Number of cliques of Paley-type graphs over finite commutative local rings

Abstract

In this work, given $(R,\frak m)$ a finite commutative local ring with identity and $k \in \mathbb{N}$ with $(k,|R|)=1$, we study the number of cliques of any size in the Cayley graph $G_R(k)=Cay(R,U_R(k))$ %and $W_R(k)=Cay(R,S_R(k))$ with $U_R(k)=\{x^k : x\in R^*\}$. Using the known fact that the graph $G_R(k)$ can be obtained by blowing-up the vertices of $G_{\mathbb{F}_{q}}(k)$ a number $|\frak{m}|$ of times, with independence sets the cosets of $\frak{m}$, where $q$ is the size of the residue field $R/\frak m$. Then, by using the above blowing-up, we reduce the study of the number of cliques in $G_R(k)$ over the local ring $R$ to the computation of the number of cliques of $G_{R/\frak{m}}(k)$ over the finite residue field $R/\frak m \simeq \mathbb{F}_q$. In this way, using known numbers of cliques of generalized Paley graphs ($k=2,3,4$ and $\ell=3,4$), we obtain several explicit results for the number of cliques over finite commutative local rings with identity.