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Let G be a finite group of order n. Must every automorphism of G have order less than n?

(David Speyer: I got this question from you long ago, but I don't know whether you knew the answer. I stil don't!)

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    $\begingroup$ Nope, I didn't know the answer. But now I do! $\endgroup$
    – David E Speyer
    Commented Oct 19, 2009 at 0:36

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Yes every automorphism has order bounded by |G|-1, provided G is not the trivial group. A reference is
M V Horoševskiĭ 1974 Math. USSR Sb. 22 584-594
which can be found at
http://www.iop.org/EJ/abstract/0025-5734/22/4/A08/

It is even shown that the upper bound is reached only for elementary abelian groups.

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    $\begingroup$ Great! Now if only I could read Russian... :) $\endgroup$
    – Reid Barton
    Commented Oct 18, 2009 at 22:20
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    $\begingroup$ The link is to an english translation ;) $\endgroup$
    – Greg Stevenson
    Commented Oct 18, 2009 at 22:40
  • $\begingroup$ So it is! (I googled for a freely available copy and found one in Russian...) $\endgroup$
    – Reid Barton
    Commented Oct 18, 2009 at 23:06
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    $\begingroup$ This was fun to read--very clever, entirely elementary arguments--thanks again. $\endgroup$
    – Reid Barton
    Commented Oct 19, 2009 at 7:22
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    $\begingroup$ The proof is given in the book of Isaacs on Finite Group Theory (p.70). $\endgroup$
    – Philip
    Commented Oct 29, 2015 at 4:00

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