G. Andrews proved that if n is a prime number then the coefficients ak
and ak+n of the product (q,q)∞/(qn,qn)∞=∑kakqk 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.Comment: Latex, 17 pages. Several formulas and references adde