On the characteristic polynomial of a supertropical adjoint matrix

Abstract

Let $\chi(A)$ denote the characteristic polynomial of a matrix $A$ over a field; a standard result of linear algebra states that $\chi(A^{-1})$ is the reciprocal polynomial of $\chi(A)$. More formally, the condition $\chi^n(X) \chi^k(X^{-1})=\chi^{n-k}(X)$ holds for any invertible $n\times n$ matrix $X$ over a field, where $\chi^i(X)$ denotes the coefficient of $\lambda^{n-i}$ in the characteristic polynomial $\det(\lambda I-X)$. We confirm a recent conjecture of Niv by proving the tropical analogue of this result.