Are the inner automorphisms the only ones that extend to every overgroup? - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-22T02:01:16Z http://mathoverflow.net/feeds/question/7782 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/7782/are-the-inner-automorphisms-the-only-ones-that-extend-to-every-overgroup Are the inner automorphisms the only ones that extend to every overgroup? Josiah Sugarman 2009-12-04T16:22:59Z 2010-01-07T00:01:19Z <p>Let H be a group. Can we find an automorphism $\phi :H\rightarrow H$ which is not an inner automorphism, so that given any inclusion of groups $i:H\rightarrow G$ there is an automorphism $\Phi: G\rightarrow G$ that extends $\phi$, i.e. $\Phi\circ i=i\circ \phi$?</p> http://mathoverflow.net/questions/7782/are-the-inner-automorphisms-the-only-ones-that-extend-to-every-overgroup/7792#7792 Answer by Leonid Positselski for Are the inner automorphisms the only ones that extend to every overgroup? Leonid Positselski 2009-12-04T17:59:13Z 2009-12-04T22:46:40Z <p>The answer is that the inner automorphisms are indeed characterized by the property of the existence of extensions to larger groups containing the original group. I learned as much from <a href="http://mathreader.livejournal.com/22934.html" rel="nofollow">this blog entry</a> (in Russian). The reference is Schupp, Paul E., "A characterization of inner automorphisms.", Proc. Amer. Math. Soc. 101 (1987), no. 2, 226--228. </p> http://mathoverflow.net/questions/7782/are-the-inner-automorphisms-the-only-ones-that-extend-to-every-overgroup/10989#10989 Answer by Vipul Naik for Are the inner automorphisms the only ones that extend to every overgroup? Vipul Naik 2010-01-07T00:01:19Z 2010-01-07T00:01:19Z <p>I struggled with the same question for quite some time and solved it for finite groups, only to then discover that it had already been solved. Schupp solved it for arbitrary groups. Martin Pettet later solved it for finite groups, and his proof works for p-groups, finite p-groups, finite pi-groups, solvable groups, etc.</p> <ol> <li><p>On inner automorphisms of finite groups by Martin R. Pettet, Proceedings of the American Mathematical Society, Volume 106,Number 1, Page 87 - 90(May 1989)</p></li> <li><p>Characterizing inner automorphisms of groups by Martin R. Pettet, Archiv der Mathematik, Volume 55,Number 5, Page 422 - 428(Year 1990)</p></li> </ol> <p>These proofs also show that analogous statements are true if we replace injective embeddings with quotient maps (i.e., the only automorphisms that can be pulled back over all quotient maps are the inner ones).</p> <p>I also have some notes on this and similar problems here: <a href="http://groupprops.subwiki.org/wiki/Extensible_automorphisms_problem" rel="nofollow">extensible automorphisms problem</a>.</p>