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LAPACK 3.12.0
LAPACK: Linear Algebra PACKage
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subroutine sstech | ( | integer | n, |
real, dimension( * ) | a, | ||
real, dimension( * ) | b, | ||
real, dimension( * ) | eig, | ||
real | tol, | ||
real, dimension( * ) | work, | ||
integer | info | ||
) |
SSTECH
Let T be the tridiagonal matrix with diagonal entries A(1) ,..., A(N) and offdiagonal entries B(1) ,..., B(N-1)). SSTECH checks to see if EIG(1) ,..., EIG(N) are indeed accurate eigenvalues of T. It does this by expanding each EIG(I) into an interval [SVD(I) - EPS, SVD(I) + EPS], merging overlapping intervals if any, and using Sturm sequences to count and verify whether each resulting interval has the correct number of eigenvalues (using SSTECT). Here EPS = TOL*MACHEPS*MAXEIG, where MACHEPS is the machine precision and MAXEIG is the absolute value of the largest eigenvalue. If each interval contains the correct number of eigenvalues, INFO = 0 is returned, otherwise INFO is the index of the first eigenvalue in the first bad interval.
[in] | N | N is INTEGER The dimension of the tridiagonal matrix T. |
[in] | A | A is REAL array, dimension (N) The diagonal entries of the tridiagonal matrix T. |
[in] | B | B is REAL array, dimension (N-1) The offdiagonal entries of the tridiagonal matrix T. |
[in] | EIG | EIG is REAL array, dimension (N) The purported eigenvalues to be checked. |
[in] | TOL | TOL is REAL Error tolerance for checking, a multiple of the machine precision. |
[out] | WORK | WORK is REAL array, dimension (N) |
[out] | INFO | INFO is INTEGER 0 if the eigenvalues are all correct (to within 1 +- TOL*MACHEPS*MAXEIG) >0 if the interval containing the INFO-th eigenvalue contains the incorrect number of eigenvalues. |
Definition at line 100 of file sstech.f.