Two families of circulant nut graphs

Abstract

A circulant nut graph is a non-trivial simple graph whose adjacency matrix is a circulant matrix of nullity one such that its non-zero null space vectors have no zero elements. The study of circulant nut graphs was originally initiated by Ba\v{s}i\'c et al.\ [Art Discrete Appl.\ Math.\ 5(2) (2021) \#P2.01], where a conjecture was made regarding the existence of all the possible pairs $(n, d)$ for which there exists a $d$-regular circulant nut graph of order $n$. Later on, it was proved by Damnjanovi\'c and Stevanovi\'c [Linear Algebra Appl.\ 633 (2022) 127--151] that for each odd $t \ge 3$ such that $t\not\equiv_{10}1$ and $t\not\equiv_{18}15$, the $4t$-regular circulant graph of order $n$ with the generator set $\{ 1, 2, 3, \ldots, 2t+1 \} \setminus \{t\})$ must necessarily be a nut graph for each even $n \ge 4t + 4$. In this paper, we extend these results by constructing two families of circulant nut graphs. The first family comprises the $4t$-regular circulant graphs of order $n$ which correspond to the generator sets $\{1, 2, \ldots, t-1\} \cup \left\{\frac{n}{4}, \frac{n}{4} + 1 \right\} \cup \left\{\frac{n}{2} - (t-1), \ldots, \frac{n}{2} - 2, \frac{n}{2} - 1 \right\}$, for each odd $t \in \mathbb{N}$ and $n \ge 4t + 4$ divisible by four. The second family consists of the $4t$-regular circulant graphs of order $n$ which correspond to the generator sets $\{1, 2, \ldots, t-1\} \cup \left\{\frac{n+2}{4}, \frac{n+6}{4} \right\} \cup \left\{\frac{n}{2} - (t-1), \ldots, \frac{n}{2} - 2, \frac{n}{2}-1 \right\}$, for each $t \in \mathbb{N}$ and $n \ge 4t + 6$ such that $n \equiv_{4} 2$. We prove that all of the graphs which belong to these families are indeed nut graphs, thereby fully resolving the $4t$-regular circulant nut graph order--degree existence problem whenever $t$ is odd and partially solving this problem for even values of $t$ as well.