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\begin{center}
\vskip 1cm{\LARGE\bf A Few New Facts about the EKG Sequence
}
\vskip 1cm
\large
Piotr Hofman and Marcin Pilipczuk \\
Department of Mathematics, Computer Science and Mechanics \\
University of Warsaw \\
Warsaw \\
Poland\\
\href{mailto:piotr.hofman@students.mimuw.edu.pl}{\tt piotr.hofman@students.mimuw.edu.pl}\\
\href{mailto:marcin.pilipczuk@students.mimuw.edu.pl}{\tt marcin.pilipczuk@students.mimuw.edu.pl} \\
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\vskip .2 in

\begin{abstract}
The EKG sequence is defined as follows: $a_1=1$, $a_2=2$ and
$a_n$ is the smallest natural number 
satisfying $\gcd (a_{n-1}, a_n) &gt; 1$
not already in the sequence.
The sequence was previously investigated by Lagarias, Rains and Sloane.
In particular, we know
that $(a_n)$ is a permutation of the natural numbers and that the
prime numbers appear in this sequence in an increasing order.

Lagarias, Rains and Sloane performed many numerical
experiments on the EKG sequence up to the $10^7$th term and came up
with several interesting conjectures.
This paper provides proofs for the core part of those conjectures.
Namely, let $(a_n')$ be the sequence $(a_n)$ with
all terms of the form $p$ and $3p$, for $p$ prime, changed to $2p$. 
First, we prove that
for any odd prime $a_n = p$ we have $a_{n-1}=2p$.
Then we prove that $\lim_{n\to\infty} \frac{a_n'}{n} = 1$, i.e.,
we have $a_n \sim n$ except for the values of $p$ and $3p$ for $p$ prime: if $a_n = p$ then
$a_n \sim \frac{n}{2}$, and if $a_n = 3p$ then $a_n \sim \frac{3n}{2}$.
\end{abstract}

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\section{Introduction}

The EKG sequence is defined as follows: $a_1=1$, $a_2=2$ and
$a_n$ is the smallest natural number not already in the sequence satisfying $\gcd(a_{n-1}, a_n)&gt;1$.
It was originally defined by Ayres and later investigated by Lagarias, Rains
and Sloane \cite{lrs}.
It appears as entry \seqnum{A064413} in the On-line Encyclopedia of Integer Sequences \cite{sloency}.
The first forty terms of the
sequence are
\vskip 0.5cm
\begin{center}
\begin{tabular}{cccccccccc}
1  &amp; 2  &amp; 4  &amp; 6  &amp; 3  &amp; 9  &amp; 12 &amp; 8  &amp; 10 &amp; 5  \\
15 &amp; 18 &amp; 14 &amp; 7  &amp; 21 &amp; 24 &amp; 16 &amp; 20 &amp; 22 &amp; 11 \\
33 &amp; 27 &amp; 30 &amp; 25 &amp; 35 &amp; 28 &amp; 26 &amp; 13 &amp; 39 &amp; 36 \\
32 &amp; 34 &amp; 17 &amp; 51 &amp; 42 &amp; 38 &amp; 19 &amp; 57 &amp; 45 &amp; 40 \\
\end{tabular}
\end{center}
\vskip 0.5cm

Lagarias, Rains and Sloane \cite{lrs} describe some basic properties of
this sequence. In particular, they prove that it is a permutation of
the natural numbers with linear asymptotic behavior; more precisely
$\frac{1}{260} n \leq a_n \leq 14n$.  We recall some of their lemmas
and theorems in Section~\ref{simplefacts}.  Their numerical experiments
resulted in several interesting conjectures. First, if $a_n=p$ and $p$
is a prime, then $a_{n-1}=2p$, which we will prove in Section~\ref{hofman}.
Second, that $a_n \sim n$ except for when $a_n = p$ is
prime; then $a_n = p\sim \frac{n}{2}$ and $a_{n+1} = 3p \sim
\frac{3n}{2}$.

We will prove the conjectures mentioned above. 
To formulate what is proved let us introduce some notation.
Let $(z_n)$ be the inverse of the permutation $(a_n)$, let $(p_i)$ be the
sequence of prime numbers in increasing
order, and let $(a_n')$ be defined as follows:
$a_n' = 2p$ if $a_n=3p$ or $a_n=p$ for $p$ prime,
$a_n'=a_n$ otherwise. 
This definition was introduced by Lagarias, Rains and Sloane \cite{lrs}.
We prove that that $\lim_{n \to \infty} \frac{a_n'}{n} = 1$.
More precisely, we will prove that there exists a universal constant
$C$ such that for all natural numbers $n$ we have
$$n - \frac{Cn}{\log \log \log n} \leq a_n \leq n + \frac{Cn}{\log \log \log n}.$$

Note that the speed of convergence is extremely slow. In particular,
this result is weaker than a more precise conjecture made by Lagarias, Rains and Sloane \cite{lrs} that
$a_n' \sim n(1 + \frac{1}{3 \log n})$. In Section~\ref{dyskusja} we discuss
some drawbacks and possible improvements of the proofs in this work.

In this work, we will use several strong results from number theory.
These facts are gathered in Section \ref{tools}.

In this work the letters $c$, $C$, $C'$, $c'$, $C_i$, $c_i$ denote constants.
If it is not specified
otherwise, they are absolute numerical constants.

\section{Number theory tools}\label{tools}

Let us define $p_n$ to be the $n$-th prime number and $\pi(n)$
the number of primes smaller than $n$.
According to Rosser \cite{pnt}, the following version of 
the prime number theorem holds:

\begin{thm}\label{szacowanie_pi}
If $n \geq 55$ then
$$\pi(n) &lt; \frac{n}{\log n - 4}.$$
\end{thm}

We conclude that

\begin{cor}\label{szacowanie_pi2}
If $n &gt; 25000 &gt; e^{10.1}$ then
$$\pi(n) &lt; \frac{2n}{\log n}$$
and
$$\pi(n) &lt; \frac{n}{6}-2$$
\end{cor}

Let us now estimate the number of different prime divisors of $n$.

\begin{lemma}\label{liczba_dzielnikow}
Any natural number $n$ has at most $C \frac{\log n}{\log \log n}$
different prime divisors.
\end{lemma}

\begin{proof}
If $n$ has at least $k$ prime divisors, then $n \geq 2 \cdot 3 \cdot \cdots \cdot p_k &gt; k!$,
so by Stirling's formula $\log n &gt; C_1 k \log k$. Let $f(x) = C_1 x \log x$.
Then
$$f\Big(C_2 \frac{\log n}{\log \log n}\Big) = C_1 C_2 \frac{\log n}{\log \log n} (\log C_2 + \log \log n - \log \log \log n) &gt; C_3 C_2 \log n.$$
Therefore $k &lt; C_4 \frac{\log n}{\log \log n}$.
\end{proof}

The following is a well-known fact from calculus.

