Tautological Intersection Numbers and Order-Consecutive Partition Sequences

TL;DR

This paper links tautological intersection numbers on moduli spaces to Ehrhart polynomials, revealing combinatorial interpretations of their vectors and proposing a conjecture on log-concavity.

Contribution

It provides an enumerative interpretation of the $f^*$- and $h^*$-vectors for specific cases and conjectures their log-concavity for all cases.

Findings

The $f^*$-vector counts order-consecutive partition sequences.

The $h^*$-vector is a binomial coefficient.

The log-concavity conjecture is verified for the case $oldsymbol{d} = (1, 1, ext{...}, 1)$.

Abstract

By recent work of Afandi, it is known that tautological intersection numbers on the moduli space of stable $n$-pointed genus $g$ curves can be arranged into families of Ehrhart polynomials, $\{L_{\vec{d}}\}$, for partial polytopal complexes. In particular, the $f^*$-vector of $L_{\vec{d}}$ is known to be integral and non-negative. In this paper, we show that both the $f^*$-vector and $h^*$-vector have an enumerative interpretation in the special case that $\vec{d} = (1, 1, \dots, 1)$. The $f^*$-vector counts order-consecutive partition sequences of $[n+1]$ and the $h^*$-vector is a binomial coefficient. Furthermore, we conjecture that, for all $\vec{d}$, the $f^*$-vector of $L_{\vec{d}}$ always forms a log-concave sequence, and we verify this conjecture in the case that $\vec{d} = (1, 1, \dots, 1)$.