Additive triples of bijections, or the toroidal semiqueens problem

Abstract

We prove an asymptotic for the number of additive triples of bijections $\{1,\dots ,n\}\to \mathbb{Z}/n\mathbb{Z}$, that is, the number of pairs of bijections ${\pi }_1,{\pi }_2:\{1,\dots ,n\}\to \mathbb{Z}/n\mathbb{Z}$ such that the pointwise sum ${\pi }_1+{\pi }_2$ is also a bijection. This problem is equivalent to counting the number of orthomorphisms or complete mappings of $\mathbb{Z}/n\mathbb{Z}$, to counting the number of arrangements of $n$ mutually nonattacking semiqueens on an $n\times n$ toroidal chessboard, and to counting the number of transversals in a cyclic Latin square. The method of proof is a version of the Hardy–Littlewood circle method from analytic number theory, adapted to the group $(\mathbb{Z}/n\mathbb{Z}{)}^n$.