2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/189814G. Andrews proved that if $n$ is a prime number then the coefficients $a_k$ and $a_{k+n}$ of the product $(q,q)_\infty/(q^n,q^n)_\infty=\sum_k a_kq^k$ have the same sign, see [A1]. We generalize this result in several directions. Our results are based on the observation that many products can be written as alternating sums of characters of Virasoro modules.Latex, 17 pages. Several formulas and references addedQuantum AlgebraCombinatoricstext