Algebraic characterizations of generating and affinely generating $Γ$-magic maps and $Γ$-distance magic labelings on regular graphs

Abstract

For an abelian group $Γ$ of order $n$, a graph $G$ of order $n$ is $Γ$-distance magic if it admits a bijection $V(G)\toΓ$ whose open-neighborhood sums are constant, and group distance magic if this holds for every such $Γ$. More generally, for any finite abelian $Γ$, a $Γ$-magic map is a map $f\colon V(G)\toΓ$ with constant open-neighborhood sums; we call $f$ generating if its labels generate $Γ$, and affinely generating if its pairwise differences do. Let $G$ be regular, let $Γ\cong\mathbb{Z}/d_1\oplus\cdots\oplus\mathbb{Z}/d_r$ with $d_1\mid\cdots\mid d_r$, put $B_Γ=\bigoplus_{i<r}\mathbb{Z}/d_i$, and let $\overline{A}_m$ be the adjacency operator induced on $(\mathbb{Z}/m)^{V(G)}$ modulo constants. We prove that $G$ admits a generating $Γ$-magic map iff $B_Γ\hookrightarrow\ker\overline{A}_{d_r}$, and an affinely generating one iff $Γ\hookrightarrow\ker\overline{A}_{d_r}$; when $|V(G)|=|Γ|$, it is $Γ$-distance magic iff the latter embedding has vertex-separating image. If the reduced adjacency operator is nonsingular over $\mathbb{Q}$, these become subgroup conditions in the reduced adjacency Smith group. Cichacz and Froncek conjectured that every distance magic graph is group distance magic. Using the affine criterion we construct a $6$-regular distance magic graph of order $27$ admitting no affinely generating $(\mathbb{Z}/3)^3$-magic map, so the conjecture fails even for affine generation. We propose the generating group distance magic conjecture, and prove it for regular distance magic graphs whenever $Γ$ is $2$-generated, hence for cube-free order. Further applications concern Cayley graphs on elementary abelian $p$-groups, Hamming relation graphs, strongly regular graphs, and symmetric designs.