Let $G=(V,E)$ be a finite, simple, undirected graph. Let $D_2(G)$ be the graph with vertex set $V$, and two vertices form an edge if and only if they have distance $2$ in the original graph $G$.

Note that, if $C_{2k+1}$ is the circle on $2k+1$ points for an integer $k\geq 2$, we have $C_{2k+1}\cong D_2(C_{2k+1})$.

Question. Is there a connected, non-regular graph $G=(V,E)$ with $|V|>2$ such that $G\cong D_2(G)$?


Yes, the hexagon ABCDEF with the additional edge of AC has this property. The first graph shows $G$ and the others show two views of $D_2(G)$.

G D_2(G) showing D_2 D_2(G) showing iso

(Graphs courtesy of NCTM)

  • 1
    $\begingroup$ Brilliant, and thanks for the nice visualization! $\endgroup$ – Dominic van der Zypen Jan 28 at 12:10

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