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Help proving matrix properties:

  1. Nov 10, 2009 #1
    1. The problem statement, all variables and given/known data

    Let A, B be both matrices with the same dimensions. Is AB^2 = (A^2)(B^2) a valid claim?

    2. Relevant equations



    3. The attempt at a solution

    I attempted to show that (AB)^2 = (AB)(AB) = A(BA)B
    and that (A^2)(B^2) = (AA)(BB) = A(AB)B, so for A(BA)B to be equal to A(AB)B, AB must be equal to BA, which is not always true.

    I discarded this approach as nothing assures me that A and B are both invertible, and thus I cannot prove that A(BA)B = A(AB)B implies BA = AB. My teacher is kinda picky about this stuff.
     
    Last edited: Nov 10, 2009
  2. jcsd
  3. Nov 10, 2009 #2

    Mark44

    Staff: Mentor

    In fact, it's not generally true that AB = BA, so if you can find a counterexample (start with 2x2 matrices), you will have shown that (AB)2 = A2B2 is not a valid claim.
     
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