A new combinatorial identity for unicellular maps, via a direct bijective approach

作者：Chapuy Guillaume^*

来源：Advances in Applied Mathematics, 2011, 47(4): 874-893.

DOI：10.1016/j.aam.2011.04.004

• Polygon gluings
• Bijection
• Harer-Zagier numbers

A unicellular map, or one-face map, is a graph embedded in an orientable surface such that its complement is a topological disk. In this paper, we give a new viewpoint to the structure of these objects, by describing a decomposition of any unicellular map into a unicellular map of smaller genus. This gives a new combinatorial identity for the number epsilon(g)(n) of unicellular maps with n edges and genus g. Unlike the Harer-Zagier recurrence formula, this identity is recursive in only one parameter (the genus).
Iterating the construction gives an explicit bijection between unicellular maps and plane trees with distinguished vertices, which gives a combinatorial explanation (and proof) of the fact that epsilon(g)(n) is the product of the n-th Catalan number by a polynomial in n. The combinatorial interpretation also gives a new and simple formula for this polynomial. Variants of the problem are considered, like bipartite unicellular maps, or unicellular maps with only cubic vertices.

• 出版日期2011-10