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\begin{center}
\vskip 1cm{\LARGE\bf Binary BBP-Formulae for Logarithms 
and Generalized Gaussian-Mersenne Primes}
\vskip 1cm
\large
Marc Chamberland \\
Department of Mathematics and Computer Science\\
Grinnell College\\
Grinnell, IA 50112\\
USA \\
\href{mailto:chamberl@math.grinnell.edu}{\tt chamberl@math.grinnell.edu} \\
\end{center}

\vskip .2 in
\begin{center}
{\bf Abstract}
\end{center}

Constants of the form
$$
C = \sum_{k=0}^\infty \frac{p(k)}{q(k)b^k}
$$
where $p$ and $q$ are integer polynomials, $\deg p &lt;\deg q$, and $p(k)/q(k)$ is
non-singular for non-negative $k$ and $b\geq 2$, have special properties. 
The $n^{th}$ digit (base $b$) of $C$ may be calculated in (essentially)
linear time without computing its preceding digits, and constants of
this form are conjectured to be either rational or normal to base $b$.

This paper constructs such formulae for constants of the form $\log p$
for many primes $p$.  This holds for all Gaussian-Mersenne primes and
for a larger class of ``generalized Guassian-Mersenne primes''.
Finally, connections to Aurifeuillian factorizations are made.  


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\section{Introduction}

The 1997 paper of Bailey, Borwein and Plouffe\cite{BBP} heralded a new era
for the computation of various transcendental constants. For  
formulae such as the alluring
$$
\pi = \sum_{k=0}^\infty \frac{1}{16^k} \left( 
\frac{4}{8k+1}- \frac{2}{8k+4} -\frac{1}{8k+5} -\frac{1}{8k+6}
\right)
$$
and more generally 
$$
C = \sum_{k=0}^\infty \frac{p(k)}{q(k)b^k}
$$
where $p$ and $q$ are integer polynomials, $\deg p &lt;\deg q$, and $p(k)/q(k)$ is
non-singular for non-negative $k$ and $b\in\Z^+$, they showed that the $n^{th}$ digit (base $b$)
may be calculated in (essentially) linear time without computing its preceding digits.
Moreover, constants of this form are conjectured to be either 
rational or normal to base $b$; see Bailey and Crandall\cite{Bailey_Crandall}.
Bailey\cite{Bailey} has recently catalogued a collection of these BBP-formulae.

Curiously, these formulae intersect with the search for prime numbers.
Recall that the Gaussian-Mersenne primes (Sloane A057429)
are the primes $p$ such that 
$$((1+i)^p - 1)((1-i)^p - 1)$$
is prime. Not only will we see that
$\log q$ has a BBP-formula for every Gaussian-Mersenne prime $q$, but also for a much broader
sequence of ``generalized Gaussian-Mersenne primes.''

Section 2 shows how evaluating cyclotomic
polynomials at particular complex values yields new BBP-formulae, which in turn
is used to motivate the definition of generalized Gaussian-Mersenne primes.
In performing such calculations, certain redundancies keep cropping up, shown
to be related to Aurifeuillian identities. 
Section 3 shows how the cyclotomic polynomials can be used to 
construct such formulae. 


