Doubling modulo odd integers, generalizations, and unexpected occurrences

Abstract

The starting point of this work is an equality between two quantities $A$ and $B$ found in the literature, which involve the {\em doubling-modulo-an-odd-integer} map, i.e., $x\in {\mathbb N} \mapsto 2x \bmod{(2n+1)}$ for some positive integer $n$. More precisely, this doubling map defines a permutation $σ_{2,n}$ and each of $A$ and $B$ counts the number $C_2(n)$ of cycles of $σ_{2,n}$, hence $A=B$. In the first part of this note, we give a direct proof of this last equality. To do so, we consider and study a generalized $(k,n)$-perfect shuffle permutation $σ_{k,n}$, where we multiply by an integer $k\ge 2$ instead of $2$, and its number $C_k(n)$ of cycles. The second part of this note lists some of the many occurrences and applications of the doubling map and its generalizations in the literature: in mathematics (combinatorics of words, dynamical systems, number theory, correcting algorithms), but also in card-shuffling, juggling, bell-ringing, poetry, and music composition.