Factorizations of nonnegative matrices

Suppose A is an n-square matrix over the real numbers such that all principal minors are nonzero. If A is nonnegative, then necessary and sufficient conditions are determined for A to be factored into a product L-U, where L is a lower triangular nonnegative matrix and U is an upper triangular nonnegative matrix with ui, = 1. These conditions are given in terms of the nonnegativity of certain almost-principal minors of A.