\section{Cyclotomic Polynomials and Generalized Gaussian-Mersenne Primes}

Perhaps the simplest BBP-formula for logarithms is the classical
$$
\log 2 = \sum_{k=1}^\infty \frac{1}{k2^k} .
$$
Bailey {\em et al.}\cite{BBP} sought to determine all integers $m$ such
that $\log m$ has a {\em binary} BBP-formula, that is, where $b=2^l$. 
Bailey and Crandall noted that the space of constants
which admit a binary BBP-formula is linear; if $C_1$ has such a formula with
base $2^{l_1}$ and $C_2$ has a formula with base $2^{l_2}$, then 
$C_1 + C_2$ has a formula with base $2^{\mbox{lcm} (l_1,l_2 )}$.
Since 
$$
\log (2^n - 1) - n\log 2 = \log \left( 1 - \frac{1}{2^n} \right) = -\sum_{k=1}^\infty \frac{1}{k2^{kn}} ,
$$
$\log(2^n-1)$ has a binary BBP-formula,
subsequently yielding formulae for $\log( 2^n +1 )$ and
the natural logarithm of any integer of the form
\begin{equation}
\label{big_frac}
\frac{ \left( 2^{a_1}-1 \right) \left( 2^{a_2}-1 \right) \cdots \left( 2^{a_h}-1 \right) }
{ \left( 2^{b_1}-1 \right) \left( 2^{b_2}-1 \right) \cdots \left( 2^{b_j}-1 \right) } .
\end{equation}
The paper \cite{BBP} gave a list of some primes which have this form.
Bailey \cite{Bailey} extended this list by using the expression
\begin{equation}
\label{Borwein_eq}
\mbox{Re}\left( \log \left( 1 \pm \frac{(1+i)^k}{2^n}\right) \right)
\end{equation}
suggested by R. Harley and J. Borwein.
This expression has a binary BBP-formula since, for example,
\begin{eqnarray*}
\log \left( 1 - \frac{1+i}{2^n}\right) &amp; = &amp;
-\sum_{j=1}^\infty \left( \frac{1+i}{2^n} \right)^j \frac{1}{j} \\
&amp; = &amp; -\sum_{j=0}^\infty \sum_{k=1}^8  \left( \frac{1+i}{2^n} \right)^{8j+k} \frac{1}{8j+k} \\
&amp; = &amp; -\sum_{j=0}^\infty \frac{1}{2^{(8n-4)j}} \sum_{k=1}^8  \left( \frac{1+i}{2^n} \right)^k \frac{1}{8j+k} . \\
\end{eqnarray*}

A crucial tool for factoring numbers of the form $b^n - 1$ is the classical
theory of cyclotomic polynomials:
\begin{equation}
\label{factoring}
b^n -1 = \prod_{d | n} \Phi_d (b)
\end{equation}
where $\Phi_d (x)$, the $d^{th}$ cyclotomic polynomial, is defined
as 
$$
\Phi_d (x) = \prod_{j=1}^{\phi(d)} (x-\zeta_j).
$$
The terms $\zeta_j$ are the primitive $d^{th}$ roots of unity
and $\phi (\cdot)$ is the Euler totient function.
Alternatively, a well-known identity for these polynomials derived using
M\"obius inversion is
\begin{equation}
\label{Mobius}
\Phi_d (x) = \prod_{k | d} (1 - x^{d/k} )^{\mu(k)}
\end{equation}
where $\mu (\cdot)$ is the M\"obius function.

In conjunction with expression (\ref{big_frac}), Bailey {\em et al.} state that 
$\log \Phi_m (2)$ admits a binary BBP-formula for all positive integers $m$. 
One may easily extend this to 
\begin{equation}
\label{pow2}
\log~ \Phi_m (2^k)
\end{equation}
for all integers $k$. However, the cyclotomic polynomials
may be used to obtain many other values. Using the M\"obius formula 
(\ref{Mobius}) with $x=(\pm 1 +i)/2^n$ 
yields
\begin{equation}
\label{root2_eq}
\mbox{Re}\left( \log~ \Phi_m \left( \frac{\pm 1 + i}{2^n} \right) \right) 
= \sum_{d|m} \mu (d) \mbox{Re}\left( \log
\left( 1 - \left( \frac{\pm 1 + i}{2^n} \right)^{m/d} \right) \right) .
\end{equation}
As in the consideration of the expression (\ref{Borwein_eq}),
the right side is a binary BBP-formula. Though it is simply a 
linear combination of expressions of the form (\ref{Borwein_eq}), the advantage
here is that implicitly some cancellation may take place.
For example, we have
$$
\mbox{Re} \left( \log \Phi_6 \left( \frac{1+i}{16} \right) \right) 
= \frac{1}{2}\left[ \log 14449 - 14\log 2 \right] ,
$$
hence 14449 joins the list.
Similarly, one may use the M\"obius formula (\ref{Mobius}) with $x=(\pm 1 +\sqrt{3}i)/2^n$ 
to obtain 
\begin{equation}
\label{root3_eq}
\mbox{Re} \left( \log~ \Phi_m \left( \frac{\pm 1 + \sqrt{3}i}{2^n} \right) \right) 
= \sum_{d|m} \mu (d) \mbox{Re}\left( \log
\left( 1 - \left( \frac{\pm 1 + \sqrt{3}i}{2^n} \right)^{m/d} \right) \right) ,
\end{equation}
again producing binary BBP-formulae. An example here is 
$$
\mbox{Re} \left( \log~ \Phi_5 \left( \frac{1+\sqrt{3}i}{4} \right) \right) 
= \frac{1}{2}\left[ \log 331 - 8\log 2 \right] ,
$$
so $331$ comes onto the list. Again, such results are linear
combinations of earlier formulae since, for example,
$$
\mbox{Re} \left( \log \left( 1 - \frac{1+\sqrt{3}i}{2}x \right) \right)
= \log (1-x^3) - \log (1-x) .
$$
Modest calculations with Maple produced the
following augmentation of Bailey's list
of primes whose logarithm admits a binary BBP-formula 
(underlined numbers are given by Bailey\cite{Bailey}):

