On Maximum Chains in the Bruhat Order of A(n,2)

Abstract

Let $\mathcal{A}(R,S)$ denote the class of all matrices of zeros and ones with row sum vector $R$ and column sum vector~$S$. We introduce the notion of an inversion in a $(0,1)$--matrix. This definition extends the standard notion of an inversion of a permutation, in the sense that both notions agree on the class of permutation matrices. We prove that the number of inversions in a $(0,1)$--matrix is monotonic with respect to the secondary Bruhat order of the class $\mathcal{A}(R,S)$. We apply this result in establishing the maximum length of a chain in the Bruhat order of the class $\mathcal{A}(n,2)$ of $(0,1)$--matrices of order $n$ in which every row and every column has a sum of~$2$. We give algorithmic constructions of chains of maximum length in the Bruhat order of $\mathcal{A}(n,2)$.