Symmetric Dellac configurations

TL;DR

This paper explores symmetric Dellac configurations, their combinatorial interpretations, and their connections to polynomial extensions of median Euler numbers, providing new insights into their enumeration and algebraic properties.

Contribution

It introduces extended Dellac configurations and links symmetric Dellac configurations to polynomial extensions of median Euler numbers, offering new combinatorial interpretations.

Findings

Cardinalities of odd and even symmetric Dellac configurations are given by specific integer sequences.

Extended Dellac configurations generate the polynomial extensions of median Euler numbers.

Symmetric Dellac configurations relate to the Poincaré polynomials of symplectic and orthogonal degenerate flag varieties.

Abstract

We define symmetric Dellac configurations as the Dellac configurations that are symmetric with respect to their centers. The symmetric Dellac configurations whose lengths are even were previously introduced by Fang and Fourier under the name of symplectic Dellac configurations, to parametrize the torus fixed points of symplectic degenerate flag varieties. In general, symmetric Dellac configurations generate the Poincar\'e polynomials of (odd or even) symplectic or orthogonal versions of the degenerate flag varieties. In this paper, we give several combinatorial interpretations of the polynomial extensions $(D_n(x))_{n \geq~0}$ of median Euler numbers, defined by Randrianarivony and Zeng, in terms of objects that we name extended Dellac configurations and which generate symmetric Dellac configurations. As a consequence, the cardinalities of the odd and even symmetric Dellac configurations are respectively given by the two adjoining sequences $(l_n)_{n \geq~0} = (1, 1, 3, 21, 267,\dots)$ and $(r_n)_{n \geq~0} = (1,2,10,98,1594,\dots)$, defined as specializations of the polynomials $(D_n(x))_{n \geq~0}$.