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\topmatter
\title
Approximate Counting via Euler transform
\endtitle

\author Helmut Prodinger \endauthor
\affil 
Department of Algebra and Discrete Mathematics \\
Technical University of Vienna, Austria
\endaffil
\address 
TU Vienna
\newline Wiedner Hauptstrasse 8--10
\newline A-1040 Vienna
\newline Austria
\endaddress 
\email proding\@email.tuwien.ac.at
\endemail
\thanks
I thank Christian Krattenthaler for his help with $q$--hypergeometric
functions
\endthanks
\endtopmatter












\document

{\bf Approximate Counting} might be  described as follows. There are
the states $1,2,\dots$, and in one step one may either advance from
state $i$ to state $i+1$ with probability $q^i$, or stay in the 
state $i$ with  probability $1-q^i$.  The interesting parameter is
the state that one reaches after $n$ random steps, starting in state 1.

The original analysis was performed by Flajolet \cite{2} and consists of an
enumerative (or algebraic) part and an asymptotic (or analytic) part.
The latter was done by the Mellin transform.
In \cite{5}, some additional
manipulations allowed to rewrite the sought
quantities in such a way that an alternative asymptotic technique
(Rice's method) could be used. See also the related papers \cite{6,7}.

In this short note we would like to emphasize how some elements of
``$q$--analysis'' (basic hypergeometric functions) allow some shortcuts
in the enumerative part. Let $H_l(x)$ be the generating function, where
the coefficient of $x^n$ is the probability that $n$ random steps have
led to state $l$. We find a rather explicit form for the super generating
function $\sum_{l\ge1}H_l(x)y^l$ which is interesting in itself and might
lead to additional insight. We only use it here to find representations
for the expection and the second factorial moment as alternating sums,
with binomial coefficients and some simple quantities, which is
essential for the use of Rice's method. One could avoid to
use such explicit formul\ae\ and, using the (new) generating 
functions, derive the asymptotics directly by the ingenious method in
\cite{4}. However, here, we only deal with the enumerative part.


\bigskip

We need a few concepts from $q$--analysis which are taken from
\cite{1}.

\noindent
$q$--{\bf Pochhammer symbol}:
$$
(a)_n:=(1-a)(1-aq)\dots(1-aq^{n-1}),\qquad (a)_0=1,
\qquad (a)_\infty=\lim_{n\to\infty}(a)_n
$$
{\bf Cauchy's formula}:
$$
\sum_{n\ge0}\frac{(a)_nt^n}{(q)_n}=\frac{(at)_\infty}{(t)_\infty}.
$$
{\bf Heine's transformation}:
$$
\sum_{n\ge0}\frac{(a)_n(b)_nt^n}{(q)_n(c)_n}=
\frac{(b)_\infty(at)_\infty}{(c)_\infty(t)_\infty}
\sum_{n\ge0}\frac{(c/b)_n(t)_nb^n}{(q)_n(at)_n}.
$$
{\bf Euler's transformation}:
If
$$
f(x)=\sum_{n\ge0}a_nx^n,
$$
then
$$
\frac{1}{1-x}f\Big(\frac{x}{x-1}\Big)=\sum_{n\ge0}
\bigg(\sum_{k=0}^n\binom{n}{k}
(-1)^ka_k\bigg) x^n.
$$
This is very easily computed directly and was used with great success in
\cite{4} and \cite{3}.
\bigskip






The generating function $H_l(x)$ was computed in \cite{2}.
Using a decomposition of a path from 1 to $l$ in stages, it is not
hard to see that
$$
H_l(x)=\frac{x^{l-1}q^{\binom{l}{2}}}{\prod\limits_{i=1}^l\big(1-x(1-q^i)
\big)}=
\frac{\frac 1x\big(\frac{x}{1-x}\big)^lq^{\binom{l}{2}}}{
\big(\frac{xq}{x-1}\big)_l}.
$$
The coefficient of $x^n$ in $\sum_{l\ge1}H_l(x)$ must be 1 for all $n$,
since each path of length $n$ must simply lead somewhere. Let us see how
we find the formula
$$
\sum_{l\ge1}H_l(x)=\frac{1}{1-x}
$$
by some properties of ``$q$-analysis''. It is equivalent to show
$$
\sum_{l\ge1}\frac{(-z)^lq^{\binom{l}{2}}}{(qz)_l}=-z,
$$
with $z=\frac{x}{x-1}$. Let us show that
$$
\sum_{l\ge0}\frac{(-z)^lq^{\binom{l}{2}}}{(qz)_l}=1-z.
$$
First, write
$$
\aligned
(-1)^lq^{\binom{l}{2}}&=(0-1)(0-q)\dots(0-q^{l-1})\\
&=
\lim_{\varepsilon\to0}\varepsilon^l(1/\varepsilon)_l.
\endaligned
$$
Then we can use the transformation of Heine and compute
$$
\sum_{l\ge0}\frac{(1/\varepsilon)_l(\varepsilon z)^l}{(qz)_l}\cdot
\frac{(q)_l}{(q)_l}
=\frac{(q)_\infty(z)_\infty}{(qz)_\infty(\varepsilon z)_\infty}
\sum_{n\ge0}\frac{(z)_n(\varepsilon z)_nq^n}{(q)_n(z)_n}.
$$
Now we can perform the limit $\varepsilon\to\infty$ on the right hand side
without problems and obtain
$$
\frac{(q)_\infty(z)_\infty}{(qz)_\infty}
\sum_{n\ge0}\frac{q^n}{(q)_n}.
$$
We use the trivial fact $(z)_\infty=(1-z)(qz)_\infty$ for the first
factor and Cauchy's identity (the special case which is attributed to
Euler), which evaluates the sum to $1/(q)_\infty$, which gives us the
desired $1-z$. Great.
\medskip

