From: Roy P Koomullil
Date: Thu, 27 Sep 2012 20:27:59 +0000
I am trying to use DGEQR2 to calculate QR factorization of a rectangular
matrix. I was able to compile the library and run it with a simple 4x3 case. I
also could see R (which is the upper triangular portion of A on exit).
However, I am having some issue on extracting Q from the matrix.
In the documentation it is written that Q is the product of elementary
reflectors (H(1)H(2)...H(k)), where H(i) = I - tau*v*v_transpose and v is
stored in the lower triangular portion of A on exit. This involves a lot of
calculations outside the library. Is there any easier way to calculate Q? I am
not sure about what are the properties of elementary reflectors and properties
of v, we can use to simplify the calculations.
Any help in this matter will be greatly appreciated.
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