On the Diameter of \(n\)th Order Degree Regular Trees

For \(n\) a positive integer and \(v\) a vertex of a graph \(G\), the \(n\)th order degree of \(v\) in \(G\), denoted by \(\text{deg}_n(v)\), is the number of vertices at distance \(n\) from \(v\). The graph \(G\) is said to be \(n\)th order regular of degree \(k\) if, for every vertex \(v\) of \(G\), \(\text{deg}_n(v) = k\). For \(n \in \{7, 8, \ldots, 11\}\), a characterization of \(n\)th order regular trees of degree \(2\) is obtained. It is shown that, for \(n \geq 2\) and \(k \in \{3, 4, 5\}\), if \(G\) is an \(n\)th order regular tree of degree \(k\), then \(G\) has diameter \(2n – 1\).