NAME

       zsteqr.f -

SYNOPSIS

   Functions/Subroutines
       subroutine zsteqr (COMPZ, N, D, E, Z, LDZ, WORK, INFO)
           ZSTEQR

Function/Subroutine Documentation

   subroutine zsteqr (character COMPZ, integer N, double precision, dimension( * ) D, double
       precision, dimension( * ) E, complex*16, dimension( ldz, * ) Z, integer LDZ, double
       precision, dimension( * ) WORK, integer INFO)
       ZSTEQR

       Purpose:

            ZSTEQR computes all eigenvalues and, optionally, eigenvectors of a
            symmetric tridiagonal matrix using the implicit QL or QR method.
            The eigenvectors of a full or band complex Hermitian matrix can also
            be found if ZHETRD or ZHPTRD or ZHBTRD has been used to reduce this
            matrix to tridiagonal form.

       Parameters:
           COMPZ

                     COMPZ is CHARACTER*1
                     = 'N':  Compute eigenvalues only.
                     = 'V':  Compute eigenvalues and eigenvectors of the original
                             Hermitian matrix.  On entry, Z must contain the
                             unitary matrix used to reduce the original matrix
                             to tridiagonal form.
                     = 'I':  Compute eigenvalues and eigenvectors of the
                             tridiagonal matrix.  Z is initialized to the identity
                             matrix.

           N

                     N is INTEGER
                     The order of the matrix.  N >= 0.

           D

                     D is DOUBLE PRECISION array, dimension (N)
                     On entry, the diagonal elements of the tridiagonal matrix.
                     On exit, if INFO = 0, the eigenvalues in ascending order.

           E

                     E is DOUBLE PRECISION array, dimension (N-1)
                     On entry, the (n-1) subdiagonal elements of the tridiagonal
                     matrix.
                     On exit, E has been destroyed.

           Z

                     Z is COMPLEX*16 array, dimension (LDZ, N)
                     On entry, if  COMPZ = 'V', then Z contains the unitary
                     matrix used in the reduction to tridiagonal form.
                     On exit, if INFO = 0, then if COMPZ = 'V', Z contains the
                     orthonormal eigenvectors of the original Hermitian matrix,
                     and if COMPZ = 'I', Z contains the orthonormal eigenvectors
                     of the symmetric tridiagonal matrix.
                     If COMPZ = 'N', then Z is not referenced.

           LDZ

                     LDZ is INTEGER
                     The leading dimension of the array Z.  LDZ >= 1, and if
                     eigenvectors are desired, then  LDZ >= max(1,N).

           WORK

                     WORK is DOUBLE PRECISION array, dimension (max(1,2*N-2))
                     If COMPZ = 'N', then WORK is not referenced.

           INFO

                     INFO is INTEGER
                     = 0:  successful exit
                     < 0:  if INFO = -i, the i-th argument had an illegal value
                     > 0:  the algorithm has failed to find all the eigenvalues in
                           a total of 30*N iterations; if INFO = i, then i
                           elements of E have not converged to zero; on exit, D
                           and E contain the elements of a symmetric tridiagonal
                           matrix which is unitarily similar to the original
                           matrix.

       Author:
           Univ. of Tennessee

           Univ. of California Berkeley

           Univ. of Colorado Denver

           NAG Ltd.

       Date:
           November 2011

Author

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