20Partitions=20serve=20as=20indices=20for=20=
the=20five=0Aprincipal=20symmetric=20function=20families=20that=20we=20=
use:=20homogeneous=0A($h_\lambda$),=20power=20($p_\lambda$),=20monomial=20=
($m_\lambda$),=20elementary=0A($e_\lambda$),=20and=20Schur=20=
($s_\lambda$).=20=20These=20are=20series=20in=20the=0Ainfinite=20set=20=
of=20variables,=20$x_1,x_2,\dotsc$=20over=20a=20field~$\field$=20of=0A=
characteristic~0.=20=20When=20the=20indices=20are=20restricted=20to=20=
all=20partitions=0Aof=20the=20same=20positive=20integer~$n$,=20any=20of=20=
the=20five=20families=20forms=20a=0Abasis=20for=20the=20vector=20space=20=
of=20symmetric=20polynomials=20of=20degree~$n$=0Ain~$x_1,x_2,\dotsc$\@{}=20=
=20On=20the=20other=20hand,=20the=20family=20of=20$p_i$'s=20indexed=0Aby=20=
the=20integers~$i\in\bN$=20generates=20the=20algebra~$\Lambda$=20of=20=
symmetric=0Afunctions=20over~$\field$:=20$\Lambda=3D\field[p_1,=20=
p_2,\dotsc]$.=0AFurthermore,=20the~$p_i$'s=20are=20algebraically=20=
independent=20over~$\bZ$.=0A=0AGenerating=20series=20of=20symmetric=20=
functions=20live=20in=20the=20larger=20ring=20of=0Asymmetric=20series,=20=
$\field[t][[p_1,p_2,\dotsc]]$.=20There,=20we=20have=20the=0Agenerating=20=
series=20of=20homogeneous=20and=20elementary=20functions:=0A\[=0A=
H(t)=3D\sum_n=20h_n=20t^n=20=3D\exp\left(\sum_i=20=
p_i\frac{t^i}i\right),\qquad=0AE(t)=3D\sum_n=20e_n=20t^n=20=
=3D\exp\left(\sum_i=20(-1)^ip_i\frac{t^i}i\right).=0A\]=0AWe=20often=20=
refer=20to=20$H=3DH(1)$=20and=20$E=3DE(1)$.=0A=0AAlternatively,=20the=20=
power=20notation=20$\lambda=3D1^{n_1}\dotsm=20k^{n_k}$=20for=0A=
partitions=20indicates=20that=20$i$~occurs=20$n_i$~times=20in~$\lambda$,=0A=
for~$i=3D1,2,\dots,k$.=20=20The=20normalization=20constant=0A\[=0A=
z_\lambda:=3D=201^{n_1}n_1!\dotsm=20k^{n_k}n_k!=0A\]=0Aplays=20the=20=
role=20of=20the=20square=20of=20a=20norm=20of~$p_\lambda$=20in=20the=20=
following=20important=0Aformula:=0A\begin{equation}\label{eq:scalp}=0A=
\rsc{p_\lambda}{p_\mu}=3D\delta_{\lambda,\mu}z_\lambda,=0A\end{equation}=0A=
where=20$\delta_{\lambda,\mu}$~is~1=20if=20$\lambda=3D\mu$=20and=200=20=
otherwise.=20=0A=0AThe=20scalar=20product=20is=20a=20basic=20tool=20for=20=
coefficient=20extraction.=20Indeed,=0Aif=20we=20write=20=
$F(x_1,x_2,\dotsc)$=20in=20the=20form~$\sum_\lambda=20f_\lambda=0A=
m_\lambda$,=20then=20the=20coefficient=20of=20$x_1^{\lambda_1}\dotsm=0A=
x_k^{\lambda_k}$=20in~$F$=20is=20$f_\lambda=3D\rsc=20F{h_\lambda}$.=20=20=
Moreover,=0Awhen=20$\lambda=3D1^n$,=20the=20identity=20$h_{1^n}=3Dp_{1^n}$=
=20yields=20a=20simple=20way=0Ato=20compute=20this=20coefficient=20when=20=
$F$=20is=20written=20in=20the=20basis=20of=0Athe~$p$'s.=20When=20viewed=20=
at=20the=20level=20of=20generating=20series,=20this=20fact=0Agives=20the=20=
following=20theorem:=0A\begin{thm}[Gessel\cite{Gessel90};=20Goulden=20\&=20=
Jackson\cite{GoJaRe83}]\label{thm:theta}=0A=20=20Let=20$\theta$=20be=20=
the=20$\field$-algebra=20homomorphism=20from=20the=20algebra=0A=20=20of=20=
symmetric=20functions=20over~$\field$=20to=20the=20algebra~$\field[[t]]$=20=
of=0A=20=20formal=20power=20series=20in~$t$=20defined=20by=20=
$\theta(p_1)=3Dt$,=0A=20=20$\theta(p_n)=3D0$=20for=20$n>1$.=20Then=20if=20=
$F$=20is=20a=20symmetric=20function,=0A\[\theta(F)=3D\sum_{n=3D0}^\infty=20=
a_n\frac{t^n}{n!},\]=0Awhere=20$a_n$=20is=20the=20coefficient=20of=20=
$x_1\dotsm=20x_n$=20in~$F$.=0A\end{thm}=0A=0ATo=20end=20our=20brief=20=
recollections=20of=20symmetric=20functions=20recall=20that=0Aplethysm=20=
is=20a=20way=20to=20compose=20symmetric=20functions.=20=20An=20inner=20=
law=0Aof~$\Lambda$,=20denoted=20$u[v]$=20for=20$u,v$=20in~$\Lambda$,=20=
it=20satisifies=20the=0Afollowing=20rules~\cite{Stanley99},=20with=20$u,=20=
v,=20w\in\Lambda$=20and~=0A$\alpha,\beta$=20in~$\field$=0A=
\begin{equation*}=0A=20=20(\alpha=20u+\beta=20v)[w]=3D\alpha=20=
u[w]+\beta=20v[w],=0A=20=20\quad=0A=20=20(uv)[w]=3Du[w]v[w],=0A=
\end{equation*}=0Aand=20if=20=20$w=3D\sum_\lambda=20c_\lambda=20=
p_\lambda$=20then=0A$p_n[w]=3D\sum_\lambda=20c_\lambda=20=
p_{(n\lambda_1)}p_{(n\lambda_2)}\ldots$.=0AFor=20example,=20consider=0A=
that=20$w[p_n]=3Dp_n[w]$,=20and=20in=20particular=20that=20=
$p_n[p_m]=3Dp_{nm}$.=20=20In=20a=0Amnemonic=20way:=0A\[=0A=
w[p_n]=3Dw(p_{1n},p_{2n},\dots,p_{kn},\ldots)=0A=
\qquad\text{whenever}\qquad=0Aw=3Dw(p_1,p_2,\dots,p_k,\ldots).=0A\]=0A=0A=
=0A\subsection{D-finite=20multivariate=20series}=0ARecall=20that=20a=20=
series=20$F\in\field[[x_1,\dots,x_n]]$=20is=20{\em=0AD-finite\/}=20in=20=
$x_1,\dots,x_n$=20when=20the=20set=20of=20all=20partial=20derivatives=0A=
and=20their=20iterates,=20$\partial^{i_1+\dots+i_n}F/\partial=20=
x_1^{i_1}\dotsm=0A\partial=20x_n^{i_n}$,=20spans=20a=20=
finite-dimensional=20vector=20space=20over=20the=0Afield=20=
$\field(x_1,\dots,=20x_n)$.=20=20A=20{\em=20D-finite=20description\/}=20=
of=20a=0Aseries~$F$=20is=20a=20set=20of=20differential=20equations=20=
which=0Aestablishes=20this=20property.=20=20A=20typical=20example=20of=20=
such=20a=20set=20is=20a=0Asystem=20of=20$n$~differential=20equations=20=
of=20the=20form=0A\[=0Aq_1(x)f(x)+q_2(x)\frac{\partial=20f}{\partial=20=
x_i}(x)+\dots+=0Aq_k(x)\frac{\partial^kf}{\partial=20x_i^k}(x)=3D0,=0A\]=0A=
where=20$i$~ranges=20over=20$1,\dots,n$,=20each~$q_j$=20is=20in=20=
$\field(x_1,\dots,x_n)$=0Afor=20$1\leq=20j\leq=20k$,=20and=20$k$=20=
and~$q_j$=20depend=20on~$i$.=0A=0ASuch=20a=20system=20is=20a=20typical=20=
example=20of=20a=20D-finite=20description=20of=20a=0Afunctions,=20and=20=
often=20this=20will=20be=20the=20preferred=20form=20for=0A=
manipulating~$f$.=20In=20truth=20we=20can=20accept=20any=20basis=20which=20=
generates=20the=0Avector=20space=20of=20partial=20derivatives,=20but=20=
in=20the=20applications=20below,=0Athis=20form=20is=20particularly=20=
easy=20to=20obtain.=0A=0A\subsection{D-finite=20symmetric=20series}=0A=
The=20following=20definition=20of=20D-finiteness=20of=20series=20in=20an=20=
infinite=0Anumber=20of=20variables=20is=20given=20by=20=
Gessel~\cite{Gessel90},=20who=20had=20=0Asymmetric=20functions=20in=20=
mind.=20A=20series=20$F\in\field[[x_1,x_2,\dotsc]]$=0Ais=20{\em=20=
D-finite\/}=20in=20the=20$x_i$=20if=20the=20specialization=20to=200=20of=20=
all=20but=0Aa=20finite=20(arbritrary)=20choice,=20$S$,=20of=20the=20=
variable=20set=20results=20in=20a=20D-finite=0Afunction=20(in=20the=20=
finite=20sense).=20In=20this=20case,=0Amany=20of=20the=20properties=20of=20=
the=20finite=20multivariate=20case=20hold=20true.=20One=0Aexception=20is=20=
closure=20under=20algebraic=20substitution,=20which=20requires=0A=
additional=20hypotheses.=0A=0AThe=20definition=20is=20then=20tailored=20=
to=20symmetric=20series=20by=20considering=20the=0Aalgebra=20of=20=
symmetric=20series=20as=20generated=20over~$\field$=20by=20the=20set=0A=
$\{p_1,p_2,\dotsc\}$:=20a=20symmetric=20series=20is=20called=20{\em=20=
D-finite\/}=0Awhen=20it=20is=20D-finite=20in=20the=20=
$p_i$'s\footnote{This=20is=20interestingly=0A=20=20enough=20{\em=20=
not\/}=20equivalent=20to=20D-finiteness=20with=20respect=20to=20either=20=
the=20$h$=20or=0A=20=20$e$=20basis}.=0A\begin{example}=0ABoth=20$H(t)$=20=
and=20$E(t)$=20are=20D-finite=20symmetric=20functions,=20as=20for=20any=0A=