\vspace{.2in}
\noindent
\underline{2},~
\underline{3},~
\underline{5},~
\underline{7},~
\underline{11},~
\underline{13},~
\underline{17},~
\underline{19},~
\underline{29},~
\underline{31},~
\underline{37},~
\underline{41},~
\underline{43},~
\underline{61},~
\underline{73},~
\underline{109},~
\underline{113},~
\underline{127}, ~ 
\newline
\underline{151},~
\underline{241},~
\underline{257},~
331 ,~
\underline{337},~
\underline{397},~
\underline{683},~
\underline{1321},~
1429,~
\underline{1613},~
\underline{2113},~
\underline{2731},~
\underline{5419},~
\\
\underline{8191},~
14449,~
26317,~
38737,~
\underline{43691},~
\underline{61681},~
65537,~
\underline{87211},~
\underline{131071},~
\underline{174763},~
246241,~
\\
\underline{262657},~
268501,~
279073,~
312709,~
\underline{524287},~
525313,~
599479,~
\underline{2796203},~
4327489,~
7416361,~
\\
\underline{15790321},~
\underline{18837001},~
\underline{22366891},~
29247661,~
47392381,~
107367629,~
536903681,~
1326700741,~
\\
\underline{4278255361},~
\underline{4562284561},~
40388473189,~
77158673929,~
118750098349,~
415878438361,~
\\
1133836730401,~
\underline{2932031007403},~
3630105520141,~
\underline{4363953127297},~
\underline{4432676798593},~
\\
4981857697937,~
108140989558681,~
140737471578113,~
1041815865690181,~
96076791871613611,~
\\
18446744069414584321,~
5302306226370307681801,~
\\
2048568835297380486760231,~
17059410504738323992180849,~
\\
84159375948762099254554456081,~
134304196845099262572814573351,~
\\
19177458387940268116349766612211,~
304832756195865229284807891468769,~
\\
1339272539833668386958920468400193,~
3652124453410972878264128353955921,~
\\
1772303994379887829769795077302561451,~
6113142872404227834840443898241613032969,~
\\
1461503031127477825099979369543473122548042956801,~
\\
867988564747274927163124868127898657976489313137639569




\vspace{.1in}

Of course, products of two primes, three primes, etc. were found to be on the list.
Keeping track of these products of primes helps generate new single primes.
For example, since $2^{11}-1 = 23\times 89$, the product 
$23\times 89$ is on the list. In addition, we have
$$
\mbox{Re} \left( \log~ \Phi_{6} \left( \frac{1+\sqrt{3}i}{2^{12}} \right) \right)
= \frac{1}{2}\left[ \log 7 + 2\log (23\times 89) + \log 599479 - 44\log 2 \right] ,
$$
so this is how 599479 was obtained. Products of two primes which are on the list include:
$$
23 \times 89,~
47 \times 178481,~
53 \times 157,~
59 \times 3033169,~
67 \times 20857,~
71 \times 122921,~
79 \times 121369,~
$$
$$
83 \times 8831418697,~
97 \times 673,~
101 \times 8101,~
% 103 \times 2143 \times 1119
137 \times 953,~
139 \times 168749965921,~
149 \times 184481113,~
% 173 \times 101653 \times 500177
181 \times 54001,~
$$
$$
193 \times 22253377,~
197 \times 19707683773,~
229 \times 457,~
% 233 \times 1103 \times 2089
223 \times 616318177,~
251 \times 4051,~
277 \times 30269,~
$$
$$
281 \times 86171,~
283 \times 165768537521,~
313 \times 1249,~
353 \times 2931542417,~
% 431 \times 9719 \times 2099869
571 \times 160465489,~
$$
$$
593 \times 231769777,~
601 \times 1801,~
631 \times 23311,~
641 \times 6700417,~
% 881 \times 3191 \times 201961
1013 \times 1657,~
1777 \times 25781083,~
$$
$$
% 2351 \times 4513 \times 13264529
3121 \times 21841,~
3761 \times 7484047069,~
5581 \times 384773,~
% 6361 \times 69431 \times 20394401
8681 \times 49477,~
10169 \times 43249589,~
$$
$$
13367 \times 164511353,~
32377 \times 1212847,~
92737 \times 649657,~
179951 \times 3203431780337,~
$$
$181549 \times 12112549$~\\

