The Second Leaper Theorem

Abstract

A $(p, q)$-leaper is a fairy chess piece that, from a square $a$, can move to any of the squares $a + (\pm p, \pm q)$ or $a + (\pm q, \pm p)$. Let $L$ be a $(p, q)$-leaper with $p + q$ odd and $C$ a cycle of $L$ within a $(p + q) \times (p + q)$ chessboard. We show that there exists a second leaper $M$, distinct from $L$, such that a Hamiltonian cycle $D$ of $M$ exists over the squares of $C$. We give descriptions of $C$ and $M$ in terms of continued fractions. We introduce the notion of a direction graph, roughly a leaper graph from which all information has been abstracted away save for the directions of the moves, and we study $C$ and $D$ in terms of direction graphs. We introduce the notion of a dual generalized chessboard, a generalized chessboard $B$ of more than one square such that the leaper graph of a leaper $L$ over $B$ is connected and isomorphic to the leaper graph of a second leaper $M$, distinct from $L$, over $B$, and we give constructions for dual generalized chessboards.