From Peter Cameron’s Blog: The symmetric group 3: Automorphisms

Here is, with Peter’s kind permission, a rebloging of Peter’s post on the automorphism group of S_n. Other very nice accounts are by the Secret blogging seminar;  John Baez,; A paper by Howard, Millson, Snowden, and Vakil; and most famously the legendary Chapter 6 (!) from the book by Cameron and Van-Lint (I dont have an electronic version for it).

My TYI 25 question about it arose from Sonia Balagopalan’s lecture in our combinatorics seminar on the 16 vertex triangulation of 4-dimensional projective space. (Here is the link to her paper.)

Peter Cameron's Blog

No account of the symmetric group can be complete without mentioning the remarkable fact that the symmetric group of degree n (finite or infinite) has an outer automorphism if and only if n=6.

Here are the definitions. An automorphism of a group G is a permutation p of the group which preserves products, that is, (xy)p=(xp)(yp) for all x,y (where, as a card-carrying algebraist, I write the function on the right of its argument). The automorphisms of G themselves form a group, and the inner automorphisms (the conjugation maps x?g-1xg) form a normal subgroup; the factor group is the outer automorphism group of G. Abusing terminology, we say that G has outer automorphisms if the outer automorphism group is not the trivial group, that is, not all automorphisms are inner.

Now the symmetric group S

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2 Responses to From Peter Cameron’s Blog: The symmetric group 3: Automorphisms

  1. Gil Kalai says:

    Yet another very nice account is given in this post by Richard Green “The exceptional symmetry” https://plus.google.com/u/0/101584889282878921052/posts/ioQW2zGjwwM

  2. Pingback: New Year’s Greeting from Franz Kafka « Log24

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