On separability of Schur rings over abelian p-groups

TL;DR

This paper investigates the conditions under which abelian p-groups are separable with respect to Schur rings, establishing new results on their structure and implications for graph isomorphism testing.

Contribution

It proves that certain abelian p-groups are separable and shows how this affects efficient isomorphism testing of Cayley graphs.

Findings

Groups $D=C_p\times C_{p^k}$ are separable for $p\in \{2,3\}$.

Separable groups allow polynomial-time isomorphism testing of Cayley graphs.

Characterization of separable abelian p-groups based on their structure.

Abstract

An $S$-ring (Schur ring) is called separable with respect to a class of $S$-rings $\mathcal{K}$ if it is determined up to isomorphism in $\mathcal{K}$ only by the tensor of its structure constants. An abelian group is said to be separable if every $S$-ring over this group is separable with respect to the class of $S$-rings over abelian groups. Let $C_n$ be a cyclic group of order $n$ and $G$ be a noncylic abelian $p$-group. From the previously obtained results it follows that if $G$ is separable then $G$ is isomorphic to $C_p\times C_{p^k}$ or $C_p\times C_p\times C_{p^k}$, where $p\in \{2,3\}$ and $k\geq 1$. We prove that the groups $D=C_p\times C_{p^k}$ are separable whenever $p\in \{2,3\}$. From this statement we deduce that a given Cayley graph over $D$ and a given Cayley graph over an arbitrary abelian group one can check whether these graphs are isomorphic in time $|D|^{O(1)}$.