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Homework Help: [z^n]^(1/m) = and != [z^(1/m)]^n

  1. Jan 11, 2010 #1
    1. The problem statement, all variables and given/known data

    Show that if m and n are positive integers, [itex]m \ne 0[/itex], and if n/m is an irreducible fraction, then the set of values of [itex]z^{n/m}[/itex] defined by [tex](z^{1/m})^n[/itex] is identical to the set of value of [itex](z^n)^{1/m}[/itex]

    I need to prove the case of a reducible fraction as well, where the two expressions aren't equal.

    3. The attempt at a solution

    I've been staring at this for a day now and I don't see where to start this beyond messing with the expressions in polar form... hints? Thanks.


    Side question:


    Does finding all three roots of each factor and then multiplying them in all combinations give all possible results of the above expression? Thanks.
  2. jcsd
  3. Jan 11, 2010 #2
    Don't forget that an complex equality like the one you have is, in fact, an equality between sets. Starting from the polar expression for z is a good idea, but remember that [tex]z^{1/m}[/tex] is the set of the m-th roots of z, and you must prove that each of its members, when raised to the power n, is a member of the set [tex]\left(z^n\right)^{1/m}[/tex], whose elements are the m-th roots of [tex]z^n[/tex].
  4. Jan 12, 2010 #3
    I get (I'm going to use cis() notation):

    [tex]z^n = r^n cis \left(n\theta \right)[/tex]
    [tex]z^{1/m} = r^{1/m} cis \left (\frac{\theta}{m} + \frac{2k\pi}{m} \right)[/tex]

    [tex](z^n)^{1/m} = r^{n/m} cis \left (\frac{n}{m}\theta + \frac{2k\pi}{m} \right)[/tex] k = 0,1,2,...,|m|-1

    [tex](z^{1/m})^{n} = r^{n/m} cis \left (\frac{n}{m}\theta + \frac{2nk\pi}{m} \right)[/tex]

    I don't think my expression for [itex](z^{1/m})^{n}[/itex] is right with the [itex]2nk[/itex] in it...
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