Reciprocals of thinned exponential series
Abstract
The reciprocal of has a power series about in which all coefficients are non-negative. Gessel [Reciprocals of exponential polynomials and permutation enumeration, Australas. J. Combin., 74, 2019] considered truncates of the power series of , i.e. polynomials of the form , and established combinatorially that the reciprocal of the truncate has a power series with all coefficients non-negative precisely when is odd. Here we extend Gessel's observations to arbitrary ``thinned exponential series''. To be precise, let and , and consider the series We consider conditions on and that ensure that the reciprocal series has all coefficients non-negative. We give combinatorial proofs for a large set of conditions, including whenever and the endpoints of the maximal consecutive intervals in are odd integers. In particular, the coefficients in the reciprocal series can be interpreted as ordered set partitions of with block size restrictions, or in terms of permutations with restricted lengths of maximally increasing runs, suitably weighted.
Cite
@article{arxiv.2303.14057,
title = {Reciprocals of thinned exponential series},
author = {David Galvin and John Engbers and Clifford Smyth},
journal= {arXiv preprint arXiv:2303.14057},
year = {2023}
}
Comments
34 pages, revised, a new section 6 proves the main result in an alternative way using the Run Theorem