#### Vol. 14, No. 5, 2021

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A look at generalized perfect shuffles

### Samuel Johnson, Lakshman Manny, Cornelia A. Van Cott and Qiyu Zhang

Vol. 14 (2021), No. 5, 813–828
##### Abstract

Standard perfect shuffles involve splitting a deck of $2n$ cards into two stacks and interlacing the cards from the stacks. There are two ways that this interlacing can be done, commonly referred to as an in shuffle and an out shuffle, respectively. In 1983, Diaconis, Graham, and Kantor determined the permutation group generated by in and out shuffles on a deck of $2n$ cards for all $n$. Diaconis et al. concluded their work by asking whether similar results hold for so-called generalized perfect shuffles. For these shuffles, we split a deck of $mn$ cards into $m$ stacks and similarly interlace the cards with an in $m$-shuffle or out $m$-shuffle, respectively. In this paper, we find the structure of the group generated by these two shuffles for a deck of ${m}^{k}$ cards, together with ${m}^{y}$-shuffles, for all possible values of $m$, $k$, and $y$. The group structure is completely determined by $k∕gcd\left(y,k\right)$ and the parity of $y∕gcd\left(y,k\right)$. In particular, the group structure is independent of the value of $m$.

 I must complain the cards are ill shuffled till I have a good hand. — Jonathan Swift (1667–1745)
##### Keywords
perfect shuffle, permutation group, symmetric group
##### Mathematical Subject Classification
Primary: 05A05, 20B35