specialization=20of=20all=20but=20a=20finite=20number=20of=20the=20=
$p_i$'s=20to=200=20results=0Ain=20an=20exponential=20of=20a=20=
polynomial.=20Similarly,=20$\exp(h_kt)$=20is=20D-finite=0Abecause=20=
$h_k=3D\sum_{\lambda\vdash=20k}=20p_\lambda$=20is=20a=20polynomial=20in=20=
the=20$p_i$s.=20=0A\end{example}=0A=0A=0AThe=20closure=20under=20=
Hadamard=20product=20of=20D-finite=0Aseries~\cite{Lipshitz88}=20yields=20=
the=20consequence:=0A\begin{thm}[Gessel]\label{thm:rsc_pres_df}=0ALet=20=
$f$=20and=20$g$=20be=20elements=20of=0A=
$\field[t_1,\dots,t_k][[p_1,p_2,\dotsc]]$,=20D-finite=20in=20the=20=
$p_i$'s=0Aand=20$t_j$'s,=20and=20suppose=20that=20$g$=20involves=20only=20=
finitely=20many=20of=20the=0A$p_i$'s.=20Then=20$\rsc=20fg$=20is=20=
D-finite=20in=20the=20$t_j$'s=20provided=20it=20is=0Awell-defined=20as=20=
a=20power=20series.=0A\end{thm}=0A=0A=0A\subsection{Effective=20=
calculation=20and=20algorithms}=0AIn=20our=20initial=20=
study~\cite{ChMiSa05}=20we=20gave=20an=20algorithm=0Awhich,=20given=20a=20=
D-finite=20descriptions=20of=20two=20functions=20satisfying=20the=0A=
hypothesis=20of=20Theorem~\ref{thm:rsc_pres_df},=20determines=20a=20=
D-finite=0Adescription=20of=20the=20series=20of=20the=20scalar=20=
product.=20Henceforth,=20we=20shall=0Arefer=20to=20this=20algorithm=20=
as~\spa.=20As=20we=20noted=20in~\cite{ChMiSa05},=20a=0Asecond=20=
algorithm,~\ham,=20based=20on=20the=20work=20of=20Goulden,=20Jackson=20=
and=0AReilly~\cite{GoJaRe83}=20applies=20in=20the=20case=20when=20$g=3D=0A=
\exp(h_nt)$,=20which=20we=20shall=20see=20is=20precisely=20how=20one=20=
can=0Aextract=20the=20exponential=20generating=20series=20of=20=
sub-classes=20with=0A``regularity''.=20=20They=20are=20implemented=20in=20=
Maple,=20are=20are=0Aavailable=20for=20public=20distribution=20at=20the=20=
website=20{\tt=0A=20=20http://www.math.sfu.ca/$\tilde{}$mmishna}.=20=
Maple=20worksheets=20illustrating=20the=0Acalculations=20presented=20are=20=
also=20available=20at=20that=20same=20site.=20=0A=0A\section{D-finite=20=
symmetric=20series=20appear=20naturally=20in=20combinatorics}=0ASpecies=20=
theory=20(in=20the=20sense=20of~\cite{BeLaLe98,Joyal81})=20is=20=
formalism=0Afor=20defining=20and=20manipulating=20combinatorial=20=
structures=20which=20relates=0Aclasses=20to=20encoding=20series.=20An=20=
important=20connection=20to=20our=20work=20here=0Ais=20that=20the=20=
series=20for=20structures=20we=20consider=20are=20D-finite=20symmetric=0A=
series,=20and=20many=20of=20the=20natural=20combinatorial=20actions=20=
preserve=0AD-finiteness=20on=20the=20level=20of=20these=20series.=0A=0A=
The=20reader=20unfamiliar=20with=20species=20is=20heartily=20encouraged=20=
to=0Aread~\cite{BeLaLe98}.=20The=20key=20features=20that=20we=20use=20=
are=20that=20for=0Aevery=20combinatorial=20family=20(species)=20$\sF$=20=
that=20one=20can=20define,=0Athere=20is=20an=20associated=20cycle=20=
index=20series=20$Z_\sF$=20and=20an=20asymmetric=0Acycle=20index=20=
series~$\Gamma_\sF$=20both=20of=20which=20are=20symmetric=0Aseries.=20=
Recall=20for=20any=20species=20\sF=20its=20cycle=20index=20series=0A=20=
$Z_\sF$=20is=20the=20series=20in=20$\C[\![p_1,=20p_2,=20\ldots]\!]$=20=
given=20by=0A\begin{equation}\label{eq:cycle_index}=0AZ_\sF(p_1,=20p_2,=20=
\ldots)=0A=20:=3D\sum_n\sum\limits_{\lambda\vdash=20n}\operatorname{Fix}=20=
\sF[\lambda]=0A=20=20=20\frac{p_1^{m_1}p_2^{m_2}\cdots=20=
p_k^{m_k}}{z_\lambda},=0A=20=20=20\end{equation}=0A%=0A=20=20=20where=20=
the=20value=20of=20$\operatorname{Fix}\sF[\lambda]$=20is=20the=0A=20=20=20=
number=20of=20structures=20of=20$\sF$=20which=20remain=20fixed=20under=20=
some=20labelling=0A=20=20=20permutation=20of=20type\footnote{A=0A=20=20=20=
=20=20permutation=20of=20type=20$(1^{m_1},=202^{m_2},=20\ldots)$=20has=20=
$m_1$=20fixed=0A=20=20=20=20=20points,=20$m_2$=20cycles=20of=20length=20=
2,=20etc.}=20$\lambda$,=20and=20$m_k$=20gives=0A=20=20=20the=20number=20=
of=20parts=20of=20$\lambda$=20equal=20to~$k$.=20=0A=0AThe=20definition=20=
of=20the=20{\em=20asymmetry=20index=20series}=20of=20a=0Aspecies~$\sF$,=20=
denoted~$\Gamma_\sF$,=20as=20introduced=20by=0ALabelle~\cite{BeLaLe98}=20=
is=20related,=20but=20more=20subtle.=20The=20series~$\Gamma$=20behaves=20=
analytically=20in=0Amuch=20the=20same=20way=20as=20the=20cycle=20index=20=
series,=20notably,=20substitution=20(in=0Aalmost=20all=20cases)=20is=0A=
reflected=20by=20plethysm,=20etc.=20Table~\ref{tab:smallspec}=20contains=20=
some=0Asmall=20examples=20of=20both=20series.=0A=0A\begin{table}[t]=0A=
\center=0A\begin{tabular}{lll|lll}\small=0AObject&Series&Symmetric=20=
function&Object&Series&Symmetric=20function\\\hline\hline=0A=
2-sets&$\Gamma_{\sE_2}$&$e_2=3D\frac{p_1^2}{2}-\frac{p_2}{2}$&2-multisets&=
$Z_{\sE_2}$&$h_2=3D\frac{p_1^2}{2}=0A+\frac{p_2}{2}$\\=0A=
3-sets&$\Gamma_{\sE_3}$&$e_3$=20=20=20=20=20=20=20=20=20=20=20=
&3-multisets&$Z_{\sE_3}$&$h_3$\\=20=0A4-sets&$\Gamma_{\sE_4}$&$e_4$=20=20=
=20=20=20=20=20=20=20=20=20&4-multisets&$Z_{\sE_4}$&$h_4$\\=0A$k$-sets=20=
&$\Gamma_{\sE_k}$&$e_k$=20=20=20=20=20=20=20=20=20=
&$k$-multisets&$Z_{\sE_k}$&=20$h_k=20$\\=0A3-cycles=20=
&$Z_{\sC_3}$&$\frac{p_1^3}{3}+\frac{p_3}{3}$=20&=20triples=20=
&$Z_{X^3}$&$p_1^3=20$\\=0A4-cycles=20=
&$Z_{\sC_4}$&$\frac{p_1^4}{4}+\frac{p_2^2}{12}+\frac{p_4}{12}$&=20=
4-arrays&$Z_{X^4}$&$p_1^4=20$\\=0A5-cycles=20=
&$Z_{\sC_5}$&$\frac{p_1^5}{5}+\frac{p_5}{30}$=20&=205-arrays&=20=
$Z_{X^5}$&$p_1^5=20$\\=0A$k$-cycles=20&$Z_{\sC_k}$&=20$\sum_{cd=3Dk}=20=
\phi(d)\frac{p_d^c}{k!}$=20&=0A$k$-arrays&$Z_{X^k}$&=20=
$p_i^k$\\\hline\hline\\=0A\end{tabular}\\=0A=0A\caption{Index=20series=20=
of=20small=20species=20and=20their=20corresponding=0A=20=20symmetric=20=
functions}=0A\label{tab:smallspec}=0A\hrule=0A\end{table}=0A=0AIn=20a=20=
fashion=20similar=20to=20the=20cycle=20index=20series,~$\Gamma_\sF$=20=
arises=0Athrough=20the=20enumeration=20of=20colourings=20of=20asymmetric=20=
$\sF$-structures.=0A=0AA=20notable=20example=20is=20the=20species=20of=20=
sets,=20$\sE$.=20Recall=20for=20any=20finite=0Aset=20$U$=20we=20have=20=
that=20$\sE[U]=3DU$.=20The=20two=20series=20above=20turn=20out=20to=20be=0A=
$Z_\sE=3D\exp(\sum_n=20p_n/n)=3D\sum_n=20h_n$=0A=
and~$\Gamma_\sE=3D\exp(\sum_n(-1)^n=20p_n/n)=3D\sum_n=20e_n$.=20=0A=0A=
The=20primary=20advantage=20of=20this=20approach,=20as=20is=20true=20=
with=20any=20generating=0Aseries=20approach,=20is=20that=20natural=20=
combinatorial=20operations=20(set,=0Acartesian=20product,=20=
substitution)=20coincide=20with=20straighforward=20analytic=0Aoperations=20=
(sum,=20product,=20plethystic=20substitution)\footnote{This=20is=0A=20=
slightly=20less=20true=20with=20the=20asymmetry=20index=20series,=20but=20=
true=20enough=20for=20our=20purposes}.=0A=0AThe=20{\em=20exponential=20=
generating=20series\/}=20of=20a=20species=20$\sF$=20is=20the=0Asum=20=
$\sF(t)=3D\sum_n=20|\sF[n]|=20\frac{t^n}{n!}$,=20where=20$|\sF[n]|$=20is=20=
the=20number=20of=0Astructures=20of=20type=20$\sF$=20on=20a=20set=20of=20=
size=0A$n$.=20The=20{\em=20ordinary=20generating=0Afunction},=20=
$\widetilde{\sF}(t)$,=20is=20the=20sum=20$\sF(t)=3D\sum_n=0A=
\operatorname{Orb}(\sF[n])=20t^n$,=20where=20=
$\operatorname{Orb}(\sF[n])$=20is=0Athe=20number=20structures=20of=20=
