Symmetric Dellac configurations

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 $(D_n(x))_{n \geq~0}$.