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Graph invariants are properties of graphs that are invariant under graph isomorphisms: each is a function ${\displaystyle f\,}$ such that ${\displaystyle f(G_{1})=f(G_{2})\,}$ whenever ${\displaystyle G_{1}\,}$ and ${\displaystyle G_{2}\,}$ are isomorphic graphs. Examples include the number of vertices and the number of edges.