Note that all the primes up to 101 are on the list,
either alone or multiplied by one other prime. 
Indeed, every odd prime $p$ is
on the list, either alone or in some multiple product of primes
since $2^{p-1} - 1$ is on the list and $p|(2^{p-1} - 1)$.
Carl Pomerance (see \cite{BBP}) showed that 23 could not be
written in the form (\ref{big_frac}); however, it is still
unknown whether $\log 23$ has a binary BBP-formula.
Related questions are extensively dealt with by Borwein, Borwein and
Galway\cite{BBG}.




An important subclass of binary BBP-formulae concerns the expression
\begin{equation}
\label{GMP-form}
\mbox{Re}\left( \log~ \Phi_m \left( \frac{ 1 + i}{2} \right) \right).
\end{equation}
Letting $m$ equal a prime $p=4k+1$, we have
\begin{eqnarray*}
\Phi_p \left( \frac{ 1 + i}{2} \right) &amp; = &amp; 1 + \left( \frac{1+i}{2} \right) +  \left( \frac{1+i}{2} \right)^2 
		+ \cdots +  \left( \frac{1+i}{2} \right)^{(4k+1)} \\
&amp; = &amp; \frac{  \left( \frac{1+i}{2} \right)  \left( \frac{1+i}{2} \right)^{4k} - 1 }{  \left( \frac{1+i}{2}  -1 \right) }\\
&amp; = &amp; 1 + i\left( 1 - \left( -\frac{1}{4} \right)^k \right),
\end{eqnarray*}
which produces
\begin{eqnarray*}
\mbox{Re} \left( \log \Phi_p \left( \frac{ 1 + i}{2} \right) \right) &amp; = &amp; \frac{1}{2} \log \left( 1 + \left( 1 - \left( -\frac{1}{4} \right)^k \right)^2 \right) \\
&amp; = &amp; \frac{1}{2} \log \left( 2\cdot 4^{2k} - 2(-4)^k + 1 \right)  - 2k\log 2 \\
&amp; = &amp; \frac{1}{2} \log \left( (1+i)^{(4k+1)} - 1 \right) \left( (1-i)^{(4k+1)} - 1 \right) - 2k\log 2 \\
&amp; = &amp; \frac{1}{2} \log \left( (1+i)^p - 1 \right) \left( (1-i)^p - 1 \right) - 2k\log 2 \\
&amp; = &amp; \frac{1}{2} \log \left( \frac{\left( (1+i)^p - 1 \right) \left( (1-i)^p - 1 \right)}{2^{4k} } \right) .
\end{eqnarray*}
A similar calculation may be done for primes of the form $p=4k-1$. We then have that if $q$ is a 
Gaussian-Mersenne prime, then $\log q$ admits a binary BBP-formula. This connection implies a larger
question: For which positive integers $m$ is the numerator of the rational expression 
\begin{equation}
\label{GMP}
\exp \left( 2 \mbox{Re} \left( \log \Phi_m \left( \frac{ 1 + i}{2} \right) \right) \right)
\end{equation}
prime? Besides the Gaussian-Mersenne primes, many composite $m$ satisfy this condition. These 
generalized Gaussian-Mersenne primes, checked for all $m &lt; 3000$,  are listed below (the regular Gaussian-Mersenne primes
are underlined). 

