[patch #9282] add minres for sparse linear systems

[Xie Rui]

Follow-up Comment #7, patch #9282 (project octave):

Dear Marco,

I looked at formulas (5.13-5.16) and (7.4), but I think it is difficult to use
them when there is a preconditioner matrix. Therefore, I adopted another
method. I accumulated the residual in every iteration just like x, to avoid
matrix-vector multiplication per step.
Moreover, I used this method and formula (7.1) to calculate resveccg, i.e. the
norm of r^c in Paige&Saunders. By the way, my x is x^M, and my resvec is the
norm of r^M in Paige&Saunders.

(file #39988)
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Additional Item Attachment:

File name: minres.m                       Size:20 KB


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