dstech.f3

!>
!>    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.
!> 

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.
!> 

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