A human proof of Gessel's lattice path conjecture

Alin Bostan (1) , Irina Kurkova (2) , Kilian Raschel (3)

1 SPECFUN - Symbolic Special Functions : Fast and Certified
2 Modélisation stochastique
3 LMPT - Laboratoire de Mathématiques et Physique Théorique

Gessel walks are planar walks confined to the positive quarter plane, that move by unit steps in any of the following directions: West, North-East, East and South-West. In 2001, Ira Gessel conjectured a closed-form expression for the number of Gessel walks of a given length starting and ending at the origin. In 2008, Kauers, Koutschan and Zeilberger gave a computer-aided proof of this conjecture. The same year, Bostan and Kauers showed, using again computer algebra tools, that the trivariate generating function of Gessel walks is algebraic. In this article we propose the first ''human proofs'' of these results. They are derived from a new expression for the generating function of Gessel walks.

Mots clés: Walks in the quarter plane; Gessel walks; Counting generating function; Elliptic functions

Domaines: Combinatoire [math.CO]; Probabilités [math.PR]

Alin Bostan, Irina Kurkova, Kilian Raschel. A human proof of Gessel's lattice path conjecture. 2014. ⟨hal-00858083v2⟩