On Irreducible Maps and Slices
Jérémie Bouttier, Emmanuel Guitter · Combinatorics Probability Computing · 2014
We consider the problem of enumeratingd-irreducible maps,i.e., planar maps all of whose cycles have length at leastd, and such that any cycle of lengthdis the boundary of a face of degreed. We develop two approaches in parallel: the natural approach via substitution, where these maps are obtained from general maps by a replacement of alld-cycles by elementary faces, and a bijective approach via slice decomposition, which consists in cutting the maps along shortest paths. Both lead to explicit expressions for the generating functions ofd-irreducible maps with controlled face degrees, summarized in some elegant ‘pointing formula’. We provide an equivalent description ofd-irreducible slices in terms of so-calledd-oriented trees. We finally show that irreducible maps give rise to a hierarchy of discrete integrable equations which include equations encountered previously in the context of naturally embedded trees.