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Counting walks in the quarter plane. (English) Zbl 1026.05004
Chauvin, Brigitte (ed.) et al., Mathematics and computer science II. Algorithms, trees, combinatorics and probabilities. Proceedings of the 2nd colloquium, Versailles-St.-Quentin, France, September 16-19, 2002. Basel: Birkhäuser. Trends in Mathematics. 49-67 (2002).
Summary: We study planar walks that start from a given point $$(i_0, j_0)$$, take their steps in a finite set of $$\mathbb{Z}^d$$, and are confined in the first quadrant $$x\geq 0$$, $$y\geq 0$$. Their enumeration can be attacked in a systematic way: the generating function $$Q(x,y; t)$$ that counts them by their length (variable $$t$$) and the coordinates of their endpoint (variables $$x$$, $$y$$) satisfies a linear functional equation encoding the step-by-step description of walks. For instance, for the square lattice walks starting from the origin, this equation reads $(xy- t(x+ y+ x^2y+ xy^2)) Q(x,y; t)= xy- xtQ(x, 0;t)- ytQ(0, y;t).$ The central question addressed in this paper is the nature of the series $$Q(x,y;t)$$. When is it algebraic? When is it $$D$$-finite (or holonomic)? Can these properties be derived from the functional equation itself?
Our first result is a new proof of an old theorem due to Kreweras, according to which one of these walk models has, for mysterious reasons, an algebraic generating function. Then, we provide a new proof of a holonomy criterion recently proved by M. Petkovšek and the author. In both cases, we work directly from the functional equation.
For the entire collection see [Zbl 0997.00016].

##### MSC:
 05A15 Exact enumeration problems, generating functions
##### Keywords:
planar walks; enumeration; generating function