Enumerating pattern avoidance for affine permutations

Abstract

In this paper we study pattern avoidance for affine permutations. In particular, we show that for a given pattern p, there are only finitely many affine permutations in ${\tilde{S}}_n$ that avoid p if and only if p avoids the pattern 321. We then count the number of affine permutations that avoid a given pattern p for each p in S_3, as well as give some conjectures for the patterns in S_4.

11 pages, 3 figures; fixed typos and proof of Proposition 6