Proving/Disproving: RQ is Tridiagonal Matrix

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SUMMARY

The discussion centers on the properties of tridiagonal matrices, specifically whether the product RQ, where A is a tridiagonal matrix decomposed into an orthogonal matrix Q and an upper-triangular matrix R (A = QR), remains tridiagonal. The conclusion drawn is that RQ does not necessarily maintain the tridiagonal structure, as demonstrated by counterexamples involving specific tridiagonal matrices and orthogonal transformations. The transformation RQ = Q-1AQ = QTAQ is pivotal in understanding this property.

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kat18
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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.
 
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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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