Combinatorics of the symmetries of ascents in restricted inversion sequences
Joanna N. Chen, Zhicong Lin · arXiv (Cornell University) · 2021
The systematic study of inversion sequences avoiding triples of relations was initiated by Martinez and Savage. For a triple $(\\rho_1,\\rho_2,\\rho_3)\\in\\{<,>,\\leq,\\geq,=,\ eq,-\\}^3$, they introduced $\\I_n(\\rho_1,\\rho_2,\\rho_3)$ as the set of inversion sequences $e=e_1e_2\\cdots e_n$ of length $n$ such that there are no indices $1\\leq i<j<k\\leq n$ with $e_i \\rho_1 e_j$, $e_j \\rho_2 e_k$ and $e_i \\rho_3 e_k$. To solve a conjecture of Martinez and Savage, Lin constructed a bijection between $\\I_n(\\geq,\ eq,>)$ and $\\I_n(>,\ eq,\\geq)$ that preserves the distinct entries and further posed a symmetry conjecture of ascents on these two classes of restricted inversion sequences. Concerning Lin's symmetry conjecture, an algebraic proof using the kernel method was recently provided by Andrews and Chern, but a bijective proof still remains mysterious. The goal of this article is to establish bijectively both Lin's symmetry conjecture and the $\\gamma$-positivity of the ascent polynomial on $\\I_n(>,\ eq,>)$. The latter result implies that the distribution of ascents on $\\I_n(>,\ eq,>)$ is symmetric and unimodal.