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Frucht's theorem : ウィキペディア英語版 | Frucht's theorem
Frucht's theorem is a theorem in algebraic graph theory conjectured by Dénes Kőnig in 1936 and proved by Robert Frucht in 1939. It states that every finite group is the group of symmetries of a finite undirected graph. More strongly, for any finite group ''G'' there exist infinitely many non-isomorphic simple connected graphs such that the automorphism group of each of them is isomorphic to ''G''. ==Proof idea== The main idea of the proof is to observe that the Cayley graph of ''G'', with the addition of colors and orientations on its edges to distinguish the generators of ''G'' from each other, has the desired automorphism group. Therefore, if each of these edges is replaced by an appropriate subgraph, such that each replacement subgraph is itself asymmetric and two replacements are isomorphic if and only if they replace edges of the same color, then the undirected graph created by performing these replacements will also have ''G'' as its symmetry group.〔, discussion following Theorem 4.1.〕
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