- Graph homomorphism
A graph homomorphism f from a graph G = (V,E) to a graph G' = (V',E'), written , is a mapping from the vertex set of G to the vertex set of G' such that implies .
The above definition is extended to directed graphs. Then, for a homomorphism , (f(u),f(v)) is an arc of G' if (u,v) is an arc of G.
If there exists a homomorphism we shall write , and otherwise. If , G is said to be homomorphic to H or H-colourable.
Two graphs G and G' are homomorphically equivalent if and .
A retract of a graph G is a subgraph H of G such that there exists a homomorphism , called retraction with r(x) = x for any vertex x of H. A core is a graph which does not retract to a proper subgraph. Any graph is homomorphically equivalent to a unique core.
The composition of homomorphisms are homomorphisms.
Graph homomorphism preserves connectedness.
Connection to coloring and girth
A graph coloring is an assignment of one of k colors to a graph G so that the endpoints of each edge have different colors, for some number k. Any coloring corresponds to a homomorphism from G to a complete graph Kk: the vertices of Kk correspond to the colors of G, and f maps each vertex of G with color c to the vertex of Kk that corresponds to c. This is a valid homomorphism because the endpoints of each edge of G are mapped to distinct vertices of Kk, and every two distinct vertices of Kk are connected by an edge, so every edge in G is mapped to an adjacent pair of vertices in Kk. Conversely if f is a homomorphism from G to Kk, then one can color G by using the same color for two vertices in G whenever they are both mapped to the same vertex in Kk. Because Kk has no edges that connect a vertex to itself, it is not possible for two adjacent vertices in G to both be mapped to the same vertex in Kk, so this gives a valid coloring. That is, G has a k-coloring if and only if it has a homomorphism to Kk.
If there are two homomorphisms , then their composition is also a homomorphism. In other words, if a graph G can be colored with k colors, and there is a homomorphism , then H can also be k-colored. Therefore, whenever a homomorphism exists, the chromatic number of H is less than or equal to the chromatic number of G.
Homomorphisms can also be used very similarly to characterize the girth of a graph G, the length of its shortest cycle, and the odd girth, the length of the shortest odd cycle. The girth is, equivalently, the smallest number g such that a cycle graph Cg has a homomorphism , and the odd girth is the smallest odd number g for which there exists a homomorphism . For this reason, if , then the girth and odd girth of G are both greater than or equal to the corresponding invariants of H.
The associated decision problem, i.e. deciding whether there exists a homomorphism from one graph to another, is NP-complete. Determining whether there is an isomorphism between two graphs is also an important problem in computational complexity theory; see graph isomorphism problem.
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