\begin{thm}\label{iloczyn_pierwszych}
Let $p_1, p_2, \ldots$ be the sequence of the prime numbers
primes in increasing order ($p_1 = 2, p_2 = 3, \ldots$). Then
$$\prod_{n=1}^\infty \Big( 1- \frac{1}{p_n}\Big) = 0.$$
\end{thm}

However, in our work we will need a stronger result; namely, 
Merten's theorem \cite{mertens}.

\begin{thm}\label{tw_mertena}
Let $p_1, p_2, \ldots$ be the sequence of prime numbers in increasing order ($p_1 = 2, p_2 = 3, \ldots$). Then
$$\lim_{n \to \infty} (\log p_n ) \prod_{n=1}^\infty \Big( 1- \frac{1}{p_n}\Big) = e^{-\gamma}$$
where $\gamma$ is the Euler-Mascheroni constant. 
\end{thm}

Since $p_n \sim n \log n$ the following is straightforward:

\begin{cor}\label{szacowanie_merten}
There exists a universal constant $C$ such that for all natural numbers $n$ the following holds:
$$\prod_{n=1}^\infty \Big(1-\frac{1}{p_n}\Big) &lt; \frac{C}{\log n}.$$
\end{cor}

{\bf{Note:}} all results cited above are independent of the Riemann hypothesis.

\section{Already known simple facts}\label{simplefacts}

As Lagarias, Rains and Sloane \cite{lrs} defined, a prime dividing both $a_{n-1}$ and $a_n$ is called a {\it controlling prime} for $a_n$.
Note that a controlling prime is not unique; if not specified otherwise, in proofs we choose any of the controlling primes.
We will define $g(n) = \gcd(a_n, a_{n-1})$.
As observed by Lagarias, Rains and Sloane \cite{lrs}, we can define the sequence in a different way.
For every prime $p$ let $B_n(p)$ be the smallest multiple of $p$ that has not appeared in the first $n$ terms
of the sequence. Then $a_{n+1}$ is smallest among all $B_n(p)$ for primes $p$ dividing $a_n$.
The following lemmas and theorems were proved by Lagarias, Rains and Sloane \cite{lrs} and for some of them we quote the proofs, since they are quite basic and
will be important in the next section.

\begin{lemma}\label{przed_p} Let $p&gt;2$ be a prime. If $a_n$ is the first term divisible by $p$, then $a_{n+1} = p$ and $a_n = pq$ where $q$ is
the smallest prime divisor of $a_{n-1}$.\end{lemma}
\begin{proof} For any $k \mid a_{n-1}$ the number $kp$ is a good candidate for $a_n$, so $a_n=qp$, where $q$ is the smallest possible controlling
prime, i.e., the smallest prime divisor. Since $q$ is a controlling prime, $q$, $2q$, \ldots, $q(p-1)$ were already used. So if $q$ was a controlling prime for $a_{n+1}$, then $a_{n+1} \geq q(p+1) &gt; p$, where $p$ is a good candidate. So $p$ will be a controlling prime and $a_{n+1}=p$.
\end{proof}

\begin{lemma}\label{rosnace_p} The prime numbers
appear in $a_n$ in increasing order. \end{lemma}
\begin{proof}
If $a_{n+1}=p$, then $a_n=qp$ is the first term divisible by $p$. For any $p' &lt; p$
we have $qp' &lt; qp = a_n$, so the term $qp'$ appeared in the sequence earlier.
\end{proof}

\begin{lemma}\label{pom_male_byly} If $\{m, 2m, \ldots, km\} \subset \{a_i: 1 \leq i \leq M\}$, then
$\{1, 2, \ldots, k\} \subset \{a_i: 1 \leq i \leq M+1\}$.\end{lemma}
\begin{proof}By induction on $k$. For $k=1$ it is obvious, since $a_1=1$. Let $\{m, 2m, \ldots, km\} \subset \{a_i: 1 \leq i \leq M\}$.
Let $a_n = km$, $n \leq M$. Let $q$ be a controlling prime for $a_n$. If $q \mid m$, then all the numbers $m, 2m, \ldots, (k-1)m$ were
good candidates for $a_n$, so they were before $a_n$, so, by the
induction hypothesis, $\{1, 2, \ldots, k-1\} \subset \{a_i: i \leq n\}$.
The number $k$ is a great candidate for $a_{n+1}$, because all smaller numbers were used, so $\{1, 2, \ldots, k\} \subset \{a_i: i \leq n+1\}$.
If $q \mid k$, then $k$ was a candidate for $a_n$, so it was before $a_n$. By the induction hypothesis $\{1, 2, \ldots, k-1\} \subset \{a_i : i \leq M+1\}$.
This completes the proof.\end{proof}

\begin{lemma}\label{male_byly} 
For any prime $p$, the numbers $1, 2, \ldots p-1$ appear in the EKG sequence before $p$.
\end{lemma}

\begin{proof}
As in the proof of Lemma~\ref{przed_p}, before $a_{n+1} = p$ for $p$ prime, there was $a_n = qp$ and
all the numbers $q, 2q, \ldots, q(p-1)$ were used before. 
Therefore, by Lemma~\ref{pom_male_byly}, all the numbers $1, 2, \ldots, p-1$ were
before $p$.
\end{proof}

\begin{thm}
For all $n$ the EKG sequence satisfies the following inequality
$$\frac{1}{260}n &lt; a_n &lt; 14n.$$
\end{thm}

\begin{cor}\label{linioweszacowanie}
If for some $a$, $b$, we have $z_a &lt; z_b$ then $a &lt; 14z_a &lt; 14z_b &lt; 14\cdot 260 b$.
\end{cor}

\section{New results}

\subsection{The fundamental lemma}

Most of the proofs in this paper are based on the following observation.

\begin{lemma}\label{wchodzenie}
Fix any real number $x$. Then for every prime $q$ there can be at most one index $n$ for which $q$ is a controlling
prime for $a_n$ and $a_{n-1} &lt; x \leq a_n$.
\end{lemma}

\begin{proof}
According to the definition, $a_n$ is the smallest multiple of $q$ not yet used in the sequence. Since $a_n \geq x$,
all multiples of $q$ smaller than $x$ were already used, and later in the sequence
there is no term divisible than $q$ smaller than $x$, i.e., if $k &gt; n$ and $q \mid a_k$, then $a_k \geq x$.
\end{proof}

This lemma has important consequences; it strongly limits
the number of times the sequence can cross the border of $x$ going upwards.
Since all the primes in the sequence appear in increasing order, if $p_{k+1}$ has not yet appeared, one can cross
the border of $x$ upwards only $k = \pi(p_{k+1}) = \Theta(\frac{p_{k+1}}{\log p_{k+1}})$ times.

\subsection{$2p$ is before $p$}\label{hofman}

In this section we will prove that each odd new prime in the EKG sequence is introduced by a subsequence $2p$, $p$, $3p$.
We will simply use Lemma~\ref{wchodzenie} and the estimate for $\pi(x)$.

