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Tridiagonal matrix

  1. Apr 24, 2006 #1
    Hi, I'm having some difficulty with the following, please help.

    Suppose A in R^{nxn} (n>3) is a tridiagonal matrix where Q is an orthogonal matrix and R is an upper-triangular matrix such that A=QR .

    Must RQ be a tridiagonal matrix?
    If yes, give a proof; otherwise, construct a counterexample.
  2. jcsd
  3. Apr 24, 2006 #2


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    RQ = Q-1AQ = QTAQ. I don't know how much that helps, but it may be a start.
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