English

Reciprocals of thinned exponential series

Combinatorics 2023-05-25 v2 Classical Analysis and ODEs

Abstract

The reciprocal of exe^{-x} has a power series about 00 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 exe^{-x}, i.e. polynomials of the form n=0r(1)nxnn!\sum_{n=0}^r (-1)^n\frac{x^n}{n!}, and established combinatorially that the reciprocal of the truncate has a power series with all coefficients non-negative precisely when rr is odd. Here we extend Gessel's observations to arbitrary ``thinned exponential series''. To be precise, let A{1,3,5,}A \subseteq \{1,3,5,\ldots\} and B{2,4,6,}B \subseteq \{2,4,6,\ldots\}, and consider the series 1aAxaa!+bBxbb!. 1-\sum_{a \in A} \frac{x^a}{a!} + \sum_{b \in B} \frac{x^b}{b!}. We consider conditions on AA and BB that ensure that the reciprocal series has all coefficients non-negative. We give combinatorial proofs for a large set of conditions, including whenever 1A1 \in A and the endpoints of the maximal consecutive intervals in ABA \cup B are odd integers. In particular, the coefficients in the reciprocal series can be interpreted as ordered set partitions of [n][n] with block size restrictions, or in terms of permutations with restricted lengths of maximally increasing runs, suitably weighted.

Keywords

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

R2 v1 2026-06-28T09:32:21.798Z