# SLAED3 (3) - Linux Manuals

slaed3.f -

## SYNOPSIS

### Functions/Subroutines

subroutine slaed3 (K, N, N1, D, Q, LDQ, RHO, DLAMDA, Q2, INDX, CTOT, W, S, INFO)
SLAED3 used by sstedc. Finds the roots of the secular equation and updates the eigenvectors. Used when the original matrix is tridiagonal.

## Function/Subroutine Documentation

### subroutine slaed3 (integerK, integerN, integerN1, real, dimension( * )D, real, dimension( ldq, * )Q, integerLDQ, realRHO, real, dimension( * )DLAMDA, real, dimension( * )Q2, integer, dimension( * )INDX, integer, dimension( * )CTOT, real, dimension( * )W, real, dimension( * )S, integerINFO)

SLAED3 used by sstedc. Finds the roots of the secular equation and updates the eigenvectors. Used when the original matrix is tridiagonal.

Purpose:

``` SLAED3 finds the roots of the secular equation, as defined by the
values in D, W, and RHO, between 1 and K.  It makes the
appropriate calls to SLAED4 and then updates the eigenvectors by
multiplying the matrix of eigenvectors of the pair of eigensystems
being combined by the matrix of eigenvectors of the K-by-K system
which is solved here.

This code makes very mild assumptions about floating point
arithmetic. It will work on machines with a guard digit in
add/subtract, or on those binary machines without guard digits
which subtract like the Cray X-MP, Cray Y-MP, Cray C-90, or Cray-2.
It could conceivably fail on hexadecimal or decimal machines
without guard digits, but we know of none.
```

Parameters:

K

```          K is INTEGER
The number of terms in the rational function to be solved by
SLAED4.  K >= 0.
```

N

```          N is INTEGER
The number of rows and columns in the Q matrix.
N >= K (deflation may result in N>K).
```

N1

```          N1 is INTEGER
The location of the last eigenvalue in the leading submatrix.
min(1,N) <= N1 <= N/2.
```

D

```          D is REAL array, dimension (N)
D(I) contains the updated eigenvalues for
1 <= I <= K.
```

Q

```          Q is REAL array, dimension (LDQ,N)
Initially the first K columns are used as workspace.
On output the columns 1 to K contain
the updated eigenvectors.
```

LDQ

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

RHO

```          RHO is REAL
The value of the parameter in the rank one update equation.
RHO >= 0 required.
```

DLAMDA

```          DLAMDA is REAL array, dimension (K)
The first K elements of this array contain the old roots
of the deflated updating problem.  These are the poles
of the secular equation. May be changed on output by
having lowest order bit set to zero on Cray X-MP, Cray Y-MP,
Cray-2, or Cray C-90, as described above.
```

Q2

```          Q2 is REAL array, dimension (LDQ2, N)
The first K columns of this matrix contain the non-deflated
eigenvectors for the split problem.
```

INDX

```          INDX is INTEGER array, dimension (N)
The permutation used to arrange the columns of the deflated
Q matrix into three groups (see SLAED2).
The rows of the eigenvectors found by SLAED4 must be likewise
permuted before the matrix multiply can take place.
```

CTOT

```          CTOT is INTEGER array, dimension (4)
A count of the total number of the various types of columns
in Q, as described in INDX.  The fourth column type is any
column which has been deflated.
```

W

```          W is REAL array, dimension (K)
The first K elements of this array contain the components
of the deflation-adjusted updating vector. Destroyed on
output.
```

S

```          S is REAL array, dimension (N1 + 1)*K
Will contain the eigenvectors of the repaired matrix which
will be multiplied by the previously accumulated eigenvectors
to update the system.
```

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
```

Author:

Univ. of Tennessee

Univ. of California Berkeley

NAG Ltd.

Date:

September 2012

Contributors:

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

Modified by Francoise Tisseur, University of Tennessee

Definition at line 185 of file slaed3.f.

## Author

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