Combinatorial interpretations of Tutte polynomials at the point $(2,-1)$

Abstract

Let $G$ be a simple connected graph, and let $T_G(x,y)$ be the Tutte polynomial of $G$. Motivated by the works in \cite{Ma}, we, in this paper, introduce the even-left spanning forests of $G$ and odd $G$-partitionable permutations, and show that $T_G(2,-1)$ is equal to both the number of even-left spanning forests of $G$ and the number of odd $G$-partitionable permutations. In particular, for a complete graph $K_n$, we prove that $T_{K_n}(2,-1)$ is the number of alternating permutations on $\{1,2,\dots ,n+1\}$, using two distinct techniques: a recurrence relation and an explicit bijection construction.