We will prove the following theorem:

\begin{thm}\label{hofman_glowny}
For any primes $p, q &gt; 2$ the term $qp$ appears in the sequence after $2p$.
\end{thm}

\begin{proof}
We will prove Theorem~\ref{hofman_glowny} by contradiction. Let us assume that $p$ is the first prime such
that for some $N$ we have $a_{N+1} = p$ and $a_N = qp$, where $q &gt; 2$. From
Lemmas~\ref{przed_p} and \ref{rosnace_p}
we know that these are the first terms divisible by $p$ and previous terms of the sequence are divisible only by smaller primes than $p$.
From Lemma~\ref{wchodzenie} we know that before the term $a_N$ the sequence can cross the border of $2p$ upwards only $\pi(p)$ times ---  for every prime
smaller than $p$ at most once. Now we will show that for large $p$, the sequence must have crossed this border downwards $\Theta(p)$ times,
which is asymptotically bigger than $\pi(p)$. We will try to be tight on the constants, to match the numerical experiments made 
by Lagarias, Rains and Sloane \cite{lrs}, and
therefore the theorem will be proved for the whole sequence.

Since $q$ is the controlling prime for $a_N = qp$, all multiples of $q$ --- $\{q, 2q, \ldots, (p-1)q\}$ have appeared before the term $a_N$.
Particularly important
for us are numbers greater than $2p$ and divisible by $2q$,
because if $a_k$ is such a number for $k &lt; N$ 
then there is the possibility that $a_{k+1}=2p$.
This cannot hold for $k \leq N$,
so for every such $a_k$ the next number is smaller than $2p$.
Among the terms $\{q, 2q, \ldots, (p-1)q\}$ there are at least $\lfloor \frac{qp-2p}{2q} \rfloor \geq \lfloor \frac{p}{6} \rfloor = \Theta(p)$ 
numbers both divisible by $2q$ and greater than $2p$. They all appeared in the sequence
before $a_N$. So we have at least $\lfloor \frac{p}{6} \rfloor$ moments before $a_N$, where the sequence goes downwards through the
border $2p$ and, due to Lemma~\ref{wchodzenie}, at most $\pi(p)$ moments before $a_N$, where the sequence goes upwards.

From Theorem~\ref{szacowanie_pi2} we know that if $p &gt; 25000$ we have $\pi(p) &lt; \lfloor \frac{p}{6} \rfloor - 1$.
The number of upward and downward crossing should differ by at most $1$, so we have a contradiction for $p &gt; 25000$.

Lagarias, Rains and Sloane \cite{lrs} computed the first $10\,000\,000$ terms of the sequence. The last prime number not greater
than $25\,000$ is $24\,989$ and it appears around the $50\,000$-th term. Up to this bound all primes $p$ were preceded by the term $2p$.
Therefore the theorem is proved for the whole sequence.
\end{proof}

From this the following theorem is an obvious corollary:
\begin{thm}\label{hofman_2p}
If $a_n = p$ and $p&gt;2$ is prime, then $a_{n-1} = 2p$.
\end{thm}
\begin{proof}
From Lemma~\ref{przed_p} we know that $a_{n-1} = qp$ for some prime $q$ and this is the first term divisible by $p$.
Therefore $q=2$, since $2p$ could not have appeared before.
\end{proof}

The above idea can be generalized. As in the proof of 
Theorem~\ref{hofman_glowny} we can use any natural number $n$ instead of 
$p$ and any fixed number $a$ instead of $2$.
This leads to the following theorem, which we do not use later, but which is an interesting result on its own.
This theorem could also be deduced from the final theorem --- $\lim_{n \to \infty} \frac{a_n'}{n} =1$, but the proof 
based on the proof of Theorem~\ref{hofman_glowny} is much more elementary.
We do not include the proof here, since it is
very similar to the proof of Theorem~\ref{hofman_glowny}.

\begin{thm}Let $a &gt; 1$ be a natural number. Then there exists a natural number $N$ such that for all natural numbers $b &gt; a$ and $n &gt; N$
the term $an$ appears in the sequence before $bn$, i.e., $z_{an} &lt; z_{bn}$. In particular, this holds if $\log N &gt; Ca^3$ for some universal
constant $C$.
\end{thm}

\comment{

\begin{proof}
Fix an integer $b &gt; a$ and assume that for some $n$ we have $a_K = bn$ and term $an$ has not yet appeared.
No prime divisor of $n$ can be a controlling prime for $a_K$, otherwise $an$ would be smaller good candidate for $a_K$.
Therefore $g(K) \mid b$. Let us concentrate on all numbers divisible by $ab$ in the set $\{an, an+1, \ldots, bn-1\}$.
All these numbers have appeared before $a_K=bn$, since $a_K$ is the smallest multiple of $g(K)$ not yet in the sequence.
After all such terms the term $an$ was a candidate for the next term, so after these terms the next term was smaller than $an$ ---
the EKG sequence crossed the border of $an$ downwards.
But, from Lemma~\ref{wchodzenie}, there was at most $\pi(an)$ indices smaller than $K$, for which the EKG sequence
crossed the border of $an$ upwards.

There are at least $\lfloor \frac{(b-a)n}{ab} \rfloor$ numbers divisible by $ab$ between $an$ and $bn$ and, according
to Lemma~\ref{szacowanie_pi2}, $\pi(an) &lt; C\frac{an}{\log an} &lt; C_1(a)\frac{n}{\log n}$, where $C_1(a)$ is a constant
depending solely on $a$.

$$\Big\lfloor \frac{(b-a)n}{ab} \Big\rfloor = \Big\lfloor \frac{1-\frac{a}{b}}{a} n\Big\rfloor.$$

This is smallest for $b=a+1$, so

$$\Big\lfloor \frac{(b-a)n}{ab} \Big\rfloor \geq \Big\lfloor\frac{n}{a(a+1)}\Big\rfloor \geq C_2(a) n,$$

\noindent where $C_2(a)$ is a constant depending solely on $a$. For large $n$, $C_2(a) n &gt; C_1(a)\frac{n}{\log n} + 1$, so
for large $n$ we have contradiction --- there are much more moments, where the sequence goes downwards the border of $an$
than where the sequence goes upwards. Therefore for large $n$, the term $bn$ must appear after term $an$. The constants
depend solely on $a$, so for all $b &gt; a$ we have uniform index $N$, when the 
claim starts to be true.
\end{proof}
  }

Lemma \ref{wchodzenie} can be somewhat enhanced, since we know that
$2p$ appears before every odd prime $p$, and there $3p$ appears
afterward. The following lemma shows that the moments when the
sequence crosses the border $x$ upwards and it is not the EKG {\it
tick} (i.e., in the subsequence of the form $q, 3q$ for $q$ prime) are
very rare.

\begin{lemma}\label{wchodzenie2}
Let $x &gt; 1$ be a real number and let $B$ be the set
$$B = \{n : a_n &lt; x \leq a_{n+1}; \ a_n\text{ is not a prime}\}.$$
Then $|B| \leq \pi(\sqrt{x})$.
\end{lemma}

\begin{proof}
Let $n \in B$. Since $a_n$ is composite, it has a prime divisor not greater than $\sqrt{a_n}$, which is
smaller than $\sqrt{x}$. Let $q$ be any such prime divisor.

