Counting Pattern-free Set Partitions I: A Generalization of Stirling Numbers of the Second Kind

A partition u of [k] = f1; 2; : : : ; kg is contained in another partition v of [l] if [l] has a k-subset on which v induces u. We are interested in counting partitions v not containing a given partition u or a given set of partitions R. This concept is related to that of forbidden permutations. A strengthening of Stanley{Wilf conjecture is proposed. We prove that the GF counting v is rational if (i) R is nite and the number of parts of v is xed or if (ii) u has only singleton parts and at most one doubleton part. In fact, (ii) is an application of (i). As another application of (i) we prove that for each k the GF counting partitions with k pairs of crossing parts belongs to Z(p1 4x). 3