2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/61203Let T(m,n) denote the number of ways to tile an m-by-n rectangle with dominos. For any fixed m, the numbers T(m,n) satisfy a linear recurrence relation, and so may be extrapolated to negative values of n; these extrapolated values satisfy the relation T(m,-2-n) = epsilon_{m,n} T(m,n), where epsilon_{m,n} is -1 if m is congruent to 2 (mod 4) and n is odd, and is +1 is otherwise. This is equivalent to a fact demonstrated by Stanley using algebraic methods. Here I give a proof that provides, among other things, a uniform combinatorial interpretation of T(m,n) that applies regardless of the sign of n.5 pages, 6 figuresCombinatorics05A15text