Principally Box-integer Polyhedra and Equimodular Matrices

A polyhedron is box-integer if its intersection with any integer box $\{\ell\leq x \leq u\}$ is integer. We define principally box-integer polyhedra to be the polyhedra $P$ such that $kP$ is box-integer whenever $kP$ is integer. We characterize them in several ways, involving equimodular matrices and box-total dual integral (box-TDI) systems. A rational $r\times n$ matrix is equimodular if it has full row rank and its nonzero $r\times r$ determinants all have the same absolute value. A face-defining matrix is a full row rank matrix describing the affine hull of a face of the polyhedron. Box-TDI systems are systems which yield strong min-max relations, and the underlying polyhedron is called a box-TDI polyhedron. Our main result is that the following statements are equivalent. - The polyhedron $P$ is principally box-integer. - The polyhedron $P$ is box-TDI. - Every face-defining matrix of $P$ is equimodular. - Every face of $P$ has an equimodular face-defining matrix. - Every face of $P$ has a totally unimodular face-defining matrix. - For every face $F$ of $P$, lin($F$) has a totally unimodular basis. Along our proof, we show that a cone $\{x:Ax\leq \mathbf{0}\}$ is box-TDI if and only if it is box-integer, and that these properties are passed on to its polar. We illustrate the use of these characterizations by reviewing well known results about box-TDI polyhedra. We also provide several applications. The first one is a new perspective on the equivalence between two results about binary clutters. Secondly, we refute a conjecture of Ding, Zang, and Zhao about box-perfect graphs. Thirdly, we discuss connections with an abstract class of polyhedra having the Integer Carath\'eodory Property. Finally, we characterize the box-TDIness of the cone of conservative functions of a graph and provide a corresponding box-TDI system.