
SLAED8(l) ) SLAED8(l)
NAME
SLAED8  merge the two sets of eigenvalues together into a single sorted set
SYNOPSIS
SUBROUTINE SLAED8( ICOMPQ, K, N, QSIZ, D, Q, LDQ, INDXQ, RHO, CUTPNT, Z, DLAMDA, Q2, LDQ2,
W, PERM, GIVPTR, GIVCOL, GIVNUM, INDXP, INDX, INFO )
INTEGER CUTPNT, GIVPTR, ICOMPQ, INFO, K, LDQ, LDQ2, N, QSIZ
REAL RHO
INTEGER GIVCOL( 2, * ), INDX( * ), INDXP( * ), INDXQ( * ), PERM( * )
REAL D( * ), DLAMDA( * ), GIVNUM( 2, * ), Q( LDQ, * ), Q2( LDQ2, * ), W( *
), Z( * )
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.
ARGUMENTS
ICOMPQ (input) 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 (output) INTEGER
The number of nondeflated eigenvalues, and the order of the related secular equa
tion.
N (input) INTEGER
The dimension of the symmetric tridiagonal matrix. N >= 0.
QSIZ (input) INTEGER
The dimension of the orthogonal matrix used to reduce the full matrix to tridiago
nal form. QSIZ >= N if ICOMPQ = 1.
D (input/output) REAL array, dimension (N)
On entry, the eigenvalues of the two submatrices to be combined. On exit, the
trailing (NK) updated eigenvalues (those which were deflated) sorted into increas
ing order.
Q (input/output) REAL array, dimension (LDQ,N)
If ICOMPQ = 0, Q is not referenced. Otherwise, on entry, Q contains the eigenvec
tors of the partially solved system which has been previously updated in matrix
multiplies with other partially solved eigensystems. On exit, Q contains the
trailing (NK) updated eigenvectors (those which were deflated) in its last NK
columns.
LDQ (input) INTEGER
The leading dimension of the array Q. LDQ >= max(1,N).
INDXQ (input) INTEGER array, dimension (N)
The permutation which separately sorts the two subproblems 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 (input/output) REAL
On entry, the offdiagonal element associated with the rank1 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 (input) INTEGER The location of the last eigenvalue in the leading sub
matrix. min(1,N) <= CUTPNT <= N.
Z (input) REAL array, dimension (N)
On entry, Z contains the updating vector (the last row of the first subeigenvector
matrix and the first row of the second subeigenvector matrix). On exit, the con
tents of Z are destroyed by the updating process.
DLAMDA (output) REAL array, dimension (N) A copy of the first K eigenvalues which
will be used by SLAED3 to form the secular equation.
Q2 (output) 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 eigen
vectors.
LDQ2 (input) INTEGER
The leading dimension of the array Q2. LDQ2 >= max(1,N).
W (output) REAL array, dimension (N)
The first k values of the final deflationaltered zvector and will be passed to
SLAED3.
PERM (output) INTEGER array, dimension (N)
The permutations (from deflation and sorting) to be applied to each eigenblock.
GIVPTR (output) INTEGER The number of Givens rotations which took place in this
subproblem.
GIVCOL (output) INTEGER array, dimension (2, N) Each pair of numbers indicates a
pair of columns to take place in a Givens rotation.
GIVNUM (output) REAL array, dimension (2, N) Each number indicates the S value to
be used in the corresponding Givens rotation.
INDXP (workspace) 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 Dvalues
and INDXP(K+1:N) points to the deflated eigenvalues.
INDX (workspace) INTEGER array, dimension (N)
The permutation used to sort the contents of D into ascending order.
INFO (output) INTEGER
= 0: successful exit.
< 0: if INFO = i, the ith argument had an illegal value.
FURTHER DETAILS
Based on contributions by
Jeff Rutter, Computer Science Division, University of California
at Berkeley, USA
LAPACK version 3.0 15 June 2000 SLAED8(l) 
