DSTEQR(3)      LAPACK routine of NEC Numeric Library Collection      DSTEQR(3)



NAME
       DSTEQR

SYNOPSIS
       SUBROUTINE DSTEQR (COMPZ, N, D, E, Z, LDZ, WORK, INFO)



PURPOSE
            DSTEQR 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 symmetric matrix can also be found
            if DSYTRD or DSPTRD or DSBTRD has been used to reduce this matrix to
            tridiagonal form.




ARGUMENTS
           COMPZ     (input)
                     COMPZ is CHARACTER*1
                     = 'N':  Compute eigenvalues only.
                     = 'V':  Compute eigenvalues and eigenvectors of the original
                             symmetric matrix.  On entry, Z must contain the
                             orthogonal 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         (input)
                     N is INTEGER
                     The order of the matrix.  N >= 0.

           D         (input/output)
                     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         (input/output)
                     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         (input/output)
                     Z is DOUBLE PRECISION array, dimension (LDZ, N)
                     On entry, if  COMPZ = 'V', then Z contains the orthogonal
                     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 symmetric matrix,
                     and if COMPZ = 'I', Z contains the orthonormal eigenvectors
                     of the symmetric tridiagonal matrix.
                     If COMPZ = 'N', then Z is not referenced.

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

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

           INFO      (output)
                     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 orthogonally similar to the original
                           matrix.



LAPACK routine                  31 October 2017                      DSTEQR(3)