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How do I prove that $A_n$ has no proper subgroup of index less than $n$ if $n\geq 5$?

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Sounds like homework. Voting to close on that basis. – Steve Huntsman May 20 2010 at 23:11
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Reads like homework. Please read mathoverflow.net/howtoask about how to ask questions, and also mathoverflow.net/faq about other sites which might be more appropriate. – José Figueroa-O'Farrill May 20 2010 at 23:12

closed as too localized by Steve Huntsman, José Figueroa-O'Farrill, Pete L. Clark, Alon Amit, fedja May 20 2010 at 23:31

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