Since $a_{n+1} \geq x$ and all multiples of $q$ are candidates for $a_{n+1}$, all multiples of $q$ smaller than $x$
must have been used before. Therefore, after $a_n$, there are no free multiples of $q$ smaller than $x$. So every
$n \in B$ {\it uses} at least one prime $q$ smaller than $\sqrt{x}$ --- there are no more unused multiples of $q$ smaller than $x$, so $|B| \leq \pi(\sqrt{x})$.
\end{proof}

\subsection{Numbers above the border}

By Lemma~\ref{wchodzenie}, we know that the sequence can cross the border of an integer $x$ upwards at most $\pi(x)$ times.
The interesting question now is: before an integer $x$ appears in the sequence, how many terms greater than $x$ can appear?

Now we are giving partial answer to that question. We need to assume that $x$ has got some small divisor,
so the numbers greater than $x$ having a common divisor with $x$ (i.e., such terms, after which $x$ is a candidate
for the next term) are quite dense.

The following lemma will be used later only for $a$ being a small prime different than $3$
and while reading one can think of $a$ as such number ---  much smaller than $n$ and prime.

\begin{lemma}\label{nadgranica1}
Let $a &gt; 1$ be an integer. Then there are at most $Ca \frac{an}{\log \log an} \leq Ca^2 \frac{n}{\log \log n}$ terms
of the EKG sequence greater than $an$ and appearing before $an$, where $C$ is a universal constant.
\end{lemma}

\begin{proof}
Fix $a &gt; 1$ and large $n$ and let $N = z_{an}$, i.e., $a_N = an$.
Let $A = \{1 \leq i &lt; N : a_i &gt; an\}$; we need to estimate $|A|$.
From Lemma~\ref{wchodzenie} we know that the sequence can cross
the border of $an$ upwards at most $\pi(an)$ times. We will consider several cases.

Let $A_1 = \{ i \in A: a \mid a_i\}$. If $i \in A_1$, then
$an$ is a candidate for $a_{i+1}$, so after such $i$ the sequence goes downwards
through the border of $an$ if $i+1 &lt; N$. Therefore $|A_1| \leq \pi(an) + 1$.

Now let $i \in A \setminus A_1$ and let $q$ be a controlling prime for $a_i$.
The idea now is as follows: either $a_i$ is quite close to the border $an$, or there is an
already used multiple of $qa$ below $a_i$. The formal argument follows.

Note that Corollary~\ref{linioweszacowanie} implies $q \leq a_i \leq C_0 \cdot an$ for some
universal constant $C_0$.

Let $f(s) = a_i - sq$. Note that $\gcd(q, a) = 1$, because otherwise $an$ would be a better candidate for $a_i$.
For all $s$ (if $f(s) &gt; 0$) $f(s)$ was already used in the sequence before $a_i$.
Among $f(1), f(2), \ldots, f(a-1)$ there is exactly one $f(s_0(a_i))$, such that $a \mid f(s_0(a_i))$.

Let $A_2 = \{i \in A \setminus A_1 : f(s_0(a_i)) \leq an\}$. For every prime $q$ there are at most $a-1$
such numbers $a_i$, for which $q$ is a controlling prime and $i \in A_2$ --- these are the first at most
$a-1$ multiples of $q$ greater than $an$. Therefore $|A_2| \leq (a-1)\pi(C_0an) \leq C_0' (a-1)\pi(an)$.

Let $A_3 = A \setminus A_1 \setminus A_2 = \{i \in A \setminus A_1: f(s_0(a_i)) &gt; an\}$.
The term $f(s_0(a_i))$ was used before $a_i$, thus for some $j &lt; i$ we have $a_j = f(s_0(a_i))$
and obviously $j \in A_1$. For how many indices $i'$ can we have $f(s_0(a_{i'})) = a_j$?
For every prime divisor $r$ of $a_j$, at most $a-1$ times: $a_{i'}$ can be
one of the numbers $a_j + r$, $a_j+2r$, \ldots, $a_j + (a-1)r$. According to
Lemma~\ref{liczba_dzielnikow},
the number $a_j$ can have at most $C_1\frac{\log a_j}{\log \log a_j}$ distinct prime divisors.

According to Corollary~\ref{linioweszacowanie} we have 
$  a_j &lt; 14 \cdot 260 an$,
so $a_j$ can have at most $C_2\frac{\log (an)}{\log \log (an)}$
distinct prime divisors, where $C_2$ is a universal constant.

Therefore
$$|A_3| \leq |A_1| \cdot (a-1) \cdot C_2 \frac{\log (an)}{\log \log (an)}.$$
Since $\pi(an) \leq C\frac{an}{\log(an)}$ for some universal constant $C$, we have

$$|A| = |A_1| + |A_2| + |A_3| \leq \pi(an) + C_0'(a-1)\pi(an) + C_2(a-1)\pi(an) \frac{\log(an)}{\log \log(an)} \leq $$
$$ \leq Ca \frac{an}{\log \log an} \leq Ca^2 \frac{n}{\log \log n}.$$
\end{proof}

The following theorem is an obvious corollary:

\begin{thm}\label{anniezadaleko}
There exists a universal constant $C$ such that for every integer $a &gt; 1$ the following holds:
$z_{an} \leq an + Ca \frac{an}{\log \log an} \leq an + Ca^2 \frac{n}{\log \log n}$.
\end{thm}

\begin{proof}
Fix $a &gt; 1$. From Lemma~\ref{nadgranica1} we know that there are at most $Ca \frac{an}{\log \log an}$ indices $i &lt; z_{an}$ such that
$a_i &gt; an$. Obviously there are at most $an$ indices $i &lt; z_{an}$ for which $a_i \leq an$. Therefore there are at most
$an + Ca\frac{an}{\log \log an} = an + o(n)$ indices $i &lt; z_{an}$.
\end{proof}

However, Lemma~\ref{nadgranica1} is not sufficient for all our purposes. It limits the number of terms greater than $an$,
but does not limit how big the terms are.
The following lemma is an improvement to Lemma~\ref{nadgranica1}. It uses almost the same
technique, but the proof is more complicated and we will use the enhanced version of Lemma~\ref{wchodzenie},
 i.e., Lemma~\ref{wchodzenie2} and Lemma~\ref{nadgranica1} itself.

\begin{lemma}\label{nadgranica2}
Let $a&gt;1$ be an integer not divisible by $3$ and let $\frac13 &gt; \varepsilon &gt; 0$. Then there exists an integer $N(a)$ such
that for all integers $n &gt; N(a)$ if the number $x &gt; (a+2\varepsilon)n$ appeared in the sequence before
term $an$, then $x$ is of the form $3p$ for some prime $p$. In particular, the sufficient condition is that $\log \log N(a) &gt; \frac{Ca^2}{\varepsilon}$
for some universal constant $C$.
\end{lemma}

\begin{proof}
First, due to Lemma~\ref{nadgranica1}, we know that there are at most $C_1a \frac{an}{\log \log (an)}$ terms greater than
$an$ before the term $an$ appears in the sequence. Choose $N(a)$ so big, that for all $n &gt; N(a)$ we have
$C_1a \frac{an}{\log \log an} &lt; 1 + \frac{\varepsilon n}{3}$,
i.e., there are more numbers divisible by $3$ in the segment $(an, (a+\varepsilon)n)$ than terms of the sequence greater than $an$ before $an$ appears.
Note that this one holds if $\log \log N(a) &gt; \frac{C_2a^2}{\varepsilon}$. 

