On the functions counting walks with small steps in the quarter plane
Publications Mathématiques de l'IHÉS, Tome 116 (2012), pp. 69-114.

Models of spatially homogeneous walks in the quarter plane 𝐙 + 2 with steps taken from a subset 𝒮 of the set of jumps to the eight nearest neighbors are considered. The generating function (x,y,z)𝒬(x,y;z) of the numbers q(i,j;n) of such walks starting at the origin and ending at of such walks starting at the origin and ending at (i,j)𝐙 + 2 after n steps is studied. For all non-singular models of walks, the functions xQ(x,0;z) and yQ(x,0;z) are continued as multi-valued functions on 𝐂 having infinitely many meromorphic branches, of which the set of poles is identified. The nature of these functions is derived from this result: namely, for all the 51 walks which admit a certain infinite group of birational transformations of 𝐂 2 , the interval ] 0 , 1 / | 𝒮 | [ of variation of z splits into two dense subsets such that the functions x 𝒬 ( x , 0 ; z ) and y 𝒬 ( 0 , y ; z ) are shown to be holonomic for any z from the one of them and non-holonomic for any z from the other. This entails the non-holonomy of ( x , y , z ) 𝒬 ( x , y ; z ) , and therefore proves a conjecture of Bousquet-Mélou and Mishna in Contemp. Math. 520:1–40(2010).

DOI : 10.1007/s10240-012-0045-7
Kurkova, Irina 1 ; Raschel, Kilian 2

1 Laboratoire de Probabilités et Modèles Aléatoires, Université Pierre et Marie Curie 4 Place Jussieu, 75252, Paris Cedex 05 France
2 Faculté des Sciences et Techniques, CNRS and Université de Tours Parc de Grandmont, 37200, Tours France
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Kurkova, Irina; Raschel, Kilian. On the functions counting walks with small steps in the quarter plane. Publications Mathématiques de l'IHÉS, Tome 116 (2012), pp. 69-114. doi : 10.1007/s10240-012-0045-7. http://www.numdam.org/articles/10.1007/s10240-012-0045-7/

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