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Homework Help: Centralizer proof

  1. Mar 31, 2010 #1
    1. The problem statement, all variables and given/known data
    If a is an element of a group and |a|=n, prove that C(a)=C(a^k) when k is relatively prime to n.

    2. Relevant equations
    If n and k are relatively prime, then there exists integers s and t such that ns+kt=1.
    The centralizer a in G, C(a) is the set of all elements in a group G that commute with a. C(a)={g an element of G|ga-ag}

    3. The attempt at a solution
    I tried proving it by double containment, but I couldn't show that C(a^K) is contained in C(a). Should I try a proof by contradiction? I will be grateful for any help.
  2. jcsd
  3. Mar 31, 2010 #2
    I think double containment is the easiest method (I have never heard the term double containment, but I'm assuming it refers to proving [itex]C(a) \subseteq C(a^K)[/itex] and [itex]C(a^K) \subseteq C(a)[/itex]).

    You want to show that if b is in C(a^K), then b is in C(a). In other words assume:
    [tex]ba^Kb^{-1} = a^K[/itex]
    i.e. a^K and b commute. You then wish to show [itex]bab^{-1}=a[/itex]. Here I have just stated the goal so we're clear about that.

    To say that K and n are relatively prime is equivalent to saying that there exists integers x, y such that xK+yn=1. Now you have:
    [tex](bab^{-1})^1 = (bab^{-1})^{xK+yn}[/tex]
    so it suffices to show:
    [tex](bab^{-1})^{xK+yn} = a[/tex]
    See if you can do this. Remember that you have:
    [tex]ba^K b^{-1} = a^K \qquad a^n = 1[/tex]
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