For such a big $n$, whenever any term of form $3p$ appears in the sequence 
before the term $an$ appears, there is an unused divisible by three candidates
for the next term smaller than $(a+\varepsilon)n$. So the next term will be smaller than the border of $(a+\varepsilon)n$.

From Lemma~\ref{wchodzenie2}, there are at most $\pi(\sqrt{(a + \varepsilon)n})$ indices $i$ smaller than $z_{an}$, for which
$a_i &lt; (a+\varepsilon)n$, and $a_{i+1} \geq (a+\varepsilon)n$ and $a_i$ is composite. In the other case,
if $a_i &lt; (a+\varepsilon)n$ and $a_{i+1} \geq(a+\varepsilon)n$ and $a_i$ is prime, the next term $a_{i+2}$ will be smaller than
$(a+\varepsilon)n$.

Suppose there is an index $i &lt; z_{an}$ for which $a_i$ is composite and $a_{i+1} &gt; (a+2\varepsilon)n$. Let $i$ be the first such index.
Since $a_i \leq (a+2\varepsilon)n$, $a_i$ has a prime divisor $q$ smaller than $\sqrt{(a+2\varepsilon)n}$. If $a_{i+1}$ is greater than $(a+2\varepsilon)n$,
then all multiples of $q$ in the segment $((a+\varepsilon)n, (a+2\varepsilon)n)$ must have been used before. In this segment there are
at least $\lfloor \frac{\varepsilon}{aq} n \rfloor$ numbers divisible by both $q$ and $a$. Let us call such terms {\it{interesting}} ones.
All {\it{interesting}} terms were used before $an$ appears in the
sequence. Therefore after such an {\it{interesting}} term, number $an$ is good candidate for the next term, 
so after such terms the sequence goes downwards through the border of $(a+\varepsilon)n$ --- the next term will be at most $an$.

Moreover, since any {\it{interesting}} term is divisible by $a$, it is not of the form of $3p$ for some prime $p$, since $p \geq\frac{an}{3} &gt; a$ and $a$ is not
 divisible by $3$. 
 
 Except for the ticks of the form $p$, $3p$, the sequence went upwards through the border
of $(a+\varepsilon)n$ at most $\pi(\sqrt{(a+\varepsilon)n})$ times, so there can be at most $\pi(\sqrt{(a+\varepsilon)n})$ {\it{interesting}} terms, i.e., from the segment $((a+\varepsilon)n, (a+2\varepsilon)n)$ divisible by $aq$. 
But for large $n$ we have
$$\frac{\varepsilon}{aq} n \geq \frac{\varepsilon}{a \sqrt{2+2\varepsilon}} \sqrt{n} &gt; \pi(\sqrt{(a+\varepsilon)n}),$$
since $\pi(x) = \Theta(\frac{x}{\log x})$. Note that this inequality holds if $\log n &gt; \frac{C_3 a^\frac{3}{2}}{\varepsilon}$ for some universal constant $C_3$.
This condition is much weaker than the bound
$\log \log n &gt; \frac{C_2 a^2}{\varepsilon}$ we assumed earlier.

Therefore if $\log \log n &gt; \frac{C_2a^2}{\varepsilon}$ 
the only terms greater than $(2+2\varepsilon)n$ are of the form $3p$ for $p$ prime.
\end{proof}

While analyzing the EKG sequence one question appears very often. Let us fix an integer $a &gt; 1$, and let
$a$ be not divisible by $3$ (in fact, the most interesting case is when $a$ is a prime different than $3$). Imagine the term $an$ has appeared in the sequence;
the question is, what is the smallest $k$ such that $ak$ has yet not appeared in the sequence (i.e., before $an$)? Lemma~\ref{slupki}, which is in fact
a simple corollary of Lemma~\ref{nadgranica2}, 
provides an answer to that question.

\begin{lemma}\label{slupki}
Let $a&gt;1$ be an integer not divisible by $3$. Let $m_a(n)$ be smallest $k$, such that
term $ak$ has not appeared in the first $n$ terms of the sequence. Let $M_a(n)$ be
the biggest $k$ such that term $ak$ has appeared in the first $n$ terms of the sequence. Then

$$\lim_{n \to \infty} \frac{M_a(n) - m_a(n)}{n} = 0.$$

More precisely, there exists a universal constant $C$ such that the following holds:
$$M_a(n) - m_a(n) \leq \frac{Can}{\log \log n}.$$
\end{lemma}

\begin{proof}
For large $n$ we can apply Lemma~\ref{nadgranica2} for $\varepsilon = \frac{C_1a^2}{\log \log n}$ obtaining that
$aM_a(n) \leq (a+2\varepsilon) m_a(n)$. 
Keeping in mind that, according to Lagarias, Rains and Sloane \cite{lrs}, the EKG sequence has linear bounds, (i.e.,
$ci &lt; a_i &lt; Ci$ for some constants $c$, $C$, so both $M_a(n) \to \infty$ and $m_a(n) \to \infty$ with $n \to \infty$ and $m_a(n) \leq Cn + a$), 
and that $\varepsilon \to 0$ as $n \to \infty$, we have for sufficiently large $n$:

$$M_a(n) - m_a(n) &lt; \frac{2\varepsilon}{a} m_a(n) &lt; \frac{C_1a^2}{a\log \log n} (Cn+a) &lt; \frac{C_2an}{\log \log n}.$$
\end{proof}

\subsection{Linear limit for even terms}

In this section we will prove that $\lim_{n \to \infty} \frac{2n}{z_{2n}} = 1$, i.e., that an even term
$2n$ is at position approximately $2n$. This will give us {\it a skeleton} for proving that all the terms
$x$ except for $p$ and $3p$ are at position approximately $x$.

From Theorem~\ref{anniezadaleko} for $a=2$ we know that term $z_{2n} \leq 2n + C \frac{n}{\log \log n}$,
which gives us $\liminf_{n \to \infty} \frac{2n}{z_{2n}} \geq 1$. What we need to prove is that
$z_{2n} \geq 2n - o(n)$. We will try to estimate the
asymptotic behavior of the $o(n)$ part in the proof.

\begin{thm}\label{techniczneeven}
$$\limsup_{n \to \infty} \frac{2n}{z_{2n}} \leq 1.$$

Moreover, there exists a universal constant $C$ such that
$$z_{2n} \geq 2n - \frac{Cn}{\log \log \log n}.$$
\end{thm}

\begin{proof}
Fix a small $\varepsilon &lt; \frac{1}{7}$. We will prove, that for large $n$, namely $\log \log \log n &gt; \frac{C}{\varepsilon}$, 
$z_{2n} \geq (2-6\varepsilon)n$. This will lead to the result.