$\sF$=20on=20a=20set=20of=20size=20$n$=20distinct=20up=20to=0A=
relabelling.=20Also=20recall=20the=20notation=20$[x^n]f(x)$=20refers=20=
to=20the=0Acoefficient=20of=20$[x^n]$=20in=20the=20expansion=20of=20=
$f(x)$.=20This=20definition=0Aextends=20likewise=20to=20monomials.=20=20=0A=
=0AThe=20next=20result=20is=20essentially=20a=20collection=20of=20known=20=
results=20and=20basic=20facts=20of=0AD-finite=20series.=20=0A=20=
\begin{thm}\label{thm:Dfin_species}=0A=20=20=20=20Suppose=20$\sF$=20is=20=
a=20=20species=20such=20that=20$Z_\sF$=20is=20a=0A=20=20=20=20D-finite=20=
symmetric=20series=20and=20write=20$p_n=3Dx_1^n+x_2^n+\ldots$.=20Then=0A=20=
=20=20=20all=20of=20the=20following=20series=20are=20D-finite=20with=20=
respect=20to=20$t$:=0A=20=20\begin{enumerate}=0A=20=20\item=20The=20=
exponential=20generating=20function=20$\sF(t)$;=0A=20=20\item=20The=20=
ordinary=20generating=0A=20=20function=20$\widetilde{\sF}(t)$,=20if=20=
the=20additional=20condition=20that=20$Z_\sF(p_1,=20p_2,=20\ldots)$=20is=20=
D-finite=20with=0A=20=20respect=20to=20the=20$x_i$=20variables=20is=20=
also=20true;=0A=20=20\item=20The=20series=20$\sum_n=20\left([x_1^k\cdots=0A=
=20=20=20=20=20=20x_n^k]Z_\sF\right)=20\frac{t^n}{n!}$,=20for=20fixed=20=
$k$;=0A=20=20\item=20=20The=20series=20=0A$\sum_n\sum_{\bar=20k\in=20=
S^n}\left(\left[=20x_1^{k_1}x_2^{k_2}\dots=0A=20=20=20=20=
x_n^{k_n}\right]=20Z_\sF\right)=20t^n/n!$,=20for=20any=20finite=20set=20=
$S\subset=20\bN$.=0A=20=20\end{enumerate}=0A\end{thm}=0A=0A=20\begin{pf}=0A=
The=20first=20two=20parts=20are=20proved=20using=20two=20basic=20results=20=
about=20cycle=0Aindex=20series:=0A\begin{equation*}=0A=20=20=
\sF(t)=3DZ_\sF(t,=200,=200,=20\ldots)=20\quad=20\text{=20and=20}=20\quad=0A=
=20=20\widetilde{\sF}(t)=3DZ_\sF(t,=20t^2,=20t^3,=20\ldots)=0A=
\end{equation*}=0AThe=20first=20specialization=20is=20well-known=20to=20=
preserve=0AD-finiteness~\cite{Stanley80}=20for=20any=20$n$.=20=0AThe=20=
additional=20condition=20on=20the=20second=20item=20is=20sufficient=20to=20=
=20prove=0Athe=20D-finiteness=20since=20the=20stated=20substitution=20is=20=
the=20same=20as=0A$x_1\mapsto=20t$,=20and=20$x_i\mapsto=200$=20=
otherwise.=20=0A=0AThe=20third=20item=20of=20the=20proposition=20is=20=
proved=20by=20the=20expression=20=0A\begin{equation*}=0A=20=20\sum_n=20=
\left([x_1^k\cdots=20x_n^k]Z_\sF\right)=20=
\frac{t^n}{n!}=3D\rsc{Z_\sF}{\exp(th_k)},=0A\end{equation*}=0Awhich=20is=20=
D-finite=20by=20Theorem~\ref{thm:rsc_pres_df}.=0A=0AThe=20final=20item=20=
=20of=20the=20proposition=20is=20true=20because=20the=20series=20is=20=
equal=20to=0A\begin{equation*}=0A=20\rsc{Z_\sF}{\exp(t\sum_{i\in=20=
S}h_i)},=0A\end{equation*}=0Awhich=20is=20also=20D-finite=20by=20=
Theorem~\ref{thm:rsc_pres_df}.=0A\end{pf}=0A=0AWe=20have=20one=20large=20=
class=20of=20species=20for=20which=20the=20cycle=20index=20series=20is=0A=
D-finite.=20All=20of=20our=20examples=20come=20from=20this=20class.=0A=
\begin{thm}=0ALet=20$\sE$=20be=20the=20species=20of=20sets=20and=20=
let~$\sP$=20be=20a=20polynomial=0Aspecies=20with=20finite=20=
support\footnote{Species=20which=0Acan=20be=20written=20as=20polynomials=20=
of=20{\em=20molecular=0A=20=20species}.=20For=20example,=20every=20=
species=20in=20Table~\ref{tab:smallspec}=20is=0Apolynomial.}.=20=
Then~$\sF=3D\sE\circ=20\sP$=20describes=20a=20species=20for=0A=
which~$Z_\sF$=20is=20a=20D-finite=20symmetric=20series=20and=20provided=20=
that=20$\sP(0)=3D0$,~$\Gamma_\sF$=20is=20also=20a=20D-finite=20symmetric=20=
series.=0A\end{thm}=0A\begin{proof}=0AIf~$\sP$=20is=20a=20polynomial=20=
species,=20then=20its=20cycle=20index=20series=20is=0Aa=20polynomial=20=
in=20the=20$p_i$'s,=20say=20$P(p_1,=20\ldots,=20p_n)$.=20Composition=20=
of=0Aspecies=20is=20reflected=20in=20the=20cycle=20index=20series=20by=20=
plethysm,=20thus=0A\[=0AZ_\sF=3D\exp(\sum_k=20p_k/k)[P(p_1,=20\ldots,=20=
p_n)]=3D\exp\left(\sum=20P(p_k,=0Ap_{2k},\dots,=20p_{nk})/k\right).=0A\]=0A=
For=20any=20specialization=20of=20all=20but=20a=20finite=20number=20of=20=
$p_i$=20to=20zero,=0Athis=20gives=20an=20exponential=20of=20a=20=
polynomial,=20which=20is=20clearly=0AD-finite.=20Thus,=20$Z_\sF$=20is=20=
D-finite.=20We=20can=20similarly=20show=20that=0A$\Gamma_\sF$=20is=20=
also=20D-finite=20under=20the=20stated=20conditions,=20since=20the=0A=
composition=20also=20results=20in=20a=20plethystic=20composition.=0A=
\end{proof}=0A=0AOur=20concluding=20remarks=20in=20=
Section~\ref{sec:conclusion}=20address=20the=0Amore=20general=20question=20=
of=20combinatorial=20criteria=20on=20a=20species=20$\sF$=0Athat=20ensure=20=
that=20$Z_\sF$=20or=20$\Gamma_\sF$=20are=20D-finite.=0A\section{Using=20=
species=20to=20describe=20regular=20graph-like=20structures}=0A=
Ultimately=20our=20goal=20is=20to=20generalize=20the=20well-studied=20=
case=20of=20$k$-regular=0Agraphs=20to=20other=20structures=20whose=20=
cycle=20index=20series=20are=20D-finite.=20To=0Ado=20so,=20we=20express=20=
the=20graph=20encoding=20by=20degree=20sequence=20as=20symmetric=0A=
series,=20and=20describe=20how=20to=20find=20such=20a=20representation=20=
in=20general=0Ausing=20species=20theory.=0A=0AIn=20=
Eq.~\eqref{eq:allgraphs}=20we=20define=20$G(x_1,=20x_2,=20\ldots)$=20as=0A=
the=20encoding=20over=20all=20graphs=20of=20their=20degree=20sequence=20=
and=20we=20express=0Athis=20as=20an=20infinite=20product.=20It=20turns=20=
out=20that=20this=20series=20is=0Aequivalent=20to=20$E[e_2]$,=20which=20=
is=20equivalent=20to=20$\Gamma_{\sE\circ=0A=20=20\sE_2}$.=20The=20=
equivalent=20series=20for=20multigraphs=20(with=20loops)=20is=20equal=0A=
to=20$H[h_2]=3DZ_{\sE\circ\sE_2}$,=20and=20thus=20suspecting=20an=20=
explanation=20via=0Aspecies,=20we=20investigate=20this=20connection.=20=
Specifically,=20=20how=20do=20we=0Aconstruct=20symmetric=20function=20=
equations=20to=20describe=20the=20generating=0Afunctions=20of=20=
different=20families=20of=20objects,=20such=20as=20hypergraphs.=0A=0AWe=20=
begin=20with=20the=20remark=20that=20$\sF=3D\sE\circ\sE_2$=20is=20{\em=20=
not\/}=20the=0Aspecies=20of=20graphs.=20It=20is=20the=20species=20of=20=
partitions=20into=202-sets.=20For=0Aexample,=20$\{\{1,4\},\{2,6\},=20=
\{3,7\},\{5,8\}\}$=20is=20an=20element=0Aof~$\sF$,=20and=20we=20should=20=
not=20think=20that=20this=20is=20the=20graph=20on=208=0Avertices,=20with=20=
four=20edges,=20rather=20it=20just=20gives=20the=20basic=20structure,=0A=
i.e.=20four=20edges.=0A=0AWe=20express=20the=20Polya=20cycle=20index=20=
in=20the=20power=20series=0Asymmetric=20function.=20As=20a=20series=20in=20=
the=20symmetric=20$x_i$=0Aindeterminates,=20it=20is=20an=20inventory=20=
of=20distinct=20(non-isomorphic)=0Acolourings=20of=20the=20elements=20of=20=
the=20species.=20=0A=20=20For=20example,=20the=20non-isomorphic=20=
colourings=20(by=20positive=0Aintegers,=20say)=20of=20the=20set=20=20=
$\{a,b\}$=20is=20the=20set=20of=20maps=0A$\{(a,b)\mapsto(i,j)\in\bN^2:=20=
i\leq=20j\}$,=20and=20the=20inventory=20of=20all=0Asuch=20colourings=20=
is=20$\sum_{i\leq=20j}=20x_ix_j=3Dh_2$.=0A=0AA=20colouring=20of=20an=20=
element=20in=20$\sE\circ\sE_2$=20gives=20rise=20to=20a=20graph=0A(See=20=
Figure~\ref{fig:graph})=20and=20two=20colourings=20are=20isomorphic=20if=20=