\begin{eqnarray*}
&amp; &amp; \underline{2},\underline{3},4,\underline{5},\underline{7},9,10,\underline{11},12,14,15,18,\underline{19},21,22,26,27,\underline{29},30,33,34,35,42,45,\\
&amp; &amp; \underline{47},49,51,54,55,58,63,65,66,69,70,\underline{73},\underline{79},85,86,87,105, 106,110,111,\underline{113},114,\\
&amp; &amp; 126,129,138,147,\underline{151},\underline{157},\underline{163},\underline{167},178,186,189,217,231,\underline{239},\underline{241},242,\\
&amp; &amp; \underline{283},319,323,350,\underline{353},363,\underline{367},375,\underline{379},385,391,\underline{457},462,522,543,566,602,621,\\
&amp; &amp; 633,651,679,741,779,819,871,885,\underline{997},1062, 1114,1126,1150,1226,1275,1317,\\
&amp; &amp; 1329,\underline{1367},1382,1434,1477,1710,1926,1970,2331,2422,2446,2995.\\
\end{eqnarray*}

Some of the primes produced by expression (\ref{GMP}) have been put on the 
previous list of primes. When $m=2995$, this produces a prime with over 
700 digits.




\section{Aurifeuillian Factorizations}


Since the cyclotomic polynomials are irreducible in $\Z[x]$, it
would seem no further factorization of $b^n - 1$ in equation (\ref{factoring}) is possible. 
However, by imposing
certain restrictions on $x$, other factorizations exist.
An example is
$$
\Phi_5 (x) = x^4 + x^3 + x^2 + x +1 = (x^2+3x+1)^2 - 5x(x+1)^2
$$
which, upon letting $x=5^{2k-1}$ and factoring the difference of squares, yields
$$
\Phi_5 ( 5^{2k-1} ) = 
[5^{4k-2} + 3\cdot 5^{2k-1} + 1 - 5^k (5^{2k-1} + 1) ]
[5^{4k-2} + 3\cdot 5^{2k-1} + 1 + 5^k (5^{2k-1} + 1) ] .
$$
These special polynomial identities were first noted by 
A. Aurifeuille and subsequently generalized by E. Lucas.
References and other examples of Aurifeuillian identities may be found
in Brillhart {\em et al.} \cite{Brillhart}, as well as their
use in factoring. Theorems regarding writing $\Phi_n (x)$ as
a difference of squares may be found in Schinzel\cite{Schinzel},
Stevenhagen\cite{Stevenhagen} and Brent\cite{Brent}. 

Earlier we saw that $\log (2^n \pm 1)$ has a binary BBP-formula.
Bailey\cite{Bailey}) notes that
$$
\mbox{Re} \left( \log \left( 1 \pm \frac{1+i}{2^n} \right) \right)
= \left( \frac{1}{2} - n\right)\log 2 + 
\frac{1}{2} \log \left( 2^{2n-1} \pm 2^n + 1 \right) ,
$$
so the two expressions $2^{2n-1} \pm 2^n + 1$ come onto the list. However,
multiplying these terms gives the classical 
Aurifeuillian identity
$$
2^{4n-2} + 1 = \left( 2^{2n-1} + 2^n + 1 \right)
\left( 2^{2n-1} - 2^n + 1 \right) .
$$
This demonstrates why some calculations used in the last section 
to generate the list of primes were redundant.
Indeed, in searching for various families of factors,
similar identities arise. We now develop other Aurifeuillian
identities, interesting for their own sake, and make connections
to expressions used in the last section. Let us 
start with a general theorem regarding cyclotomic polynomials.

\begin{theorem}
\label{theo_cyclotomic}
Let $m,n\in\Z^+$ satisfy $\gcd (m,n) = 1$ and at least
one of $m$ or $n$ is greater than 2. Then
\begin{equation}
\label{main}
\Phi_{mn}(x) = \prod_{j=1}^{\phi(m)} \Phi_n (x\zeta_j )
\end{equation}
where $\zeta_j$ are the primitive $m^{th}$ roots of unity.
\end{theorem}

\begin{proof}
Since the degree of $\Phi_n (x)$ is $\phi(n)$, the degree of the 
left side polynomial of (\ref{main}) is $\phi(mn) = \phi(m)\phi(n)$, 
matching the degree of the right. 
The left polynomial is monic, while the leading coefficient of the right side is
\begin{eqnarray*}
\prod_{j=1}^{\phi(m)} {\zeta_j} ^{\phi(n)}
&amp; = &amp; 
\left( \prod_{j=1}^{\phi(m)} \zeta_j \right)^{\phi(n)}\\
&amp; = &amp; 1
\end{eqnarray*}
so the right is also monic. It remains to show that
the roots of each side are the same. 