First, from Lemma~\ref{slupki}, we can assume that $n$ is sufficiently large, so that all even numbers smaller than $(2-\varepsilon)n$
were already used in the sequence before the term $2n$. For this we need only $\log \log n &gt; \frac{C}{\varepsilon}$, which is one $\log$ weaker than the assumption made
in this theorem.

Let $p_1, p_2, \ldots$ be a sequence of the prime numbers in increasing order ($p_1 = 2$ and $p_2 = 3$). By Merten's theorem (Theorem~\ref{tw_mertena} and 
Corollary~\ref{szacowanie_merten})
$$\prod_{n=1}^K \Big(1-\frac{1}{p_n}\Big) &lt; \frac{C}{\log K}.$$

Let $K$ be sufficiently large (namely $\log K &gt; \frac{C}{\varepsilon}$) such that
$$\prod_{n=3}^K \Big(1-\frac{1}{p_n}\Big) &lt; \varepsilon.$$

Notice, that we omitted $p_1=2$ and $p_2=3$ in this product. 

Let $P = \prod_{n=3}^K p_i$. Since $p_i \leq C i \log i$, 
$$P \leq (CK \log K)^K \leq (CK)^{2K} \leq e^{\frac{2C}{\varepsilon} e^{\frac{C}{\varepsilon}}} \leq e^{e^\frac{C'}{\varepsilon}}.$$
Therefore $\log \log P \leq \frac{C}{\varepsilon}$. Since we assumed that $\log \log \log n &gt; \frac{C}{\varepsilon}$, we can assume that
$n &gt; \frac{P}{\varepsilon} + P$ by choosing appropriate universal constants.

Let us concentrate on numbers smaller than $(2-3\varepsilon)n$, not divisible by any of the numbers $p_3, p_4, \ldots, p_K$.
Let $s = \lfloor \frac{(2-3\varepsilon)n}{P} \rfloor &gt; \frac{1}{\varepsilon}$. Then in every segment $(iP, (i+1)P]$ for
$i = 0, 1, \ldots, s-1$ there are exactly $P \cdot \prod_{i=3}^K (1-\frac{1}{p_i}) &lt; P\varepsilon$ numbers not divisible by any
$p_3, p_4, \ldots, p_K$. So in $[1, (2-3\varepsilon)n]$ there are at most $sP\varepsilon + ((2-3\varepsilon)n-sP) &lt; 2n\varepsilon + P &lt; 3\varepsilon n$ numbers not
divisible by any of $p_3, p_4, \ldots, p_K$.

Since $n &gt; \frac{P}{\varepsilon} + P$, then $n &gt; \frac{2p_K}{\varepsilon}$. Then, since all even numbers smaller than $(2-\varepsilon)n$ appear
in the sequence before index $z_{2n}$, for all $3 \leq i \leq K$ some number divisible by $2p_i$ greater than $(2-2\varepsilon)n$ appears
in the sequence before index $z_{2n}$. Now we are going to use Lemma~\ref{slupki} to estimate, that for all $3 \leq i \leq K$ all numbers 
divisible by $p_i$ smaller than $(2-3\varepsilon)n$ were used in the sequence 
before the term $2n$. Let $3 \leq i \leq K$. To use Lemma~\ref{slupki}, 
we need $\frac{C p_i n}{\log \log n} &lt; \varepsilon n$. However, $p_i \leq p_K \leq CK\log K &lt; e^\frac{C'}{\varepsilon}$, so we need to prove
that $\frac{C e^\frac{C'}{\varepsilon}}{\varepsilon} &lt; \log \log n$. By applying logarithm to both sides we get $\frac{C''}{\varepsilon} &lt; \log \log \log n$, which
was our assumption.

But, as we proved before, there are at most $3\varepsilon n$ numbers not divisible by any of the $p_3, p_4, \ldots, p_K$ smaller than
$(2-3\varepsilon)n$. In total, there are at most $(3\varepsilon + 3\varepsilon)n = 6 \varepsilon n$ numbers smaller than $2n$, than
might not have been used in the sequence before index $z_{2n}$. Therefore, for large $n$, $z_{2n} &gt; (2-6\varepsilon)n$, which
completes the proof.
\end{proof}

\begin{cor}\label{granicaparzystych}
$$\lim_{n\to\infty} \frac{2n}{z_{2n}} = 1.$$

More precisely, there exists a universal constant $C$ such that for all $n$:

$$2n - \frac{Cn}{\log \log \log n} \leq z_{2n} \leq 2n + \frac{Cn}{\log \log n}.$$
\end{cor}

Since for every prime $p &gt; 3$, we have $z_p = z_{2p}+1 = z_{3p}-1$, and from 
Corollary~\ref{granicaparzystych},
$\lim_{i \to \infty} \frac{2p_i}{z_{2p_i}} = 1$, the following corollary is obvious:

\begin{cor}\label{granicapierwszych}
$$\lim_{i\to\infty} \frac{p_i}{z_{p_i}} = \frac12$$
$$\lim_{i\to\infty} \frac{3p_i}{z_{3p_i}} = \frac32$$
\end{cor}

From Corollary~\ref{granicaparzystych} we can conclude a
few other facts that can be useful in our final proof.

\begin{cor}\label{parzyste_niezadaleko}
Let $x &gt; 0$. Then there exists a universal constant $C$ such that all even terms smaller than $x$ appear
in the EKG sequence before index $x + \frac{Cx}{\log \log x}$.
\end{cor}

\begin{cor}\label{parzyste_niezablisko}
Let $x &gt; 0$. Then there exists a universal constant $C$ such that all even terms greater than
$x + \frac{Cx}{\log \log \log x}$ appear in the sequence after index $x$.
\end{cor}

\begin{proof}
This is straightforward corollary from Corollary~\ref{granicaparzystych} --- 
if $2n &gt; x + \frac{Cx}{\log \log \log x}$ then $z_{2n} &gt; 2n - \frac{C'n}{\log \log \log n} &gt; x$ by adjusting the
constant $C$.
\end{proof}

\subsection{Linear limit for all terms}

In this final section we will prove that if $(e_i)$ is a sequence of all {\bf{odd}} natural numbers except these of the form $3p$ and $p$ for $p$
prime, in increasing order, then $\lim_{i \to \infty} \frac{e_i}{z_{e_i}} = 1$. This, together with linear bounds proved by Lagarias, Rains and Sloane \cite{lrs} and Corollary~\ref{granicaparzystych},
gives us the desired result: $\lim_{n\to\infty} \frac{a_n'}{n} = 1$.

First we will prove that any $e_i$ could not appear in the sequence too early, i.e., it can appear at position
$e_i - o(e_i)$. This will be a quite straightforward corollary from 
Corollary~\ref{granicapierwszych}: if some $e_i$
appears, there must have been a not much smaller even term before.

