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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