The roots of $\Phi_{mn} (x)$ are simply $e^{ki2\pi/mn}$ with
$\gcd( k,mn) = 1$. We will show that each of these $\phi (mn)$
distinct roots is also a root of the right side.
To expand the right side, first note that each
$\zeta_j$ has the form $e^{li2\pi/m}$ for some $l$ satisfying
$\gcd( l,m ) = 1$. This combines to give
$$
x\zeta_j = e^{(k+ln)i2\pi/mn}
$$
so it is suffices to show that for each $k$ there exists
an $l$ such that 
$$
\gcd\left( \frac{k+ln}{m}, n \right) = 1.
$$
Since $\gcd( k, mn)=1$, we have $\gcd(k,n)=1$. 
This implies that $\gcd(k+ln,n) = 1$ and,
using the Euclidean algorithm with $\gcd( m,n)=1$, there 
exists an $l$ such that
$k+ln$ is a multiple of $m$. This completes the proof.
\end{proof}

With the identities
$$
\Phi_{2^k n} (x) = \Phi_n \left( -x^{2^{k-1}}\right), ~~~n\mbox{ odd}
$$
and 
$$
\Phi_{pn} (x) = \Phi_n (x^p),~~~p~\mbox{prime}, ~p\not| n ,
$$
we construct several examples.


\begin{example}
The case $m=4$ was foreshadowed by Schinzel\cite[formula (12)]{Schinzel}.
Letting $n$ be odd in Theorem \ref{theo_cyclotomic} gives
$$
\Phi_n (-x^2) = \Phi_{4n} (x) = \Phi_n (ix) \Phi_n ( -ix) .
$$
Replacing $x$ with $ix$ gives
$$
\Phi_n  (x^2 ) = \Phi_n (x) \Phi_n (-x) .
$$
\end{example}


\begin{example}
Letting $m=8$, $n$ odd, we have
\begin{equation}
\label{ex1}
\Phi_{8n} (x) = 
\left[ \Phi_n \left( x\frac{1+i}{\sqrt{2}}\right)
\Phi_n \left( x\frac{1-i}{\sqrt{2}}\right) \right]
\left[ \Phi_n \left( x\frac{-1+i}{\sqrt{2}}\right)
\Phi_n \left( x\frac{-1-i}{\sqrt{2}}\right) \right] .
\end{equation}
Replacing $x$ with $\sqrt{2}x$ yields
\begin{eqnarray}
\nonumber
\lefteqn{ \Phi_n (-4x^4) = \Phi_{4n} (2x^2) } &amp; &amp; \\
\label{AF1}
&amp; &amp; =
\left[ \Phi_n ( x(1+i))
\Phi_n ( x(1-i)) \right]
\left[ \Phi_n ( x(-1+i))
\Phi_n ( x(-1-i)) \right] .
\end{eqnarray}
If $x$ is real,
the two right side expressions must be integer polynomials 
since they are each the product of complex conjugates.
Example subcases with $x=2^k$ are\\
\noindent {\bf n=1}: 
$$
2^{4k+2} + 1 = (2^{2k+1} + 2^{k+1} + 1) (2^{2k+1} - 2^{k+1} + 1)
$$
\noindent {\bf n=15}: 
$$
2^{32k+16} + 2^{28k+14} -2^{20k+10} 
- 2^{16k+8} - 2^{12k+6} + 2^{4k+2} + 1
= L\cdot R
$$
where
\begin{eqnarray*}
L,R &amp; = &amp; 
2^{16k+8} \pm 2^{15k+8} + 2^{14k+7} \pm 2^{13k+7}
+ 2^{12k+7} \pm 2^{11k+7} + 3\cdot 2^{10k+5} 
\pm 2^{9k+6} 
\\
&amp; &amp; 
+ 3\cdot 2^{8k+4} \pm 2^{7k+5} + 3\cdot 2^{6k+3}
\pm 2^{5k+4} + 2^{4k+3} \pm 2^{3k+2} + 2^{2k+1} \pm 2^{k+1} + 1
\end{eqnarray*}