Now let us attack the generating function of the expectations,
$$
\sum_{l\ge 1}l\, H_l(x).
$$
For that, we consider the super generating function
$$
H(x,y)=\sum_{l\ge 0}H_l(x)y^l,
$$
differentiate it w.r.t. $y$ and evaluate at $y=1$. We obtain
$$
H(x,y)=\frac{1}{x}
\sum_{l\ge0}\frac{(-yz)^lq^{\binom{l}{2}}}{(qz)_l},\qquad
z=\frac{x}{x-1}.
$$
Using Heine's transformation as before, we get
$$
\sum_{l\ge0}\frac{(-yz)^lq^{\binom{l}{2}}}{(qz)_l}
=\frac{(q)_\infty(yz)_\infty}{(qz)_\infty}
\sum_{n\ge0}\frac{(z)_nq^n}{(q)_n(yz)_n}.
$$
For $y=1$ we could evaluate the sum immediately. Now we transform the sum
again by ``Heine'', with $a=0$, $b=z$, $c=yz$ and $t=q$ and find
(for the sum only)
$$
\frac{(z)_\infty}{(yz)_\infty(q)_\infty}
\sum_{n\ge0}\frac{(y)_n(q)_nz^n}{(q)_n},
$$
which gives
$$
H(x,y)=\frac 1x(1-z)\sum_{n\ge0}(y)_nz^n,\qquad
z=\frac{x}{x-1}.
$$
Now observe that
$$
\frac{d}{dy}(y)_n\Big|_{y=1}=-(q)_{n-1}, \qquad n\ge1, 
$$
so that the generating function of the expectations $E(x)$ is
$$
E(x)=-\frac{1}{x}(1-z)\sum_{n\ge0}(q)_nz^{n+1}=
(1-z)^2\sum_{n\ge0}(q)_nz^{n}.
$$
According to Euler's transformation we have to extract the coefficient
of $z^n$ in 
$$
\frac{1}{1-z}\cdot (1-z)^2\sum_{n\ge0}(q)_nz^{n}=
(1-z)\sum_{n\ge0}(q)_nz^{n},
$$
which is
$$
(q)_n-(q)_{n-1}=-q^n(q)_{n-1}
$$
for $n\ge 1$ and 1 for $n=0$.
Hence the expected value $E_n$ is
$$
E_n=1-\sum_{k=1}^n\binom{n}{k}(-1)^kq^k(q)_{k-1},
$$
a formula already reported in \cite{5}.

\medskip

To obtain the generating function $E_2(x)$
for the second factorial moments 
we have to differentiate twice w.r.t. $y$ and evaluate at $y=1$.
Observe that for $n\ge 2$
$$
\frac{d^2}{dy^2}(y)_n\Big|_{y=1}=2(q)_{n-1}\sum_{k=1}^{n-1}\frac{q^k}{1-q^k}.
$$
Let us abbreviate
$$
T_n=\sum_{k=1}^{n}\frac{q^k}{1-q^k}.
$$
Then
$$
E_2(x)=\frac 1x(1-z)\cdot2\sum_{n\ge1}(q)_nT_nz^{n+1}.
$$
As before, we have to extract the coefficient of $z^n$ in
$$
\frac{1}{1-z}E_2(x)=-2(1-z)\sum_{n\ge1}(q)_nT_nz^{n},
$$
which is
$$
-2\Big((q)_nT_n-(q)_{n-1}T_{n-1}\Big)=2q^n(q)_{n-1}\Big(T_{n-1}-1\Big).
$$
Hence the second factorial moment $E_n^{(2)}$ is given by
$$
E_n^{(2)}=\sum_{k=1}^n\binom{n}{k}(-1)^k\cdot 
2q^k(q)_{k-1}\Big(T_{k-1}-1\Big).
$$
This is equivalent to the formula given in \cite{5}.
\medskip

As stated before, asymptotics follow, as demonstrated in \cite{5},
by Rice's method. However, since we have the explicit forms of
the generating functions of the moments, an approach like the one
in \cite{4} would give the asymptotics even a little bit more
directly. We do not work it out, because the computations are somehow
similar to the ones which occur with Rice's method.

\Refs

\ref \no 1 \by G. E. Andrews \book The Theory of Partitions
\yr 1976 \publ Addison Wesley \endref

\ref \no 2  \by P. Flajolet \pages 113--\-134 \paper Approximate Counting:
A Detailed Analysis \yr 1985 \vol 25 \jour BIT \endref

\ref \no 3  \by P. Flajolet, G. Labelle, 
L. Laforest, and B. Salvy \paper 
Hypergeometrics and the Cost Structure of Quadtrees
\yr 1995  \jour Random Structures and Algorithms\toappear\endref

\ref \no 4  \by P. Flajolet and B. Richmond
 \pages 305--320 \vol 5 \yr 1992 \paper 
Generalized Digital Trees and their Difference--Differential Equations
\jour Random Structures and Algorithms \endref






\ref \no 5 \by P.  Kirschenhofer and H. Prodinger\paper Approximate
Counting: An Alternative Approach \jour RAIRO Informatique Th\'eorique
et Applications \vol 25 \yr 1991 \pages 43--48 \endref

\ref \no 6\paper 
A coin tossing algorithm for counting large numbers of events 
 \by P.Kirschenhofer and H. Prodinger
\pages 531--545\vol42
\jour Mathematica Slovaca  \yr 1992 
\endref


\ref \no 7 \paper 
Hypothetic Analyses: Approximate Counting in the Style of  Knuth,
Path Length in the Style of Flajolet
\jour Theoretical Computer Science\vol 100\pages 243--251 \by 
H. Prodinger \yr 1992 
\endref

\endRefs














\enddocument
