DSTEV - compute all eigenvalues and, optionally, eigenvec­
       tors of a real symmetric tridiagonal matrix A


SYNOPSIS

       SUBROUTINE DSTEV( JOBZ, N, D, E, Z, LDZ, WORK, INFO )

           CHARACTER     JOBZ

           INTEGER       INFO, LDZ, N

           DOUBLE        PRECISION D( * ), E( * ), WORK( * ),  Z(
                         LDZ, * )


PURPOSE

       DSTEV  computes all eigenvalues and, optionally, eigenvec­
       tors of a real symmetric tridiagonal matrix A.


ARGUMENTS

       JOBZ    (input) CHARACTER*1
               = 'N':  Compute eigenvalues only;
               = 'V':  Compute eigenvalues and eigenvectors.

       N       (input) INTEGER
               The order of the matrix.  N >= 0.

       D       (input/output) DOUBLE PRECISION array, dimension
               (N)
               On entry, the n diagonal elements of the tridiago­
               nal matrix A.  On exit, if INFO = 0, the eigenval­
               ues in ascending order.

       E       (input/output) DOUBLE PRECISION array, dimension
               (N)
               On entry, the (n-1) subdiagonal  elements  of  the
               tridiagonal  matrix A, stored in elements 1 to N-1
               of E; E(N) need not be set, but  is  used  by  the
               routine.    On   exit,   the  contents  of  E  are
               destroyed.

       Z       (output) DOUBLE PRECISION array, dimension (LDZ,
               N)
               If  JOBZ  =  'V', then if INFO = 0, Z contains the
               orthonormal eigenvectors of the matrix A, with the
               i-th  column  of Z holding the eigenvector associ­
               ated with D(i).  If JOBZ = 'N', then Z is not ref­
               erenced.

       LDZ     (input) INTEGER
               The  leading  dimension of the array Z.  LDZ >= 1,
               and if JOBZ = 'V', LDZ >= max(1,N).

       WORK      (workspace)   DOUBLE   PRECISION    array,
               dimension (max(1,2*N-2))


       INFO    (output) INTEGER
               = 0:  successful exit
               < 0:  if INFO = -i, the i-th argument had an ille­
               gal value
               >  0:   if  INFO = i, the algorithm failed to con­
               verge; i off-diagonal elements of E did  not  con­
               verge to zero.


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