Diameter Constraints in 2-distance Graphs

For any finite, simple graph $G = (V,E)$, its $2$-distance graph $G_2$ is a graph having the same vertex set $V$ where two vertices are adjacent if and only if their distance is $2$ in $G$. Connectivity and diameter properties of these graphs have been well studied. For example, it has been shown that if ${\rm diam}(G) = k \geq 3$ then $\lceil \frac{1}{2} k \rceil \leq {\rm diam}(G_2)$, and that this bound is sharp. In this paper, we prove that ${\rm diam}(G_2) = \infty$ (that is, $G_2$ is disconnected) or otherwise ${\rm diam}(G_2) \leq k + 2$. In addition, we show that this inequality is sharp for any even $k$, a result that we verify for some higher orders through judicious use of a \textsc{sat} solver.