Later, we will prove that $e_i$ could not appear in the sequence too late, i.e., it can appear at position $e_i + o(e_i)$.
In this proof once again we will be estimating how many times do the sequence cross some border upwards or downwards. We will
prove, that not much bigger even term was not used before the term $e_i$.

\begin{thm}\label{limsup_ostateczny}
$$\limsup_{i \to \infty} \frac{e_i}{z_{e_i}} \leq 1.$$

More precisely, there exists a universal constant $C$ such that for all $e_i$
$$z_{e_i} \geq e_i - \frac{Ce_i}{\log \log \log e_i}.$$
\end{thm}

\begin{proof}
Fix $i$ and let $n = z_{e_i}$, i.e., $a_n = e_i$. Since $e_i$ is not of the form of $p$ or $3p$, then
$a_{n-1}$ is composite. Lagarias, Rains and Sloane \cite{lrs} proved that $a_{n-1} &lt; 14(n-1) &lt; 14 n &lt; 14 \cdot 260 a_n = 14\cdot 260 e_i$.
Therefore $a_{n-1}$ has a prime divisor $q$ satisfying $q \leq \sqrt{a_{n-1}} &lt; \sqrt{14\cdot 260 e_i}$.
Since $a_n = e_i$, all multiples of $q$ smaller than $e_i$ must have been used in the sequence before the term $a_n$.
Let $x(i)$ be biggest integer divisible by $2q$ smaller than $e_i$. Since $q \mid x(i)$, $z_{x(i)} &lt; n$. Obviously
$x(i) \geq e_i - 2q &gt; e_i - 2\sqrt{14\cdot 260} \sqrt{e_i}$.
But $x(i)$ is even, so from Corollary~\ref{granicaparzystych}
we have $z_{x(i)} \geq x(i) - \frac{Cx(i)}{\log \log \log x(i)}$ for some universal constant $C$.
Therefore

$$z_{e_i} = n &gt; z_{x(i)} \geq x(i) - \frac{Cx(i)}{\log \log \log x(i)} &gt; e_i - C \sqrt{e_i} - \frac {Ce_i}{\log \log \log e_i} \geq e_i - \frac{C'e_i}{\log \log \log e_i}.$$
\end{proof}

\begin{thm}\label{liminf_ostateczny}
$$\liminf_{i \to \infty} \frac{e_i}{z_{e_i}} \geq 1.$$

More precisely, there exists a universal constant $C$ such that for all $e_i$
$$z_{e_i} \leq e_i + \frac{Ce_i}{\log \log \log e_i}.$$
\end{thm}

\begin{proof}
We will prove the theorem by contradiction. Assume there exists an increasing unbounded sequence $0 &lt; c_0 &lt; c_i \to \infty$ such that
for all $I$ there exists $i &gt; I$ such that $z_{e_i} &gt; (1+7\varepsilon_i)e_i$ where
$\varepsilon_i = \frac{c_i}{\log \log \log e_i}$. We can assume that $\varepsilon_i &lt; \frac{1}{7}$, i.e., $c_i \leq \frac{1}{7}\log \log \log e_i$.
Let $e_i$ be a very large term satisfying $z_{e_i} &gt; (1+7\varepsilon_i)e_i$.

We will start by using Corollaries \ref{parzyste_niezadaleko} and \ref{parzyste_niezablisko} several times. We can use them
since $\limsup_{i \to \infty} \frac{\frac{c_i}{\log \log \log e_i}}{\frac{C}{\log \log \log e_i}} = \infty$ and
$\frac{C}{\log \log \log e_i}$ is the worst asymptotic bound in Corollaries \ref{parzyste_niezadaleko} and \ref{parzyste_niezablisko}.

First, using Corollary~\ref{parzyste_niezadaleko} we can assume that $e_i$ is large enough so that all even terms
smaller than $e_i$ appeared before index $(1+\varepsilon_i)e_i$. In other words, for all $2n &lt; e_i$ we have $z_{2n} &lt; (1+\varepsilon_i)e_i$.

Second, using Corollary~\ref{parzyste_niezadaleko} a second time, we can assume that $e_i$ is large enough so that
all even terms smaller than $(1+6\varepsilon_i)e_i$ have appeared before index $(1+7\varepsilon_i)e_i$. In other words,
for all $2n &lt; (1+6\varepsilon_i)e_i$ we have $z_{2n} &lt; (1+7\varepsilon_i)e_i &lt; z_{e_i}$.

Third, using Corollary~\ref{parzyste_niezablisko} we can assume that $e_i$ is large enough so that
all even terms greater than $(1+2\varepsilon_i)e_i$ have appeared after index $(1+\varepsilon_i)e_i$. In other words,
for all $2n &gt; (1+2\varepsilon_i)e_i$ we have $z_{2n} &gt; (1+\varepsilon_i)e_i$.

To sum up, we have an interval of indices (i.e., a part of sequence) between $(1+\varepsilon_i)e_i$ and $(1+7\varepsilon_i)e_i$,
where all even terms between $(1+2\varepsilon_i)e_i$ and $(1+6\varepsilon_i)e_i$ appear and no even term smaller than $e_i$ appear.
Let us call these even terms between $(1+2\varepsilon_i)e_i$ and $(1+6\varepsilon_i)e_i$ the {\it{important}} terms. 

Since $e_i$ is composite, it has prime divisor $q(i)$ not greater than $\sqrt{e_i}$. Therefore, among all {\it{important}} terms,
we have at least 
$$\Big\lfloor \frac{4\varepsilon_i e_i}{2q(i)} \Big\rfloor \geq \lfloor 2\varepsilon_i \sqrt{e_i} \rfloor \geq \varepsilon_i \sqrt{e_i} \geq \frac{c_0\sqrt{e_i}}{\log \log \log e_i}$$
terms divisible by $q(i)$. Let us call these terms {\it{very important}} ones. 

Every {\it{very important}} term is not of the form $2p$ for $p&gt;3$ prime, since it is divisible by $2q(i)$ and greater than $2q(i)$. And obviously
after every {\it{very important}} term the number $e_i$ is a candidate for the
next term. Therefore we have at least $\frac{c_0\sqrt{e_i}}{\log \log \log e_i}$ moments
between indices $(1+\varepsilon_i)n$ and $(1+7\varepsilon_i)n$, when the EKG sequence crosses the border of $e_i$ downwards. Notice, that none of these moments
are subsequences of the form $2p, p$, since a {\it{very important}} term is not of the form $2p$.

From Lemma~\ref{wchodzenie2} we know, that before index $z_{e_i}$ the EKG sequence can cross the border of $e_i$ upwards at most
$\pi(\sqrt{e_i})$ times, except for the subsequences of the form $p, 3p$ for $p$ prime. 

