Given two graphs and , the maximum common edge subgraph problem is the problem of finding a graph with as many edges as possible which is isomorphic to both a subgraph of and a subgraph of . The maximum common edge subgraph problem on general graphs is NP-complete as it is a generalization of subgraph isomorphism: a graph is isomorphic to a subgraph of another graph if and only if the maximum common edge subgraph of and has the same number of edges as . Unless the two inputs and to the maximum common edge subgraph problem are required to have the same number of vertices, the problem is APX-hard. See also Maximum common subgraph isomorphism problem Subgraph isomorphism problem Induced subgraph isomorphism problem References Computational problems in graph theory