DMTCS Proceedings, Fifth Colloquium on Mathematics and Computer Science

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Are even maps on surfaces likely to be bipartite?

Guillaume Chapuy


It is well known that a planar map is bipartite if and only if all its faces have even degree (what we call an even map). In this paper, we show that rooted even maps of positive genus g chosen uniformly at random are bipartite with probability tending to 4-g when their size goes to infinity. Loosely speaking, we show that each of the 2g fondamental cycles of the surface of genus g contributes a factor 1 / 2 to this probability. We actually do more than that: we obtain the explicit asymptotic behaviour of the number of even maps and bipartite maps of given genus with any finite set of allowed face degrees. This uses a generalisation of the Bouttier-Di Francesco-Guitter bijection to the case of positive genus, a decomposition inspired by previous works of Marcus, Schaeffer and the author, and some involved manipulations of generating series counting paths. A special case of our results implies former conjectures of Gao.

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