one=0Ais=20a=20graph=20relabelling=20of=20the=20other.=20The=20monomial=20=
encoding=20a=20colouring=20indicates=0Ahow=20many=20time=20each=20colour=20=
was=20used,=20that=20is,=20in=20how=20many=20edges=20the=0Acolour=20=
appears,=20that=20is,=20the=20degree=20of=20the=20vertex=20represented=20=
by=20the=0Acolour.=0A=20=0A\begin{figure}=0A\center=0A=
\includegraphics[width=3D5cm]{graph1.eps}=0A\caption{The=20graph=20=
associated=20to=20the=20coloured=20set=20partition=20$\{=20\{1_3,4_2\},=20=
\{2_2,6_4\},\{3_4,7_3\},\{5_2,8_1\}\}$}=0A\label{fig:graph}=0A\hrule=0A=
\end{figure}=0A=0AWe=20restate=20this=20correspondence.=20The=20species=20=
$\sE\circ\sE_2$=20indicates=0Athe=20structure--=20sets=20of=20pairs.=20=
The=20cycle=20index=20series=0A$Z_{\sE\circ\sE_2}$=20encodes=20=
non-isomorphic=20colourings=20of=20elements,=0Awhich=20are=20in=20turn=20=
equivalent=20to=20labelled=20multi-graphs.=20The=20sets=20of=20pairs=0A=
indicate=20edges,=20and=20the=20colours=20indicate=20vertices.=0A=0AFor=20=
many=20applications,=20like=20regular=20graphs,=20we=20would=20like=20to=20=
count=0Acolourings=20without=20repetition.=20In=20this=20case,=20we=20do=20=
not=20allow=0Arepetition=20of=20a=20colour=20in=20a=20given=20object,=20=
hence=20to=20encode=20a=20$k$-set,=0Aeach=20colour=20appears=20exactly=20=
once,=20and=20this=20is=20precisely=20the=20notion=20of=0Aasymmetry=20in=20=
the=20asymmetry=20cycle=20index,=20and=20thus=20we=20use=20$\Gamma$=0A=
instead=20of=20$Z$.=20Remark,=0Aand=20thus=0A\begin{equation*}=0A=20=20=
\Gamma_{\sE_k}=20=3D=20e_k=20\quad\text{and=20thus,=20}=20=
\quad\Gamma_\sE=3DE.=0A\end{equation*}=0ATaking=20the=20same=20species=20=
$\sE\circ\sE_2$=20as=20above,=20and=20using=20the=0Aasymmetry=20index=20=
series=20with=20a=20similar=20argument,=20we=20get=20that=0A=
$\Gamma_{\sE\circ\sE_2}=3D\EE[e_2]$=20encodes=20simple=20graphs=20=
without=20loops=0Aon=20the=20set=20of=20colours=20precisely=20as=20is=20=
determined=20by=0AEq.~\eqref{eq:allgraphs}:=20=
$E[e_2]=3D\prod_{i<j}(1+x_ix_j)$.=20=20This=20gives=0Aus=20a=20way=20to=20=
have=20{\em=20direct=20access=20to=20monomial=20encodings=20of=0A=20=20=
combinatorial=20objects\/},=20as=20symmetric=20functions=20expressed=20=
in=20common=0Abases,=20like=20the=20power=20sum=20basis.=20=20These=20=
two=20series=20are=20compatible=20and,=0Aone=20can=20show=20that=20=
graphs=20with=20loops=20are=20encoded=20by~$E[h_2]$,=20and=0Agraphs=20=
with=20multiple=20edges,=20but=20no=20loops=20are=20given=20by~$H[e_2]$.=0A=
=0AMore=20generally,=20we=20can=20consider=20any=20structure=20which=20=
is=20built=20as=20a=20set=0Aof=20objects=20from=20a=20finite=20set=20of=20=
classes.=20Figure~\ref{fig:multi}=20shows=0Aa=20more=20general=20object=20=
built=20as=20a=20set=20of=20cycles=20and=20sets.=20=20In=20this=0A=
framework=20it=20is=20encoded=20by=20the=20=
monomial~$x_1^2x_2^2x_3x_4^2x_5^2$,=20and=0Athus=20we=20see=20that=20=
regularity=20in=20this=20situation=20refers=20to=20the=20number=20of=0A=
times=20each=20label=20appears=20in=20one=20of=20the=20smaller=20=
substructures.=0A\begin{figure}=0A\center=0A=
\includegraphics[height=3D3.5cm]{multi1.eps}=0A\caption{A=20structure=20=
composed=20of=20a=20set=20of=20smaller=20structures=20(cycles=0A=20=20=
and=203-sets).}=0A\label{fig:multi}=0A\hrule=0A\end{figure}=0A=0AUsing=20=
this=20framework=20we=20can=20examine=20other=20species=20of=20=
structures=20built=0Aup=20from=20smaller=20objects.=20These=20species=20=
are=20such=20that=20both=20$Z_\sF$=0Aand~$\Gamma_\sF$=20give=20rise=20to=20=
interesting=20combinatorial=20objects.=0A=0AWe=20can=20produce=20=
enumerative=20results=20for=20objects=20all=20of=20the=20same=0Aflavour:=20=
labelled=20sets=20of=20objects=20in=20which=0Athere=20is=20a=20certain=20=
regularity.=20We=20begin=20with=20a=20natural=20generalization=0Aof=20=
$k$-regular=20graphs,=20and=20then=20we=20consider=20other=20types=20of=0A=
objects=20such=20as=20hyper-graphs,=20and=20directed=20graphs.=0A=0A=
\subsection{$S$-regular=20graphs}=0AA=20graph=20is=20{\em=20$S$-regular}=20=
if=20the=20set=20of=20vertex=20degrees=20in=20the=20graph=0Ais=20a=20=
subset=20of=20$S$.=20For=20example,=20a=20graph=20is=20$\{i,j\}$-regular=20=
if=20every=20vertex=0Ais=20of=20degree=20$i$=20or=20$j$.=0A=0AIt=20does=20=
not=20seem=20that=20the=20asymptotic=20enumeration=20of=20these=20=
objects=20has=20been=0Adirectly=20considered=20before.=20It=20is,=20in=20=
some=20sense,=20a=20variation=20of=20the=20=0Aasymptotic=20number=20of=20=
labelled=20graphs=20with=20a=20given=20degree=20sequence,=0Awhich=20has=20=
been=20considered=20by=20Bender=20and=20Canfield~\cite{BeCa78}=20and=0A=
McKay=20and=20Wormald~\cite{McWo90},=20and=20may=20very=20well=20be=20=
computable=20from=0Athis.=20=0A=0A%=20If=20$i\neq=20j$,=20then=20there=20=
are=20$2^3$=20possible=20degree=20sequences=20for=20a=0A%=20labelled=20=
$\{i,j\}$-regular=20graphs.=20One=20of=20these=20is=20$(i,i,j)$,=20=
encoded=0A%=20by=20$x_1^ix_2^ix_3^j$.=20Remark,=20this=20differs=20from=20=
a=20graph=20with=20degree=0A%=20sequence=20$(i,j,i)$,=20since=20we=20are=20=
considering=20labelled=20graphs.=20In=0A%=20the=20case=20of=20=
$k$-regular=20graphs,=20the=20coefficient=20of=20$x_1^kx_2^k\dots=0A%=20=
x_n^k$=20in=20$E[e_2]$=20is=20the=20same=20as=20the=20coefficient=20of=20=
$m_{k^n}$,=20thus=0A%=20computing=20the=20latter=20gives=20the=20former.=20=
However,=20we=0A%=20want=20to=20count=20the=20total=20number=20of=20=
graphs=20encoded=20by=20monomials=20with=0A%=20only=20$i$s=20and=20$j$s=20=
as=20powers,=20that=20is,=20the=20sum=20of=20all=20of=20their=0A%=20=
coefficients.=20Now,=20two=20monomials=20with=20the=20same=20number=0A%=20=
of=20$i$s=20and=20$j$s=20are=20both=20summands=20in=20the=20appropriate=20=
monomial=0A%=20symmetric=20function,=20and=20thus,=20to=20get=20the=20=
proper=20total=20we=20must=20scale=0A%=20the=20coefficient=20of=20the=20=
monomial=20by=20the=20number=20of=20different=20degree=0A%=20sequences=20=
it=20represents.=20If=20there=20are=20$k$=20$i$s=20and=20$n-k$=20$j$s,=20=
then=0A%=20this=20factor=20is=20all=20the=20ways=20to=20shuffle=20the=20=
$i$s=20and=20the=20$j$s:=0A%=20$\binom{n}{k}$.=20=0A=0AThus,=20the=20=
scalar=20product=20which=20represents=20the=20generating=0Aseries=20for=20=
the=20number=20of=20$\{i,=20j\}$-regular=20graphs=20is=20given=20by=0A=
incorporating=20this=20factor,=20which=20ultimately=20greatly=20=
simplifies=20the=0Acalculation.=20The=20exponential=20generating=20=
series=20for=20the=20number=20of=0A$\{i,j\}$-regular=20graphs=20is=20=
given=20by=0A\begin{align*}=0AG_{i,j}(t)&=3D=0A%=20=20=20\left\langle=20=
E[e_2],=20=0A%=20=20=20=20=20\sum_{n\geq=200}\frac{t^n}{n!}\sum_{k=3D0}^n=20=
\binom{n}{k}=20h_i^kh_j^{n-k}=0A%=20=20=20\right\rangle=20\\=0A%=20=
&=3D\left\langle=20E[e_2],=0A%=20=20=20=20=20\sum_{n\geq=20=
0}\sum_{k=3D0}^n=20\frac{h_i^kt^k}{k!}\frac{h_jt^{n-k}}{(n-k)!}=0A%=20=20=
=20\right\rangle=20\\=0A%=20&=3D=0A\big\langle=20E[e_2],=20=
\exp\left(t(h_i+h_j)\right)=20=0A=20=20\big\rangle.=0A\end{align*}=0A=0A=
This=20is=20clearly=20D-finite,=20and=20computable=20using=20=
\spa~(although=0Anot~\ham).=20Furthermore,=20by=20a=20similar=20=
