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.

COMPZ

          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

          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 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

          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 orthogonally similar to the original


                matrix.

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

November 2011