In the mathematical field of graph theory, a core is a notion that describes behavior of a graph with respect to graph homomorphisms.
such that Two graphs are said to be homomorphism equivalent or hom-equivalent if they have isomorphic cores.
Every finite graph has a core, which is determined uniquely, up to isomorphism.
The core of a graph G is always an induced subgraph of G. If
It is NP-complete to test whether a graph has a homomorphism to a proper subgraph, and co-NP-complete to test whether a graph is its own core (i.e. whether no such homomorphism exists) (Hell & Nešetřil 1992).