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In the mathematical field of graph theory a graph homomorphism is a mapping between two graphs that respects their structure. More concretely it maps adjacent vertices to adjacent vertices. ==Definitions== A graph homomorphism from a graph to a graph , written , is a mapping from the vertex set of to the vertex set of such that implies . The above definition is extended to directed graphs. Then, for a homomorphism , is an arc of if is an arc of . If there exists a homomorphism we shall write , and otherwise. If , is said to be homomorphic to or -colourable. If the homomorphism is a bijection whose inverse function is also a graph homomorphism, then is a graph isomorphism. Two graphs and are homomorphically equivalent if and . A retract of a graph is a subgraph of such that there exists a homomorphism , called retraction with for any vertex of . A core is a graph which does not retract to a proper subgraph. Any graph is homomorphically equivalent to a unique core. 抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)』 ■ウィキペディアで「graph homomorphism」の詳細全文を読む スポンサード リンク
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