computation,=20we=20have=20the=0Afollowing=20result.=0A\begin{thm}=20=0A=20=
=20The=20number=20of~$S$-regular=20graphs=20is=20D-finite=20for=20any=0A=20=
=20finite~$S\subset\N$,=20and=20its=20exponential=20generating=20series=20=
is=20given=0A=20=20by=20the=20scalar=20product=20of=20symmetric=20=
functions,=0A\begin{equation*}=0A=20=20G_S(t)=3D\left\langle=20E[e_2],=20=
\exp\left(\sum_{i\in=20S}=20h_it\right)=0A=20=20\right\rangle.=0A=
\end{equation*}=0A\end{thm}=0A=0ATable~\ref{tab:kjreg}=20offers=20the=20=
initial=20counting=20sequence=20for=20some=0Asmall=20values=20of=20$k$=20=
and=20$j$.=20Each=20one=20corresponds=20to=20a=20known=0Adifferential=20=
equation=20satisfied=20by=20it=20generating=20function.=20In=0A=
Table~\ref{tab:graph}=20we=20compute=20the=20asymptotic=20number=20of=20=
some=20of=0Athese=20graphs.=20=0A=0AWe=20make=20one=20simple=20=
observation.=20The=20$\{k,k+2\}$-regular=20graphs=20are=0Aisomorphic=20=
to=20the=20$k+2$-regular=20graphs=20with=20loops,=20by=20simply=20adding=0A=
loops=20to=20the=20vertices=20of=20degree=20$k$.=20This=20gives=20a=20=
family=20of=20identities=0A\begin{equation*}=0A=20=20\left\langle=20=
E[e_2],\exp\left(t(h_i+h_{i+2})\right)\right\rangle=0A=20=3D\left\langle=20=
E[h_2],=20\exp(th_{i+2})\right\rangle.=0A\end{equation*}=0A=0A=
%-----------------------------------------------------------------=0A=
\begin{table}=0A=
%-----------------------------------------------------------------=0A=
\center=0A\begin{tabular}{cl}=0A$i,j$=20&=20Initial=20terms=20in=20the=20=
counting=20sequence\\=0A1,2=20&=0A=
1,0,1,4,18,112,820,6912,66178,708256,8372754,108306280,1521077404=0A\\=0A=
1,3&=20=0A1,=200,=201,=200,=208,=200,=20730,=200,=20188790,=200,=20=
102737670,=200,=20102172297920,0=0A\\=0A1,4&=0A=
1,0,1,0,3,6,30,1011,38920,1920348,116400186,8580463110,757574641296=0A\\=0A=
2,3=20&=0A=
1,0,0,1,10,112,1760,35150,848932,24243520,805036704,30649435140=0A\\=0A=
2,4&=0A1,0,0,1,3,38,730,20670,781578,37885204,2289786624,168879532980=0A=
\\=0A3,4&=0A=
1,0,0,0,1,26,820,35150,1944530,133948836,11234051976,1127512146540=0A\\=0A=
\end{tabular}\\=0A=0A\caption{Counting=20sequences=20for=20=
$\{i,j\}$-regular=20graphs=20for=20small=0A=20=20values=20of~$i$=20=
and~$j$.}=0A\label{tab:kjreg}=0A\hrule=0A\end{table}=0A=
%-----------------------------------------------------------------=0A=0A=
\subsection{Set=20covers=20and=20uniform=20hypergraphs}=20We=20now=20=
illustrate=20the=0Amethod=20on=20another=20family=20of=20objects,=20=
which=20results=20in=20set=20covers=20and=0Auniform=20hypergraphs.=0AAn=20=
$n$-set=20is=20a=20set=20of=20cardinality~$n$.=0A=0A=
\begin{defn}[$k$-cover=20of=20a=20set]=0A=20=20A=20collection=20of=20=
$r$-sets=20$\mathcal{B}=3D\{B_1,=20\ldots,=20B_r\}$=20is=20an=20{\em=0A=20=
=20$r$-cover}=20of~$S$=20if~$\bigcup_{i=3D1}^r=20B_i=20=3DS$.=20=
If~$S=3D[n]=3D\{1,2,\dots,=20n\}$,=20then=20it=20is=20an=20$r$-cover=20=
of=20$n$.=20=20A=20cover=20is=20{\em=0A=20=20restrictive\/}=20if=20all=20=
of=20the=20$B_i$=20are=20distinct.=20=20An=20$r$-cover=20is=0A=20=20{\em=20=
$k$-regular}=20if=20any=20given=20element=20occurs=20in=20exactly=20=
$k$-subsets.=0A\end{defn}=0A=0AA=20combinatorial=20argument=20shows=20=
that=20the=20number=20of=20distinct=20covers=20for=0Aa=20set=20of=20$n$=20=
elements=20is=0A\begin{equation*}=0A=20=20=
\frac12\sum_{k=3D0}^n(-1)^k\binom{n}{k}2^{2^n-k},=0A\end{equation*}=0A=
which=20is=20clearly=20not=20P-recursive=20(equivalently,=20its=20=
generating=20series=0Ais=20not=20D-finite.)=0A=0ADevitt=20and=20=
Jackson~\cite{DeJa82}=20give=20a=20generating=20function=20for=20the=0A=
number=20of=20$k$-regular=20$r$-covers=20of=20$[n]$,=20a=20notion=20=
introduced=20by=0AComtet~\cite{Comtet68}.=20Further,=20they=20prove=20=
that=20the=20number=20of=0Aarithmetic=20operations=20required=20to=20=
actually=20calculate=20the=20number=20of=0A$k$-covers=20of=20an=20$n$=20=
set=20by=20their=20method=20is=20bounded=20by=20$cn^k=0A\log=20n$.=20=
Results=20for=20fixed=20$k$,=20specifically=20$k=3D2,=203$=20were=20=
treated=20by=20Comtet~\cite{Comtet68}=20and=20Bender~\cite{Bender74a}=20=
respectively.=0A=0AWe=20can=20derive=20enumeration=20formulas.=20For=20=
example,=20a=20$k$-regular=20graph=0Aon~$n$=20vertices=20is=20a=20=
restrictive=20$k$-cover=20of=20$[n]$=20into=202-sets.=20In=0Ageneral,=20=
calculating=20the=20generating=20function=20for=20restrictive=0A=
$k$-covers=20of=20$[n]$=20into=20$j$-sets=20can=20be=20expressed=20as=0A=
\[=0A\rsc{\Gamma_{\sE\circ\sE_j}(p_1,=20p_2,=20\ldots)}{\sum_n=20=
h_{k}^nt^n}=3D=0A\rsc{\EE[e_j]}{\sum_n=20h_{k}^nt^n}.=0A\]=0ATo=20=
determine=20$k$-covers=20with=20mixed-cardinality=20sets,=20say=20both=20=
$i$=20and=0A$j$,=20we=20calculate=0A=
\[\rsc{\Gamma_{\sE\circ(\sE_i+\sE_j)}(p_1,=20p_2,=0A=20=20=
\ldots)}{\sum_n=20h_{k}^nt^n}=3D=20\rsc{\EE[e_i+e_j]}{\sum_n=20=
h_{k}^nt^n}.\]=0A=0AThis=20yields=20the=20following=20simple=20=
consequence=20of=0ATheorem~\ref{thm:Dfin_species}.=20=0A\begin{cor}=0A=20=
=20Let=20$S$=20be=20a=20finite=20set=20of=20integers.=20=20For=20fixed=20=
$n$,=20and=20fixed=20$k$,=20the=20exponential=20generating=0A=20=20=
function=20for=20$k$-regular=20$S$-covers=20of=20sets=20is=20D-finite,=20=
and=20is=20given=20by=20the=20scalar=20product=0A\begin{equation*}=0A=
\left\langle=20E[\sum_{s\in=20S}=20e_s],=20\exp(h_kt)\right\rangle.=0A=
\end{equation*}=0A\end{cor}=0A=0A\begin{example}=0AWe=20can=20express=20=
the=20problem=20of=20counting=20distinct=0Arestrictive=202-covers=20of=20=
a=20set=20of=20cardinality=20$n$=20by=20sets=20of=0Acardinality=20less=20=
than=205=20as=20a=20scalar=20product.=20Denote=20the=20exponential=20=
generating=0Afunction=20of=20such=20set=20covers,=20by=20$S(t)$.=20We=20=
have,=0A%=0A\begin{equation*}=0AS(t)=3D\rsc{E[e_1+e_2+e_3+e_4]}{\exp(t=20=
h_2)}.=0A\end{equation*}=0A=0AThis=20problem=20is=20perfectly=20suited=20=
to=20either=20of=20our=20algorithms.=20We=20can=0Adetermine=20this=20=
differential=20equation,=20and=20the=20initial=20terms=20of=20the=0A=
counting=20sequence:=0A\[=201,=200,=201,=208,=2080,=201037,=2017200,=20=
350682,=208544641,=20243758420,=208010360039.\]=0A\end{example}=0A=0AIt=20=
is=20worthwhile=20to=20remark=20that=20for=20a=20fixed~$j$,=20the=20set=20=
coverings=20by=20$j$-sets=20are=20equivalent=20to=20loopless=20=
$j$-uniform=20hypergraphs=20without=20multiplicities.=20These=20are=20=
encoded=20by=20$E[e_j]$.=20If=20we=20wish=20to=20encode=20hypergraphs=20=
with=20loops,=20we=20replace=20$e_j$=20by=20$h_j$,=20and=20if=20we=20=
wish=20to=20encode=20hypergraphs=20with=20multiplicities=20we=20replace=20=
$E$=20by=20$H$.=20=0A=0A=
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%=0A=0A=
%-----------------------------------------------------------------=0A=
\section{Asymptotic=20analysis}=20\label{sec:asympt}=0A=
%-----------------------------------------------------------------=0ANow=20=
that=20we=20have=20established=20how=20to=20determine=20the=20=
differential=0Aequations=20satisfied=20by=20regular=20families=20of=20=
combinatorial=20objects,=20we=0Aprocess=20these=20differential=20=
equations=20to=20obtain=20asymptotic=20enumeration=0Aresults.=0A=0A=
Asymptotic=20enumeration=20of=20regular=20graphs=20is=20a=20topic=20that=20=
