Counting Partitions of a Fixed Genus

  • Robert Cori
  • Gábor Hetyei
Keywords: Set partitions, Noncrossing partitions, Genus of a hypermap


We show that, for any fixed genus $g$, the ordinary generating function for the genus $g$ partitions of an $n$-element set into $k$ blocks is algebraic. The proof involves showing that each such partition may be reduced in a unique way to a primitive partition and that the number of primitive partitions of a given genus is finite. We illustrate our method by finding the generating function for genus $2$ partitions, after identifying all genus $2$ primitive partitions, using a computer-assisted search.

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