sstech.f - Man Page

subroutine sstech (n, a, b, eig, tol, work, info)
SSTECH

SSTECH

Purpose:

    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.

Parameters

N

          N is INTEGER
          The dimension of the tridiagonal matrix T.

A

          A is REAL array, dimension (N)
          The diagonal entries of the tridiagonal matrix T.

B

          B is REAL array, dimension (N-1)
          The offdiagonal entries of the tridiagonal matrix T.

EIG

          EIG is REAL array, dimension (N)
          The purported eigenvalues to be checked.

TOL

          TOL is REAL
          Error tolerance for checking, a multiple of the
          machine precision.

WORK

          WORK is REAL array, dimension (N)

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.

Author

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

Definition at line 100 of file sstech.f.