Divisibility by 2 and 3 of certain Stirling numbers
Abstract
The numbers ep(k,n) defined as min(nup(S(k,j)j!): j >= n) appear frequently in algebraic topology. Here S(k,j) is the Stirling number of the second kind, and nup(-) the exponent of p. The author and Sun proved that if L is sufficiently large, then ep((p-1)pL + n -1, n) >= n-1+nup([n/p]!). In this paper, we determine the set of integers n for which equality holds in this inequality when p=2 and 3. The condition is roughly that, in the base-p expansion of n, the sum of two consecutive digits must always be less than p.
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