# slaed8.f man page

slaed8.f

## Synopsis

### Functions/Subroutines

subroutine **slaed8** (ICOMPQ, K, **N**, QSIZ, D, Q, LDQ, INDXQ, RHO, CUTPNT, Z, DLAMDA, Q2, LDQ2, W, PERM, GIVPTR, GIVCOL, GIVNUM, INDXP, INDX, INFO)**SLAED8** used by sstedc. Merges eigenvalues and deflates secular equation. Used when the original matrix is dense.

## Function/Subroutine Documentation

### subroutine slaed8 (integer ICOMPQ, integer K, integer N, integer QSIZ, real, dimension( * ) D, real, dimension( ldq, * ) Q, integer LDQ, integer, dimension( * ) INDXQ, real RHO, integer CUTPNT, real, dimension( * ) Z, real, dimension( * ) DLAMDA, real, dimension( ldq2, * ) Q2, integer LDQ2, real, dimension( * ) W, integer, dimension( * ) PERM, integer GIVPTR, integer, dimension( 2, * ) GIVCOL, real, dimension( 2, * ) GIVNUM, integer, dimension( * ) INDXP, integer, dimension( * ) INDX, integer INFO)

**SLAED8** used by sstedc. Merges eigenvalues and deflates secular equation. Used when the original matrix is dense.

**Purpose:**

SLAED8 merges the two sets of eigenvalues together into a single sorted set. Then it tries to deflate the size of the problem. There are two ways in which deflation can occur: when two or more eigenvalues are close together or if there is a tiny element in the Z vector. For each such occurrence the order of the related secular equation problem is reduced by one.

**Parameters:**-
*ICOMPQ*ICOMPQ is INTEGER = 0: Compute eigenvalues only. = 1: Compute eigenvectors of original dense symmetric matrix also. On entry, Q contains the orthogonal matrix used to reduce the original matrix to tridiagonal form.

*K*K is INTEGER The number of non-deflated eigenvalues, and the order of the related secular equation.

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

*QSIZ*QSIZ is INTEGER The dimension of the orthogonal matrix used to reduce the full matrix to tridiagonal form. QSIZ >= N if ICOMPQ = 1.

*D*D is REAL array, dimension (N) On entry, the eigenvalues of the two submatrices to be combined. On exit, the trailing (N-K) updated eigenvalues (those which were deflated) sorted into increasing order.

*Q*Q is REAL array, dimension (LDQ,N) If ICOMPQ = 0, Q is not referenced. Otherwise, on entry, Q contains the eigenvectors of the partially solved system which has been previously updated in matrix multiplies with other partially solved eigensystems. On exit, Q contains the trailing (N-K) updated eigenvectors (those which were deflated) in its last N-K columns.

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

*INDXQ*INDXQ is INTEGER array, dimension (N) The permutation which separately sorts the two sub-problems in D into ascending order. Note that elements in the second half of this permutation must first have CUTPNT added to their values in order to be accurate.

*RHO*RHO is REAL On entry, the off-diagonal element associated with the rank-1 cut which originally split the two submatrices which are now being recombined. On exit, RHO has been modified to the value required by SLAED3.

*CUTPNT*CUTPNT is INTEGER The location of the last eigenvalue in the leading sub-matrix. min(1,N) <= CUTPNT <= N.

*Z*Z is REAL array, dimension (N) On entry, Z contains the updating vector (the last row of the first sub-eigenvector matrix and the first row of the second sub-eigenvector matrix). On exit, the contents of Z are destroyed by the updating process.

*DLAMDA*DLAMDA is REAL array, dimension (N) A copy of the first K eigenvalues which will be used by SLAED3 to form the secular equation.

*Q2*Q2 is REAL array, dimension (LDQ2,N) If ICOMPQ = 0, Q2 is not referenced. Otherwise, a copy of the first K eigenvectors which will be used by SLAED7 in a matrix multiply (SGEMM) to update the new eigenvectors.

*LDQ2*LDQ2 is INTEGER The leading dimension of the array Q2. LDQ2 >= max(1,N).

*W*W is REAL array, dimension (N) The first k values of the final deflation-altered z-vector and will be passed to SLAED3.

*PERM*PERM is INTEGER array, dimension (N) The permutations (from deflation and sorting) to be applied to each eigenblock.

*GIVPTR*GIVPTR is INTEGER The number of Givens rotations which took place in this subproblem.

*GIVCOL*GIVCOL is INTEGER array, dimension (2, N) Each pair of numbers indicates a pair of columns to take place in a Givens rotation.

*GIVNUM*GIVNUM is REAL array, dimension (2, N) Each number indicates the S value to be used in the corresponding Givens rotation.

*INDXP*INDXP is INTEGER array, dimension (N) The permutation used to place deflated values of D at the end of the array. INDXP(1:K) points to the nondeflated D-values and INDXP(K+1:N) points to the deflated eigenvalues.

*INDX*INDX is INTEGER array, dimension (N) The permutation used to sort the contents of D into ascending order.

*INFO*INFO is INTEGER = 0: successful exit. < 0: if INFO = -i, the i-th argument had an illegal value.

**Author:**-
Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

**Date:**December 2016

**Contributors:**Jeff Rutter, Computer Science Division, University of California at Berkeley, USA

Definition at line 245 of file slaed8.f.

## Author

Generated automatically by Doxygen for LAPACK from the source code.

## Referenced By

The man page slaed8(3) is an alias of slaed8.f(3).