Let us look at the subsequence $p, 3p$ for $p$ prime that appeared in the EKG sequence after index $(1+\varepsilon_i)e_i$ and before index $(1+7\varepsilon_i)e_i$. The term before
was obviously $2p$ and, since every even term smaller than $e_i$ has appeared before index $(1+\varepsilon_i)e_i$, we
have that $2p &gt; e_i$, except for maybe one moment where $z_{2p} = \lfloor (1+\varepsilon_i)e_i \rfloor$. 
Otherwise we have $p &lt; e_i &lt; 2p &lt; 3p$, so the EKG sequence crossed here the border of $e_i$ first downwards and then upwards.

Therefore between indices $(1+\varepsilon_i)e_i$ and $(1+7\varepsilon_i)e_i$ the number of times the sequence crosses the border of $e_i$ upwards and downwards, without taking into
account subsequences of the form $2p, p$ and $p, 3p$ for $p$ prime, should differ by at most two. The other ones just form pairs --- subsequences of the form 
$2p, p, 3p$ with $p &lt; e_i &lt; 2p &lt; 3p$. But we proved that we have at least $\frac{c_0\sqrt{e_i}}{\log \log \log e_i}$
downward crossings and at most $\pi(\sqrt{e_i}) = O(\frac{\sqrt{e_i}}{\log e_i})$ upward crossings, which is a contradiction. This completes the proof.
\end{proof}

To sum up, in the end, we have proved the following theorem, by
Theorems \ref{limsup_ostateczny}, \ref{liminf_ostateczny} and
Corollaries \ref{granicaparzystych} and \ref{granicapierwszych}:

\begin{thm}\label{ostateczne}
$$\lim_{n\to\infty}\frac{a_n'}{n} = 1.$$

More precisely, there exists a universal constant $C$ such that
$$n - \frac{Cn}{\log \log \log n} \leq a_n' \leq n + \frac{Cn}{\log \log \log n}.$$
\end{thm}

\section{Conclusion}\label{dyskusja}

In this paper we presented a quite elementary proof that in the EKG sequence odd primes appear in the subsequences $2p$, $p$, $3p$ (Theorem \ref{hofman_2p}) and, 
building some technical tools on top of observation made in Lemma~\ref{wchodzenie} and Lemma~\ref{wchodzenie2}, we managed to prove asymptotic behavior of the EKG sequence,
namely, that $\lim_{n\to\infty}\frac{a_n'}{n} = 1$. We tried to estimate the
speed of the convergence through the proofs to see if we could use these techniques to cope with
the stronger conjecture made by Lagarias, Rains and Sloane \cite{lrs}:

\begin{conjecture}[Lagarias, Rains, Sloane]\label{lrscon}
$$a_n' \sim n\Big(1 + \frac{1}{3\log n}\Big).$$
\end{conjecture}

The proved result, Theorem~\ref{ostateczne}, is far from this goal. We were able to prove a very weak convergence speed --- $\frac{C}{\log \log \log n}$ tail.
This is the reason we were not able to apply any argument similar to those 
supporting Conjecture~\ref{lrscon}. Let us repeat the argument. 
In the EKG sequence integers appear more or less in the increasing order, except for terms of the form $p$ and $3p$. If $a_n = m$, then earlier in the sequence
we had all integers smaller than $m$ except for numbers of the form $p$ and $3p$ smaller than $m$, but we had all $p$ and $3p$ for $2p \leq m$. 
This leads to the
Conjecture~\ref{lrscon} by
$$n \sim m - \pi(m) - \pi\Big(\frac{m}{3}\Big) + 2\pi\Big(\frac{m}{2}\Big) \sim m - \frac{m}{3 \log m}.$$

Unfortunately, in this argument we need to investigate sets of missing terms (numbers of the form $p$ and $3p$) that are of the size $\frac{Cn}{\log n}$.
Such sizes are lost comparing to $\frac{Cn}{\log \log \log n}$ part. Moreover, in this paper we get weaker convergence speed ($\frac{Cn}{\log \log n}$) very early,
namely in the Lemma~\ref{nadgranica1}, so we were not able to use this technique. 

To get closer to proving Conjecture~\ref{lrscon} using techniques presented in this paper, one needs to improve the convergence speed.
Note that there are two places where we gain one more $\log$ in the
speed. One is Lemma~\ref{nadgranica1}, where we need to multiply $\pi(x)$ by the number of different prime divisors of $x$, which might be $\frac{\log x}{\log \log x}$.
The proof of Lemma~\ref{nadgranica1} is quite tricky and we were not able to improve it.

The second place where we lose out
is the proof of Theorem~\ref{techniczneeven}, probably the most technical proof in this paper. The estimations made there are quite rough, 
therefore we think they might be somehow improved. However, we do not know how to do it now.

To sum up, although this paper provides some new tools and techniques to cope with the EKG sequence, and proves the core part of the conjectures made by Lagarias, Rains and Sloane \cite{lrs}, 
there is still quite a significant hole between what is done and what is conjectured. The aforementioned Conjecture~\ref{lrscon} seems to us to be the most interesting question remaining.

\section*{Acknowledgments}

We would like to say a huge ``thank you'' to few our professors and
colleagues, without whom this work wouldn't exist. Chronologically
first, to Professor Jaroslaw Grytczuk and to Andrzej Grzesik for
showing us the EKG sequence.  Later, to our RPG Fellowship, Marek
Grabowski, Kaja Malek, Michal Pilipczuk and Jakub Wojtaszczyk, for
patience in listening to our proofs and concepts instead of playing
Earthdawn. Finally, to Joanna Jaszunska and Jakub Wojtaszczyk for
pointing out many errors and an almost infinite number of missing articles
in the paper. And finally, to Professor Jerzy Urbanowicz, for being
our mentor while working on this problem.

\begin{thebibliography}{1}

\bibitem{lrs}
J.~C. Lagarias, E.~M. Rains, and N.~J.~A. Sloane.
\newblock The {EKG} sequence.
\newblock {\em Experimental Math.}, {\bf 11} (2002), 437--446.

\bibitem{mertens}
F.~Mertens.
\newblock Ein {B}eitrag zur analytischen {Z}ahlentheorie.
\newblock {\em J. reine angew. Math.}, {\bf 78} (1874), 46--62.

\bibitem{pnt}
B.~Rosser.
\newblock Explicit bounds for some functions of prime numbers.
\newblock {\em American J. Math.}, {\bf 63} (1941), 211--232.

\bibitem{sloency}
N.~J.~A. Sloane.
\newblock {The On-Line Encyclopedia of Integer Sequences}. \\
\newblock \href{http://www.research.att.com/\~njas/sequences/}{\tt http://www.research.att.com/\~{}njas/sequences/}.

\end{thebibliography}


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\noindent 2000 {\it Mathematics Subject Classification}:
Primary 11B83.

\noindent \emph{Keywords: } EKG sequence, integer sequence, prime numbers.

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\noindent (Concerned with sequence
\seqnum{A064413}.)

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\vspace*{+.1in}
\noindent
Received March 4 2008;
revised version received  September 20 2008.
Published in {\it Journal of Integer Sequences}, October 2 2008.

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\noindent
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\htmladdnormallink{Journal of Integer Sequences home page}{http://www.cs.uwaterloo.ca/journals/JIS/}.
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