has=20received=20a=20great=20deal=20of=20attention.=20Indeed,=20as=20=
Gropp~\cite{Gropp92}=0Apoints=20out,=20the=20basic=20problem=20of=20=
regular=20graph=20enumeration=20was=0Aconsidered=20before=20graphs=20=
were=20even=20``invented'',=20over=20120=20years=20ago.=0AWe=20first=20=
see=20some=20explicit=20results=20for=20graphs=20with=20fixed=20degree=0A=
sequences=20in=20the=20work=20of=20Read~\cite{Read58,Read60},=20however,=20=
these=20are=0Arumoured=20to=20be=20``difficult=20to=20penetrate''.=20=
Nonetheless,=20one=20can=0Adetermine=20an=20asymptotic=20expression=20=
for=20the=20number=20of=203-regular=0Agraphs.=20Bender=20and=20=
Caufield~\cite{BeCa78}=20produce=20the=20first=20general=0Aasymptotic=20=
formula=20for=20=20the=20number=20of=20$k$-regular=20graphs=20on=20$n$=0A=
vertices,=20and=20Bollob\'as=20produces=20a=20similar=20result=20by=20a=20=
more=0Aprobabilistic=20approach=20that=20generalizes=20with=20ease=20to=20=
treat=0Ahypergraphs.=20Next,=20=20work=20by=20McKay~\cite{McKay90}=20and=20=
McKay=20and=0AWormald~\cite{McWo91}=20consider=20the=20problem=20of=20=
$k$=20which=20is=20not=20fixed,=0Abut=20rather=20a=20function=20of=20=
$n$,=20and=20they=20achieve=20a=20formula=20which=20they=0Abelieve=20to=20=
be=20true=20in=20general,=20valid=20as=20$n\to\infty=20\text{=20=
uniformly=20for=20}=201\leq=20k=3Do(n\sp=20{1/2})$=0A\begin{equation}=0A=
g_{k}(n)\sim=20\frac=20{(nk)!}{(nk/2)!2\sp=20{nk/2}(k!)\sp=20=
n}\exp\bigg(-\frac=20{k\sp=0A=20=202-1}4-\frac=20{k\sp=203}{12n}+O(k\sp=20=
2/n)\bigg).=0A\end{equation}=0A=0AThis=20resembles=20Bollob\'as'=20=
asymptotic=20formula~\cite{Bollobas80}=20for=20labelled=20$k$-regular=20=
$r$-uniform=20hypergraphs,=20on=20$n$=20vertices=20when=20$\frac{nk}{r}$=20=
is=20an=20integer=0A\begin{equation*}=0A=
g_{k}^{(r)}(n)\sim\frac{(nk)!}{(nk/r)!(r!)^{(nk/r)}(k!)^n}\exp\left(-(r-1)=
(k-1)/2\right).=0A\end{equation*}=0AHe=20also=20gives=20a=20formula=20=
for=20hypergraphs=20in=20which=20hyperedges=20only=20have=20single=20=
vertex=20intersections,=20which=20gives=20the=20constant=20in=20the=20=
McKay=20and=20Wormald=20formula=20for=20$r=3D2$.=20=0A=0AThe=20=
asymptotic=20enumeration=20problem=20of=20regular=20graphs=20has=20been=20=
treated=0Awith=20a=20variety=20of=20methods,=20such=20as=20the=20=
multidimensional=20Cauchy=0Aintegral=20technique=20mentioned=20=
earlier~\cite{McKay90},=20a=20``switching''=0Atechnique=20based=20on=20=
inclusion=20exclusion~\cite{McWa03,McWo91},=20and=20some=20direct=0A=
combinatorial=20arguments=20on=20the=20equivalent=20problem=20of=20=
symmetric=20(0,1)=0Amatrices=20with=20fixed=20row=20sum~\cite{BeCa78}.=0A=
=0ABollob\'as~\cite{Bollobas82}=20remarks=20that=20the=20number=20of=0A=
$k$-regular=20unlabelled=20graphs=20grows=20asymptotically=20=
like~$l_n/n!$=20as~$n$=0Atends=20to=20infinity=20and=20where=20$l_n$=20=
is=20the=20number=20of=20labelled=20regular=0Agraphs.=20Intuitively,=20=
this=20is=20due=20to=20the=20fact=20that,=20for=20most=20large=0Agraphs=20=
with=20no=20isolated=20vertices,=20and=20at=20most=20one=20vertex=20of=20=
maximal=0Adegree,=20the=20automorphism=20group=20consists=20of=20only=20=
the=20identity=20automorphism.=20=0A=0A=0AThe=20enumeration=20of=20other=20=
configurations=20is=20relatively=20untreated.=0AGessel=20=
remarked~\cite{Gessel90}=20that=20the=20exponential=20generating=20=
functions=20of=20$k$-regular=20$r$-uniform=20hypergraphs=20(with=20and=20=
without=20loops,=20with=20and=20without=20multiplicities)=20are=20=
D-finite=20and=20the=20differential=20equations=20they=20satisfy=20are=20=
obtainable=20via=20the=20scalar=0Aproduct.=20=
Domoco{\c{s}}~\cite{Domocos96}=20determines=20a=20scalar=20product=20=
form=20for=0Athe=20generating=20series=20of=20minimal=20coverings=20that=20=
are=20multipartite=0Ahypergraphs.=0A=0AHere=20we=20continue=20to=20treat=20=
a=20variety=20of=20configurations.=20The=20results=20are=0Atabulated=20=
in=20Table~\ref{tab:graph}=20and=20Table~\ref{tab:other}=20and=20allow=20=
for=0Aa=20comparison=20across=20objects=20rather=20than=20regularity=20=
parameter.=20All=20of=20the=20results=20were=20automatically=20=
generated.=20=0A=0A\subsection{Technique}=0AOur=20method=20is=20a=20=
classical=20singularity=20analysis=20of=20formal=20solutions=20of=20the=20=
linear=0Adifferential=20equations.=20It=20is=20precisely=20the=20same=20=
method=20we=20used=20in=20our=0Aanalysis=20of=20$k$-uniform=20Young=20=
tableaux~\cite{ChMiSa05},=20and=0Athus=20we=20do=20not=20repeat=20the=20=
details=20here.=20Instead,=20after=20a=20short=0Adescription=20of=20the=20=
major=20steps,=20we=20present=20the=20fruits=20of=20our=0Aanalyses.=20A=20=
Maple=20worksheet=20of=20the=20computations=20is=20available=20at\\=20=
{\tt=0A=20=20http://www.math.sfu.ca/$\tilde{=20}$mmishna/programs.html}.=20=
=20=0A=0AIn=20the=20simplest=20cases,=20essentially=20the=20cases=20we=20=
could=20analyze=20directly=20with=20combinatorial=20arguments,=20we=20=
can=20solve=20the=20differential=20equation=20and=20do=20an=20asymptotic=20=
analysis=20on=20the=20solution.=20=20In=20the=20more=20complex=20cases,=20=
we=20first=20convert=20our=20differential=20equation=20to=20the=20=
recurrence=20satisfied=20by=20the=20coefficients.=20Our=20series=20are=20=
D-finite,=20and=20thus=20such=20a=20sequence=20is=20bounded=20by=20a=20=
rational=20power=20of=20$n!$,=20and=20thus,=20we=20scale=20our=20=
sequence=20until=20it=20is=20convergent,=20and=20this=20allows=20us=20a=20=
more=20precise=20analysis.=20We=20convert=20this=20recurrence=20back=20=
to=20a=20differential=20equation,=20and=20determine=20the=20roots=20of=20=
the=20polynomial=20which=20is=20the=20coefficient=20of=20the=20leading=20=
term.=20=46rom=20this=20we=20can=20calculate=20the=20dominant=20=
singularity,=20and=20determine=20a=20power=20series=20solution=20to=20=
the=20differential=20equation=20around=20this=20point.=20=46rom=20this,=20=
we=20analyze=20the=20solution=20to=20determine=20an=20asymptotic=20=
expression=20=20for=20the=20coefficients.=20This=20can=20be=20done=20=
automatically=20using=20tools=20from=20Maple,=20specifically=20DEtools=20=
and=20gfun.=20Finally,=20by=20generating=20sufficiently=20many=20terms=20=
in=20the=20sequence,=20we=20compare=20with=20the=20formula=20to=20=
determine=20a=20value=20for=20the=20constant.=20=20=0A=0AIn=20the=20=
tables=20that=20follow,=20the=20Sloane=20sequence=20number=20refer=20to=20=
the=0Acounting=20sequence=20as=20indexed=20in=20the=20Sloane=20On-line=20=
Encyclopedia=20of=0AInteger=20Sequences~\cite{Sloane}.=20=0A=0A=
Furthermore,=20we=20mention=20only=20the=20dominant=20term=20of=20the=20=
asymptotic=0Aexpression,=20but=20we=20could=20get=20the=20subsequent=20=
terms,=20save=20for=20the=0Aappropriate=20constants.=20This=20is=20a=20=
consequence=20of=20the=20principal=20weakness=0Aof=20our=20method--=20we=20=
cannot=20generate=20exact=20expressions=20for=20the=0Aconstants.=0A=0AWe=20=
remark=20that,=20as=20presented=20in=20Table~\ref{tab:graph},=20graphs=20=
of=20different=20types=20have=20the=20same=20asymptotic=20development,=20=
but=20differ=20only=20in=20the=20constants.=20=0A=
%-----------------------------------------------------------------=0A=
\begin{table}=0A=
