[Math] Example of a simple graph isomorphic to a permutation group.

graph theorygroup-theorypermutations

I'm taking a first course in graph theory this semester and I'm working trough Graph Theory with Applications by J.A. Bonday and U.S.R. Murty. I can't find an answer to question 1.2.12(f):

(a) Show, using execise 1.2.5 that an automorphism of a simple graph $G$ can be regarded as a permutation on $V$ which preserves adjacency,and that the set of such permutations form a group $\Gamma(G)$ (the automorphism group of $G$) under the usual operation of composition.

(e) Consider the permutation group $\Lambda $ with elements (1)(2)(3), (1, 2, 3) and (1, 3, 2). Show that there is no simple graph $G$ with vertex set {1, 2, 3} such that $\Gamma(G)=\Lambda$.

(f) Find a simple graph G such that $\Gamma(G)\cong\Lambda$.

Best Answer

At first I thought a solution wouldn't be too hard to find: simply take the triangle graph and attach various subgraphs to each node. This doesn't work, because it either: doesn't restrict any symmetries, or restricts too many symmetries.

It's apparently not as easy as it sounds to construct graphs with cyclic automorphism groups, but a little googling returned this Mathworld page. Just find the word "cyclic" on the page to see some examples.

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