ZLAED7 computes the updated eigensystem of a diagonal


 matrix after modification by a rank-one symmetric matrix. This


 routine is used only for the eigenproblem which requires all


 eigenvalues and optionally eigenvectors of a dense or banded


 Hermitian matrix that has been reduced to tridiagonal form.


   T = Q(in) ( D(in) + RHO * Z*Z**H ) Q**H(in) = Q(out) * D(out) * Q**H(out)


   where Z = Q**Hu, u is a vector of length N with ones in the


   CUTPNT and CUTPNT + 1 th elements and zeros elsewhere.


    The eigenvectors of the original matrix are stored in Q, and the


    eigenvalues are in D.  The algorithm consists of three stages:


       The first stage consists of deflating the size of the problem


       when there are multiple eigenvalues or if there is a zero in


       the Z vector.  For each such occurrence the dimension of the


       secular equation problem is reduced by one.  This stage is


       performed by the routine DLAED2.


       The second stage consists of calculating the updated


       eigenvalues. This is done by finding the roots of the secular


       equation via the routine DLAED4 (as called by SLAED3).


       This routine also calculates the eigenvectors of the current


       problem.


       The final stage consists of computing the updated eigenvectors


       directly using the updated eigenvalues.  The eigenvectors for


       the current problem are multiplied with the eigenvectors from


       the overall problem.

N

          N is INTEGER


         The dimension of the symmetric tridiagonal matrix.  N >= 0.

CUTPNT

          CUTPNT is INTEGER


         Contains the location of the last eigenvalue in the leading


         sub-matrix.  min(1,N) <= CUTPNT <= N.

QSIZ

          QSIZ is INTEGER


         The dimension of the unitary matrix used to reduce


         the full matrix to tridiagonal form.  QSIZ >= N.

TLVLS

          TLVLS is INTEGER


         The total number of merging levels in the overall divide and


         conquer tree.

CURLVL

          CURLVL is INTEGER


         The current level in the overall merge routine,


         0 <= curlvl <= tlvls.

CURPBM

          CURPBM is INTEGER


         The current problem in the current level in the overall


         merge routine (counting from upper left to lower right).

D

          D is DOUBLE PRECISION array, dimension (N)


         On entry, the eigenvalues of the rank-1-perturbed matrix.


         On exit, the eigenvalues of the repaired matrix.

Q

          Q is COMPLEX*16 array, dimension (LDQ,N)


         On entry, the eigenvectors of the rank-1-perturbed matrix.


         On exit, the eigenvectors of the repaired tridiagonal matrix.

LDQ

          LDQ is INTEGER


         The leading dimension of the array Q.  LDQ >= max(1,N).

RHO

          RHO is DOUBLE PRECISION


         Contains the subdiagonal element used to create the rank-1


         modification.

INDXQ

          INDXQ is INTEGER array, dimension (N)


         This contains the permutation which will reintegrate the


         subproblem just solved back into sorted order,


         ie. D( INDXQ( I = 1, N ) ) will be in ascending order.

IWORK

          IWORK is INTEGER array, dimension (4*N)

RWORK

          RWORK is DOUBLE PRECISION array,


                                 dimension (3*N+2*QSIZ*N)

WORK

          WORK is COMPLEX*16 array, dimension (QSIZ*N)

QSTORE

          QSTORE is DOUBLE PRECISION array, dimension (N**2+1)


         Stores eigenvectors of submatrices encountered during


         divide and conquer, packed together. QPTR points to


         beginning of the submatrices.

QPTR

          QPTR is INTEGER array, dimension (N+2)


         List of indices pointing to beginning of submatrices stored


         in QSTORE. The submatrices are numbered starting at the


         bottom left of the divide and conquer tree, from left to


         right and bottom to top.

PRMPTR

          PRMPTR is INTEGER array, dimension (N lg N)


         Contains a list of pointers which indicate where in PERM a


         level's permutation is stored.  PRMPTR(i+1) - PRMPTR(i)


         indicates the size of the permutation and also the size of


         the full, non-deflated problem.

PERM

          PERM is INTEGER array, dimension (N lg N)


         Contains the permutations (from deflation and sorting) to be


         applied to each eigenblock.

GIVPTR

          GIVPTR is INTEGER array, dimension (N lg N)


         Contains a list of pointers which indicate where in GIVCOL a


         level's Givens rotations are stored.  GIVPTR(i+1) - GIVPTR(i)


         indicates the number of Givens rotations.

GIVCOL

          GIVCOL is INTEGER array, dimension (2, N lg N)


         Each pair of numbers indicates a pair of columns to take place


         in a Givens rotation.

GIVNUM

          GIVNUM is DOUBLE PRECISION array, dimension (2, N lg N)


         Each number indicates the S value to be used in the


         corresponding Givens rotation.

INFO

          INFO is INTEGER


          = 0:  successful exit.


          < 0:  if INFO = -i, the i-th argument had an illegal value.


          > 0:  if INFO = 1, an eigenvalue did not converge

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

June 2016