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Homework Help: A short one on symmetric matrices

  1. Jun 25, 2007 #1


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    This isn't really homework, but close enough. I suppose this is quite simple, but my head's all tangled up for today. Anyways,
    Given the real symmetric matrix
    LTL = UDUT, find L.

    I suppose L = +- D1/2UT, and it's clear this choice of L satisfies the given equation.

    But can it be proven that the above L actually follows from the given equation? i.e.
    LTL = UDUT = (D1/2UT)T(D1/2UT) <=> L = +- D1/2UT?
    Am I making any sense?
  2. jcsd
  3. Jun 25, 2007 #2


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    So you are asking if L is unique? No. Take L and U to be different unitary matrices and D=1. Then T(L).L=1=U.D.T(U) (T=transpose). But it certainly isn't necessarily true that L=+/-T(U).
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