Graph invariants are properties of graphs that are invariant under graph isomorphisms: each is a function f{\displaystyle f\,} such that f(G1)=f(G2){\displaystyle f(G_{1})=f(G_{2})\,} whenever G1{\displaystyle G_{1}\,} and G2{\displaystyle G_{2}\,} are isomorphic graphs. Examples include the number of vertices and the number of edges.
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