Some subpolytopes of the Birkhoff polytope. Marques de Sá, Eduardo — 2006 ELA. The Electronic Journal of Linear Algebra [electronic only]
Hidden relations for the Smith invariants of a matrix sum Marques de Sá, Eduardo — 1990 Portugaliae mathematica
On the invariant factors of sums of integral matrices Marques de Sá, Eduardo — 1989 Portugaliae mathematica