% Another way to manilpulate equation (\ref{ex1}) is to replace $x$ with
% $x (1+i)/\sqrt{2}$ to produce
% $$
% \Phi_n (x^4) = \Phi_{8n} \left( \frac{1+i}{\sqrt{2}} x \right) = 
% \left[ \Phi_n (ix) \Phi_n (-ix) \right] \Phi_n (x) \Phi_n (-x)
% $$
% For real $x$, the square-bracketed term is also an integer polynomial. Sub-cases
% include the trivial case {$\bf n=1$}: $x^4 - 1 = (x^2 +1)(x-1)(x+1)$ and
% the less obvious $\bf n=15$ case:
% \begin{eqnarray*}
% \lefteqn{ x^{32} - x^{28} + x^{20} - x^{16} + x^{12} - x^4 + 1 = } &amp; &amp; \\
% &amp; &amp; ( x^8 - x^7 + x^5 - x^4 + x^3 - x + 1)
% ( x^8 + x^7 - x^5 - x^4 - x^3 + x + 1) \\
% &amp; &amp; \times ( x^{16} + x^{14} - x^{10} - x^8 - x^6 + x^2 + 1 ) .
% \end{eqnarray*}

Getting back to the redundancies in our earlier calculations, let $x=1/2^k$ in (\ref{AF1}) to produce
$$
\mbox{Re} \left( \log~ \Phi_{4n} \left( \frac{1}{2^{2k-1}}\right) \right)
= 2\left[ \mbox{Re} \left( \log~ \Phi_{n} \left( \frac{1+i}{2^k}\right) \right)
+ \mbox{Re} \left( \log~ \Phi_{n} \left( \frac{-1+i}{2^k}\right) \right) \right] .
$$
This shows how terms from (\ref{pow2}) and (\ref{root2_eq}) appear in some factorizations.
\end{example}
 

\begin{example}
Letting $m=12$ and $2,3\not| n$, we have
\begin{equation}
\label{ex2}
\Phi_{12n} (x) =
\Phi_n \left( x\frac{\sqrt{3}+i}{2}\right)
\Phi_n \left( x\frac{\sqrt{3}-i}{2}\right) 
\Phi_n \left( x\frac{-\sqrt{3}+i}{2}\right)
\Phi_n \left( x\frac{-\sqrt{3}-i}{2}\right) 
\end{equation}
Replacing $x$ with $2\sqrt{3}x$ gives
$$
\Phi_{6n} (12x^2) 
=
\left[ 
\Phi_n \left( x(3+\sqrt{3}i) \right)
\Phi_n \left( x(3-\sqrt{3}i) \right)
\right]
\left[ 
\Phi_n \left( x(-3+\sqrt{3}i) \right)
\Phi_n \left( x(-3-\sqrt{3}i) \right)
\right]
$$
Again, the two right side expressions must be integer polynomials.
Example subcases with $x=2^{k-1}$ are\\
\noindent {\bf n=1}: 
$$
9\cdot 2^{4k} - 3\cdot 2^{2k} + 1 =
( 3\cdot 2^{2k} - 3\cdot 2^k + 1)
( 3\cdot 2^{2k} + 3\cdot 2^k + 1)
$$

\noindent {\bf n=5}: 
\begin{eqnarray*}
\lefteqn{
2^{16k}3^8 + 2^{14k}3^7 - 2^{10k}3^5 
- 2^{8k} 3^4 - 2^{6k} 3^3 + 2^{2k} 3  + 1
} &amp; &amp; \\
&amp; &amp; ( 2^{8k} 3^4 + 2^{7k} 3^4 + 2^{6k+1} 3^3 + 2^{5k} 3^3
+ 2^{4k} 3^2 + 2^{3k} 3^2 + 2^{2k+1} 3 + 2^k 3 + 1 ) \\
&amp; &amp; \times(  2^{8k} 3^4 - 2^{7k} 3^4 + 2^{6k+1} 3^3 - 2^{5k} 3^3 
+ 2^{4k} 3^2 - 2^{3k} 3^2 + 2^{2k+1} 3 - 2^k 3 + 1
)
\end{eqnarray*}

