# CONFLUENTES MATHEMATICI

Length derivative of the generating function of walks confined in the quarter plane
Confluentes Mathematici, Volume 13 (2021) no. 2, pp. 39-92.

In the present paper, we use difference Galois theory to study the nature of the generating function counting walks with small steps in the quarter plane. These series are trivariate formal power series $Q\left(x,y,t\right)$ that count the number of walks confined in the first quadrant of the plane with a fixed set of admissible steps, called the model of the walk. While the variables $x$ and $y$ are associated to the ending point of the path, the variable $t$ encodes its length. In this paper, we prove that in the unweighted case, $Q\left(x,y,t\right)$ satisfies an algebraic differential relation with respect to $t$ if and only if it satisfies an algebraic differential relation with respect $x$ (resp. $y$). Combined with [2, 3, 4, 9, 11], we are able to characterize the $t$-differential transcendence of the $79$ models of walks listed by Bousquet-Mélou and Mishna.

Accepted:
Revised after acceptance:
Published online:
DOI: 10.5802/cml.77
Classification: 05A15,  30D05,  39A06
Keywords: Random walks, Difference Galois theory, Transcendence, Valued differential fields.
Thomas Dreyfus 1; Charlotte Hardouin 2

1 Institut de Recherche Mathématique Avancée UMR 7501, Université de Strasbourg et CNRS, 7 rue René Descartes, 67084 Strasbourg, France
2 Université Paul Sabatier - Institut de Mathématiques de Toulouse, 118 route de Narbonne, 31062 Toulouse, France
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Thomas Dreyfus; Charlotte Hardouin. Length derivative of the generating function of walks confined in the quarter plane. Confluentes Mathematici, Volume 13 (2021) no. 2, pp. 39-92. doi : 10.5802/cml.77. https://cml.centre-mersenne.org/articles/10.5802/cml.77/

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