Spectral Theory of the Toroidal 3D Queen Graph

Abstract: We study the adjacency spectrum of the toroidal three-dimensional queen graph $G_n$ on $(\mathbb{Z}_n)^3$. Since $G_n$ is a Cayley graph on an abelian group, its adjacency matrix is diagonalized by Fourier characters. For each frequency $a\in(\mathbb{Z}_n)^3$, the corresponding eigenvalue is $λ(a)=nμ(a)-13$, where $μ(a)$ counts the queen directions orthogonal to $a$ modulo $n$. In the generic odd case, meaning $n$ odd with $3\nmid n$, the possible values of $μ(a)$ are exactly $0,1,2,3,4,$ and $13$, and each multiplicity is given by an explicit polynomial in $n$. The proof combines a geometric classification of frequency points by orthogonality type with two global counting identities.