To show how equation (\ref{ex2}) produces redundancies, replace $x$ with
$x (\sqrt{3} + i)/2$ to obtain
$$
\Phi_{6n} \left( \frac{1 + \sqrt{3}i}{2} x^2 \right)  = 
\Phi_{12n} \left( \frac{\sqrt{3} + i}{2} x \right)  = 
\Phi_n \left( \frac{1 + \sqrt{3}i}{2} x \right)
\Phi_n \left( \frac{-1 - \sqrt{3}i}{2} x \right)
\Phi_n (x)
\Phi_n (-x) .
$$
With $x = 1/2^{k-1}$ this yields the relationship
\begin{eqnarray*}
\mbox{Re}~ \left( \log \Phi_{6n} \left( \frac{1 + \sqrt{3}i}{2^{2k-1}} \right) \right) &amp; = &amp; 
\mbox{Re}~ \left( \log \Phi_n \left( \frac{1 + \sqrt{3}i}{2^k} \right) \right)
+ \mbox{Re}~ \left( \log \Phi_n \left( \frac{-1 + \sqrt{3}i}{2^k}  \right) \right) \\
&amp; &amp; + \log \Phi_n \left(\frac{1}{2^{k-1}}\right) + 
\log \Phi_n \left(\frac{-1}{2^{k-1}}\right) .
\end{eqnarray*}
This shows how terms from (\ref{pow2}) and (\ref{root3_eq}) appear in some factorizations.

\end{example}


\vspace{0.2in}
\noindent {\bf Acknowledgement}: 
I would like thank David Bailey, Jonathan Borwein and Samuel Wagstaff 
for supportive dialgoue. Special thanks go to Arnold Adelberg for
helping me obtain Theorem \ref{theo_cyclotomic} in its general form. 

% The authors also wish to express their thanks to the 
% anonymous referees for valuable suggestions and information
% regarding the literature.

\begin{thebibliography}{1}

\bibitem{Bailey}
D. H. Bailey,
\newblock A compendium of BBP-type formulas for mathematical constants.
\newblock Preprint, \href{http://crd.lbl.gov/~dhbailey/dhbpapers/index.html}{\tt http://crd.lbl.gov/$\tilde{\mbox{ }}$dhbailey/dhbpapers/index.html}, (2000).

\bibitem{BBP}
D. H. Bailey, P. B. Borwein, and S. Plouffe.
\newblock On the rapid computation of various polylogarithmic constants.
\newblock {\em Math.\ Comp.} {\bf 66} (1997), 903--913.

\bibitem{Bailey_Crandall}
D. H. Bailey and R. E. Crandall,
\newblock On the random character of fundamental constant expansions.
\newblock {\em Experiment.\  Math.} {\bf 10} (2001), 175--190.

\bibitem{BBG}
D. Borwein, J. M. Borwein, and W. F. Galway.
\newblock Finding and excluding b-ary Machin-type BBP formulae.
\newblock {\em Canadian J. Math.} submitted, (2003).
CECM Preprint 2003:195.

\bibitem{Brent}
R.~Brent.
\newblock Computing Aurifeuillian factors.
\newblock In {\em Computational Algebra and Number Theory},
\newblock {Mathematics and its Applications} Vol.\ 325,
Kluwer, Dordrecht, (1995), pp.\ 201--212.

\bibitem{Brillhart}
J.~Brillhart,~D.H.~Lehmer,~J.L.~Selfridge,~B.~Tuckerman, and S.S.~Wagstaff,Jr.
\newblock {\em Factorizations of $b^n\pm 1$}.
\newblock American Mathematical Society, Providence, 1983.

\bibitem{Schinzel}
A.~Schinzel.
\newblock On primitive prime factors of $a^n-b^n$.
\newblock {\em Proc. Cambridge Phil. Soc.} {\bf 58} (1962), 555--562.

\bibitem{Stevenhagen}
P.~ Stevenhagen.
\newblock On Aurifeuillian factorizations.
\newblock {\em Proc. Kon. Akad. Wetensch.} {\bf 90} (1987), 451--468.

\end{thebibliography}

\bigskip
\hrule
\bigskip

\noindent 2000 {\it Mathematics Subject Classification}:
Primary 11Y05; Secondary 11A41, 11B99, 11T22, 11Y60.

\noindent \emph{Keywords: primes, Gaussian-Mersenne, BBP, Aurifeuillian.}

\bigskip
\hrule
\bigskip

\noindent (Concerned with sequence
\seqnum{A057429}.)

\bigskip
\hrule
\bigskip

\vspace*{+.1in}
\noindent
Received July 15, 2003;
revised version received  October 24, 2003.
Published in {\it Journal of Integer Sequences},
October 25, 2003.
\bigskip
\hrule
\bigskip

\noindent
Return to
\htmladdnormallink{Journal of Integer Sequences home page}{http://www.math.uwaterloo.ca/JIS/}.
\vskip .1in


\end{document}

                                                                                

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