%-----------------------------------------------------------------=0A=
\center=0A\begin{tabular}{|p{3cm}||l|l|l|l|}\hline=0A$k$=20&=20&=201=20&=20=
2&=203\\\hline\hline=0AFormula=20=20&=20=20=
\begin{minipage}{1cm}{\mbox{}\\=20\vspace{1.5cm}}=0A=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20\end{minipage}=0A=20=20=20=20=20=20=20=20=20=20=20&=20=
=20=20$\displaystyle\left(\frac12\right)^{\frac=20n2}\frac{n!}{(n/2)!}$=20=
=20=0A=20=20=20=20=20=20=20=20=20=20=20&=20=
$\displaystyle\frac{n!}{\sqrt{n}}$=0A=20=20=20=20=20=20=20=20=20=20=20&=20=
$\displaystyle\left(\frac32\right)^{\frac=20n2}\frac{n!=20(n/2)!}{n}$\\=0A=
\hline\hline=0A=
%------------------------------------------------------------------------=0A=
Simple=20graphs&=20Sloane=20\#=20=20&=20A001147=20&=20A001205=20&=20=
A002829\\=20=0A$E[e_2]$&=20Constant=20&=201=20=20=20=20=20=20=20&=20=
$e^{-\frac34}/\sqrt{\pi}$=20&=20.043\\=0A\hline=0A=
%------------------------------------------------------------------------=0A=
With=20loops=20=20&=20Sloane=20\#=20=20=20&=20A001147=20&=20A108246=20&=20=
A110039\\=0A$E[h_2]$&=20Constant=20&=201=20=20=20=20=20=20=20&=20=20=
$e^{-\frac14}/\sqrt{\pi}$=20&=20.318=20\\=20=0A\hline=0A=
%------------------------------------------------------------------------=0A=
Multigraphs=20&=20Sloane=20\#=20=20=20&=20A001147=20&=20A002137=20&=20=
A108243\\=20=0A$H[e_2]$&=20Constant=20&=201=20=20=20=20=20=20=20&=20=
$e^{\frac14}/\sqrt{\pi}$=20&=20.318\\=0A\hline=0A=
%------------------------------------------------------------------------=0A=
Multigraphs=20with=20loops=20&=20Sloane=20\#=20=20=20&=20A001147=20&=20=
A002135=20&=20A108247\\=20=0A$H[h_2]$&=20Constant=20&=201=20=20=20=20=20=20=
=20&=20$e^{\frac34}/\sqrt{\pi}$=20&=202.35\\=0A\hline=0A=
%------------------------------------------------------------------------=0A=
\end{tabular}\\=0A=0A\caption{Asymptotic=20enumeration=20formulas=20for=20=
different=20classes=20of=0A=20=20$k$-regular=20graphs.=20Formulas=20for=20=
1-=20and=203-=20regular=20are=20valid=20only=0A=20=20for=20even=20$n$.}=0A=
=0A\label{tab:graph}=0A\hrule=0A\end{table}=0A=
%-----------------------------------------------------------------=0A=
\begin{table}=0A=
%-----------------------------------------------------------------=0A=
\center=0A\begin{tabular}{|l|c|l|l|l|}\hline=0A$k$=20or=20$S$&=20=
Restrictions=20&=20Sloane=20\#=20&=20Formula=20&=20Constant=20\\\hline=0A=
\hline=0A=
%------------------------------------------------------------------------=0A=
\multicolumn{5}{|c|}{$S$-regular=20graphs}\\\hline=0A$\{1,2\}$=20&=20&=20=
A00986=20&=20$\displaystyle=20n^{-\frac=2012}e^{\sqrt{2n}}n!$=20&=0A=
$e^\frac{-3}2/2\sqrt=20\pi$\\=0A$\{2,3\}$=20&&=20A110040=20&=20=
$\displaystyle=20n^{-\frac34}=20\left(\frac{\sqrt=0A=20=203}{2}\right)^n=20=
e^{\sqrt{3n}}=20n!^{3/2}$=20&=200.007=20\\=0A$\{1,3\}$=20&=20$n=3D0\mod=20=
2$=20&=20A110039=20&=20$\displaystyle=20=
n^{-1}\left(\frac32\right)^{n/2}n!=20(n/2)!$=20=20&=200.43\\=0A=
$\{1,2,3\}$=20&=20&=20A110041=20&=20=20$\displaystyle=20n^{-\frac=0A=20=20=
34}\left(\frac{\sqrt=203}2\right)^n=20e^{\sqrt{3n}}n!^{3/2}=20$&=20=
0.05\\=0A\hline=0A=
%------------------------------------------------------------------------=0A=
\multicolumn{5}{|c|}{$k$-regular=203-uniform=20hyper-graphs:=20=
$E[e_3]$}\\\hline=0A1=20&=20$n=3D0\mod=203$=20&A025035&=20$\displaystyle=20=
\left(\frac1{3!}\right)^{n/3}\frac{n!}{(n/3)!}$=20&=201=20\\=0A2=20&=20=
$n=3D0\mod=203$=20&A110100&=20$\displaystyle=20=
n^{-1}\left(\frac32\right)^{n/3}n!(n/3)!=20$=20&=200.175\\=0A3=20&=20=20=20=
&A110101&=20$\displaystyle=20n^{-1}\left(\frac34\right)^nn!^2$=20&=20=
0.037\\=0A\hline=0A=
%------------------------------------------------------------------------=0A=
\multicolumn{5}{|c|}{$k$-regular=204-uniform=20hyper-graphs:=20=
$E[e_4]$}\\\hline=0A1=20&$n=3D0=20\mod=204$&=20A110102&$\displaystyle=20=
\left(\frac=201{4!}=20\right)^{n/4}=20\frac{n!}{(n/4)!}$=20&=201\\=0A2=20=
&$n=3D0=20\mod=202$&A110103&$\displaystyle=20n^{-1}\left(\frac=20=
23\right)^{n/2}=20n!=20(n/2)!$&0.100=20\\=0A\hline=0A%=20=
%------------------------------------------------------------------------=0A=
%=20\multicolumn{5}{|c|}{$k$-regular=203-Cyclic=20word=20covers:=20=
$E[Z_{\sC_3}]$=20(no=20repeats)}\\\hline=0A%=201=20&$n=3D0\mod=203$&=20=
A052502=20=20=20=20&=20$\displaystyle=20\left(\frac13\right)^{n/3}=20=
\frac{n!}{(n/3)!}$=20=20&=201\\=0A%=202=20&$n=3D0\mod=20=
3$&A110104&$\displaystyle=20n^{-1}6^{n/3}n!(n/3)!=20$=20=20=20=20=20=20=20=
&=200.477\\=0A%=203=20&=20&A110105&$\displaystyle=20=
n^{-1}\left(\frac32\right)^n=20(n!)^2$=20=20=20&0.275\\=0A%=20\hline=0A%=20=
%------------------------------------------------------------------------=0A=
%=20\multicolumn{5}{|c|}{$k$-regular=203-Cyclic=20word=20covers:=20=
$H[Z_{\sC_3}]$}\\\hline=0A%=201=20&$n=3D0\mod=203$=20&A052502=20=20=20=20=
=20&$\displaystyle=20\left(\frac13\right)^{n/3}=20\frac{n!}{(n/3)!}$=20&=20=
1\\=0A%=202=20&$n=3D0\mod=203$=20&A110106&$\displaystyle=20=
n^{-1}6^{n/3}n!(n/3)!$=20=20=20=20=20=20&=200.477\\=0A%=203=20&=20=20=20=
&A108242=20=20=20=20=20&$\displaystyle=20n^{-1}\left(\frac32\right)^n=20=
(n!)^2$=20=20&0.276\\=0A%=20\hline=0A\end{tabular}=0A=0A\smallskip=0A=0A=
\caption{Asymptotic=20enumeration=20of=20different=20classes=20of=20=
regular=20objects}=0A\label{tab:other}=0A=0A\hrule=0A\end{table}=0AThere=20=
are=20a=20few=20observations=20we=20can=20make=20based=20on=20this=20=
data.=20We=20see=0Athat=20allowing=20repetitions=20influences=20only=20=
the=20constant=20of=20the=20asymptotic=0Aexpansion,=20and=20often=20only=20=
slightly.=20We=20see=20this=20again=20in=20the=20cycle=0Acovers=20in=20=
Table~\ref{tab:other}.=20The=20formulas=20look=20different=20than=20the=0A=
graph=20formulas=20we=20presented=20earlier,=20however=20if=20we=20=
expand=20the=0Afactorials=20with=20Stirling's=20formula=20for=20$n!$,=20=
we=20quickly=20see=20that=20they=0Aare=20the=20same.=20=0A=0AAlthough=20=
we=20are=20able=20to=20compute=20the=20differential=20equations=20for=20=
the=0Agenerating=20functions=20of=20the=20classes=20of=204-regular=20=
(multi-)=20graphs=20(with=0Aloops),=20their=20asymptotic=20analysis=20is=20=
more=20complicated=20to=20do=20in=20an=0Aautomated=20fashion,=20because=20=
a=20saddle=20point=20analysis=20arises.=20This=20is=20an=20obvious=20=
starting=0Apoint=20for=20future=20work.=20Most=20of=20the=20tools=20are=20=
already=20implemented,=0Ait=20is=20mostly=20a=20question=20of=20=
understanding=20them,=20and=20determining=20how=20to=0Abest=20automate=20=
them.=0A=0AWe=20could=20make=20further=20observations=20by=20considering=20=
directed=20versions=20of=0Aany=20of=20these=20structures.=20To=20=
consider=20directed=20graphs,=20we=20need=20only=0Aconsdider=20=
$\sE\circ\sL_2$,=20where=20$\sL_k$=20is=20the=20species=20of=20lists=20=
of=0Alength=20$k$,=20and=20we=20could=20generalize=20hypergraphs=20in=20=
a=20number=20of=20ways;=0Aby=20putting=20an=20order,=20or=20even=20an=20=
orientation=20on=20each=20``edge''=20using=20the=0Aspecies=20of=20cycles=20=
or=20lists,=20as=20in=20the=20directed=20graph=20case.=20=0A=0A=
\section{Comments,=20conclusions=20and=20perspectives}=0A=
\subsection{Asymptotic=20expansions=20of=20different=20families=20of=20=
