TESTING/EIG/dstech.f(3)    Library Functions Manual    TESTING/EIG/dstech.f(3)

NAME
       TESTING/EIG/dstech.f

SYNOPSIS
   Functions/Subroutines
       subroutine dstech (n, a, b, eig, tol, work, info)
           DSTECH

Function/Subroutine Documentation
   subroutine dstech (integer n, double precision, dimension( * ) a, double
       precision, dimension( * ) b, double precision, dimension( * ) eig,
       double precision tol, double precision, dimension( * ) work, integer
       info)
       DSTECH

       Purpose:


               Let T be the tridiagonal matrix with diagonal entries A(1) ,...,
               A(N) and offdiagonal entries B(1) ,..., B(N-1)).  DSTECH 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
               DSTECT).  Here EPS = TOL*MAZHEPS*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 DOUBLE PRECISION array, dimension (N)
                     The diagonal entries of the tridiagonal matrix T.

           B

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

           EIG

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

           TOL

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

           WORK

                     WORK is DOUBLE PRECISION array, dimension (N)

           INFO

                     INFO is INTEGER
                     0  if the eigenvalues are all correct (to within
                        1 +- TOL*MAZHEPS*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 dstech.f.

Author
       Generated automatically by Doxygen for LAPACK from the source code.

LAPACK                          Version 3.12.0         TESTING/EIG/dstech.f(3)