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[Lapack] Bug: xSTEDC eigenvalues not sorted

Hi,

according to the LAPACK Doxygen documentation, xSTEDC guarantees to
return the eigenvalues in ascending order (parameter D on exit). To
this end, it sorts the eigenvalues if necessary.

If there is only a single subproblem of size n, no sorting algorithm is
called (cf. {s,d}tedc.f, line 450). Furthermore, xLAED0
({s,d}tedc.f:400) does not guarantee to return eigenvalues in ascending
order. Thus, xSTEDC may return eigenvalues that are not in ascending
order.

As a fix, I propose always sorting the eigenvalues (patch attached).

The bug can be triggered in practice (see compressed file in matrix
market format).

Sincerely
Christoph Conrads
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