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The center of a graph $G$ is the set of vertices that minimize the largest distance to vertices in $G$, e.g., in the graph below, that radius is $4$:           Define the center $C$ as the subgraph …

Let $G$ be a nonplanar graph (undirected) of $n$ nodes and $e$ edges, with $e \le 3n-6$. Define a rewiring move as replacing edge $(a,b)$ with edge $(a,c)$. The result must be a simple graph (no loops …

Let $G$ be a plane $3$-connected graph; so it partitions the plane into regions bounded by faces. Let $\mathrm{deg}_v$ be the sequence of vertex degrees of $G$, and $\mathrm{deg}_f$ be the sequence of …

Let $\cal T$ be a monohedral, edge-to-edge tiling of the plane, with prototile $T$ a simple polygon, and with one edge $e^*$ of $T$ distinguished. Associate a graph $G=G_{\cal T}$ with $\cal T$ as fol …

In an equitable coloring of a graph $G$, the number of vertices in each color class differ by at most $1$. For example, left below is not an equitable coloring, while the right graph is equitably colo …