Generalizations of an Expansion Formula for Top to Random Shuffles

Abstract

In the top to random shuffle, the first a cards are removed from a deck of n cards 12 \cdots n and then inserted back into the deck. This action can be studied by treating the top to random shuffle as an element B_a, which we define formally in Section 2, of the algebra Q[S_n]. For a = 1, Adriano Garsia in "On the Powers of Top to Random Shuffling" (2002) derived an expansion formula for B_1^k for k \leq n, though his proof for the formula was non-bijective. We prove, bijectively, an expansion formula for the arbitrary finite product B_{a_1}B_{a_2} \cdots B_{a_k} where a_1, \ldots, a_k are positive integers, from which an improved version of Garsia's aforementioned formula follows. We show some applications of this formula for B_{a_1}B_{a_2} \cdots B_{a_k}, which include enumeration and calculating probabilities. Then for an arbitrary group G we define the group of G-permutations S_n^G := G \wr S_n and further generalize the aforementioned expansion formula to the algebra Q[S_n^G] for the case of finite G, and we show how other similar expansion formulae in Q[S_n] can be generalized to Q[S_n^G].