functions}=0ACoefficients=20of=20taylor=20expansions=20of=20algebraic=20=
functions=20have=20a=20known=20kind=20of=20expansion,=20that=20can=20in=20=
fact=20we=20used=20to=20establish=20the=20transcendence=20of=20some=20=
series~\cite{Flajolet87}.=20We=20can=20also=20describe=20asymptotic=20=
discrepancy=20criteria=20for=20coefficients=20of=20D-finite=20functions.=20=
=0A=0AAs=20we=20remarked=20earlier,=20coefficients=20of=20D-finite=20=
series=20are=20also=20restricted=20in=20their=20asymptotic=20growth.=20A=20=
more=20complete=20version=20of=20this=20criteria=20is=20following=20=
theorem=20presented=20in=20Wimp=20and=20Zeilberger~\cite{WiZe85}.=0A=
\begin{thm}[Wimp=20and=20Zeilberger]=20Suppose=20that=20=
$f(t)=3D\sum_{n\geq=200}=20f_n=20t^n$=20is=20a=0A=20=20D-finite=20series=20=
in=20in=20$\C[[t]]$.=20Then,=20for=20sufficiently=20large=20$n$,=0A=20=20=
the=20coefficients=20$f_n$=20have=20an=20asymptotic=20expansion=20which=20=
is=20a=20sum=20of=20terms=20of=20the=20form=0A=
\[\lambda(n!)^{r/s}\exp(Q(n^{1/m}))\omega^nn^\alpha(\log=20n)^k,\]=20=0A=20=
=20where=20$r,=20s,=20m,=20k\in\N$,=20$Q$=20is=20a=20polynomial=20and=20=
$\lambda,=0A=20=20\omega,=20\alpha$,=20are=20complex=20numbers.=0A=
\end{thm}=0AWe=20may=20ask=20ourselves,=20have=20our=20examples=20=
encompassed=20the=20full=20asymptotic=20potential=20of=20D-finite=20=
sequences?=20=20We=20are=20very=20curious=20about=20the=20combinatorial=20=
structure=20of=0Afamilies=20which=20do=20have=20such=20expansions.=20Are=20=
D-finite=0Aspecies=20sufficient=20to=20consider?=0A=0AFor=20example,=20=
in=20our=20earlier=20study=20of=20$k$-uniform=20Young=0A=
Tableaux~\cite{ChMiSa05},=20which=20are=20enumerated=20by=20the=20scalar=20=
product=0A$\left\langle=20H[e_1+e_2],=20\exp(h_kt)\right\rangle$=20have=20=
a=20conjectured=0Aform=20(verified=20for=20$k=3D1..4$)=20of=0A=
\begin{equation*}=0Ay_n^{[k]}\sim=0A=
\frac1{\sqrt2}\left(\frac{e^{k-2}}{2\pi}\right)^{k/4}=0A=
n!^{k/2-1}\left(\frac{k^{k/2}}{k!}\right)^n\frac{\exp(\sqrt{kn})}{n^{k/4}}=
,=0A\qquad=20n\rightarrow\infty.=0A\end{equation*}=0AThis=20is=20an=20=
exact=20conjecture,=20more=20complete=20than=20the=20examples=20we=20=
have=20presented=20here,=0Aalthough=20it=20results=20from=20the=20same=20=
kind=20of=20calculation,=20and=20presumably=20if=20we=0Acompleted=20the=20=
complex=20saddle=20point=20analyses=20required=20for=20the=0A$k=3D4$=20=
cases,=20we=20might=20be=20able=20to=20guess=20such=20a=20form=20for=20=
our=20examples.=20=20=20=20=0A=0AIt=20is=20also=20of=20interest=20to=20=
note=20that=20while=20in=20some=20cases=0Aoperations=20of=20summation=20=
and=20integration=20preserve=20D-finiteness,=20$G_r$,=0Athe=20class=20of=20=
{\em=20all\/}=20regular=20graphs=20is=20not=20D-finite.=20The=20same=20=
is=0Atrue=20of=20all=20the=20classes=20we=20have=20presented=20here:=20=
Although=20for=20any=20$k$,=0Athe=20subclass=20of=20$k$-regular=20=
objects=20is=20D-finite,=20the=20larger=20subclass=0Aof=20regular=20=
objects=20is=20not.=0AThis=20is=20interesting=20and=20can=20help=20us=20=
refine=20our=20notion=20of=20D-finiteness.=20=0A=0A\subsection{D-finite=20=
species?}=20\label{sec:conclusion}=0AWe=20have=20thus=20far=20restrained=20=
ourselves=20from=20defining=20a=20notion=20of=0AD-finite=20species.=20=
Ideally,=20such=20a=20theory=20would=20contain=20two=20main=0A=
components:=20A=20``D-finite=20=20species''=20should=20satisfy=20some=20=
sort=20of=20system=20combinatorial=0Adifferential=20equations=20with=20=
polynomial=20coefficients;=20and=20symmetric=0Aseries=20of=20such=20=
species=20should=20be=20D-finite.=20We=20would=20then=20expect=20to=20be=0A=
able=20to=20have=20theorems=20of=20the=20form:=0A\begin{enumerate}=0A=
\item\label{eq:cis1}=20If=20\sF=20and=20\sG=20are=20D-finite=20species,=20=
then=20so=20are=0A=20=20=20=20=20$\sF+\sG$,=20and=20$\sF\cdot=20\sG$,=20=
$\sF'$;=0A\item\label{eq:cis2}=20If=20\sG=20is=20a=20polynomial=20=
species,=20(in=20particular,=0Aif=20its=20cycle=20index=20series=20is=20=
a=20polynomial),=20then=20$\sF\circ\sG$=20is=20a=0AD-finite=20species;=0A=
\item\label{eq:cis3}=20If=20\sF=20satisfies=20an=20``algebraic=20=
equation''=20of=0A=20=20species,=20including=20for=20instance,=20=
equations=20of=20the=20form=0A=20=20$\sF=3D\sX\mathsf{P}(\sX,=20\sF)$=20=
for=20polynomial=20species=20$\mathsf{P}$,=0Athen=20$\sF$=20is=20a=20=
D-finite=20species.=0A\end{enumerate}=0AA=20candidate=20definition=20is=20=
given=20in~\cite{Mishna03},=20however=20more=20work=0Aremains=20to=20be=20=
done.=20=0A=0AFinally,=20much=20work=20has=20been=20done=20to=20=
characterize=20combinatorial=20classes=0Aof=20objects=20with=20rational=20=
and=20algebraic=20generating=20series=0A(see~\cite{MBM06}=20for=20a=20=
recent=20summary),=20and=0Ahopefully=20this=20work=20is=20a=20step=20=
towards=20such=20a=20characterization=20for=0AD-finite=20generating=20=
functions.=20We=20are=20encouraged=20by=20a=20recent=20thought=0Aof=20=20=
Flajolet,=20Gerhold=20and=20Salvy~\cite{FlGeSa05},=0A\begin{quote}=0A\em=20=
Almost=20every=20thing=20is=20non-holonomic=20unless=20it=20is=20=
holonomic=20by=20design.\\=0A\end{quote}=0A(Series=20are=20holonomic=20=
if=20and=20only=20if=20they=20are=20D-finite.)=20They=20follow=0Athis=20=
with=20the=20remark=20that=20there=20are=20several=20surprising=20=
exceptions=20to=0Athis=20rule,=20notably=20$k$-regular=20graphs.=20=
Hopefully=20we=20have=20demonstrated=0Athat=20this=20isn't=20so=20=
surprising;=20That=20in=20fact,=20there=20are=20deep=20reasons=0A=
underlying=20the=20D-finiteness=20of=20objects=20with=20this=20sort=20of=20=
regularity,=0Aand=20that=20furthermore,=20this=20D-finiteness=20can=20be=20=
exploited=20in=20an=0Aautomatic=20way.=0A=0A=
\subsubsection*{Acknowledgements}=20The=20author=20wishes=20to=20thank=20=
Fran\c=20cois=0ABergeron,=20Bruno=20Salvy,=20Fr\'ederic=20Chyzak,=20and=20=
indirectly=20Fr\'ederic=20Jouhet,=20for=20useful,=20instructive=0A=
discussions.=20=0A=0A\bibliographystyle{acm}=0A=
\begin{thebibliography}{10}=0A=0A%=20\bibitem{AuLaLe02}=0A%=20{\sc=20=
Auger,=20P.,=20Labelle,=20G.,=20and=20Leroux,=20P.}=0A%=20\newblock=20=
Combinatorial=20addition=20formulas=20and=20applications.=0A%=20=
\newblock=20{\em=20Adv.=20in=20Appl.=20Math.=2028},=203-4=20(2002),=20=
302--342.=0A=0A=20\bibitem{Bender74a}=0A=20{\sc=20Bender,=20E.~A.}=0A=20=
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Discrete=20Math.=209\/}=20(1974),=20301--311.=0A=0A=20\bibitem{BeCa78}=0A=
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46},=202=20(1987),=20291--305.=0A=0A%=20\bibitem{Bergeron89}=0A%=20{\sc=20=
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Rational=20and=20algebraic=20series=20in=20combinatorial=20enumeration.=0A=
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of=0A=20=20Mathematicians=202006.=20To=20appear=20in=20the=20proceedings=20=
of=20the=20ICM.}=20(2006)=0A=0A\bibitem{ChMiSa05}=0A{\sc=20Chyzak,=20F.,=20=
Mishna,=20M.,=20and=20Salvy,=20B.}=0A\newblock=20Effective=20scalar=20=
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Appl.,=20111\/}=20(1985),=20162--176.=0A=0A\end{thebibliography}=0A=
\end{document}=0A=

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