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Group Action on a Set

  1. Dec 1, 2008 #1
    Let g= [tex] \left( \begin{array}{ccccc}

    1 & 2 & 3 & 4 & 5 \\

    2 & 5 & 4 & 1 & 3 \end{array} \right)

    [/tex] be an element of [tex]S_{5}[/tex] and a set S={1,2,3}.

    The theorem of a group action says "If a group G acts on a set, this action induces a homomorphism G->A(S), A(S) is the group of all permutations of the set S."

    When I apply the above action g on a set S, [tex]1 \mapsto 2, 2 \mapsto 5, 3 \mapsto 4 [/tex], which is not a permutation of a set S.

    A group action on a set possibly does not induce a set of its own permutation on set S?

    Any opinion will be appreciated.
  2. jcsd
  3. Dec 2, 2008 #2
    The problem is that S5 doesn't act on {1, 2, 3} (with the usual action of permutation groups), since 2 is in X, but g(2) = 5 is not in {1, 2, 3}. A group action of G on a set X must send an element of G and an element of X to an element of X; what you show is a map that does not map into {1, 2, 3}, so it cannot be an action on {1, 2, 3}.
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