TESTING/EIG/sstech.f(3)

NAME
TESTING/EIG/sstech.f

SYNOPSIS

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

Function/Subroutine Documentation

  subroutine sstech (integer n, real, dimension( * ) a, real, dimension( * ) b, real, dimension( * ) eig, real tol, real, dimension( * ) work, integer info)

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.

Author

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