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Any complete group (that is, with trivial center and outer automorphism group) is isomorphic to its automorphism group. The inverse is not true, as the dihedral group of order 8 is isomorphic to its automorphism group but isn't complete.

Problem 15.29 in the Kourovka Notebook asks to find another exemple of a p-group, but I was wondering, is any other exemple known? Is there any other finite (or even infinite) group (not necessarily a p-group) which is isomorphic to its automorphism group but isn't complete?

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The infinite dihedral group $G=D_{\infty}$ also is isomorphic to its own outomoprhism group, but is not complete - see the article A Note on Groups with Just-Infinite Automorphism Groups.

For finite groups, not $p$-groups, not complete, it is said here: "Whether there exist other groups isomorphic to their automorphism groups is an open problem". For more results, see the article On a question about automorphisms of finite p-groups by G. Cutolo. See also this MO-question.

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