# dstein.f

Section: LAPACK (3)
Updated: Tue Nov 14 2017
Page Index

dstein.f

## SYNOPSIS

### Functions/Subroutines

subroutine dstein (N, D, E, M, W, IBLOCK, ISPLIT, Z, LDZ, WORK, IWORK, IFAIL, INFO)
DSTEIN

## Function/Subroutine Documentation

### subroutine dstein (integer N, double precision, dimension( * ) D, double precision, dimension( * ) E, integer M, double precision, dimension( * ) W, integer, dimension( * ) IBLOCK, integer, dimension( * ) ISPLIT, double precision, dimension( ldz, * ) Z, integer LDZ, double precision, dimension( * ) WORK, integer, dimension( * ) IWORK, integer, dimension( * ) IFAIL, integer INFO)

DSTEIN

Purpose:

``` DSTEIN computes the eigenvectors of a real symmetric tridiagonal
matrix T corresponding to specified eigenvalues, using inverse
iteration.

The maximum number of iterations allowed for each eigenvector is
specified by an internal parameter MAXITS (currently set to 5).
```

Parameters:

N

```          N is INTEGER
The order of the matrix.  N >= 0.
```

D

```          D is DOUBLE PRECISION array, dimension (N)
The n diagonal elements of the tridiagonal matrix T.
```

E

```          E is DOUBLE PRECISION array, dimension (N-1)
The (n-1) subdiagonal elements of the tridiagonal matrix
T, in elements 1 to N-1.
```

M

```          M is INTEGER
The number of eigenvectors to be found.  0 <= M <= N.
```

W

```          W is DOUBLE PRECISION array, dimension (N)
The first M elements of W contain the eigenvalues for
which eigenvectors are to be computed.  The eigenvalues
should be grouped by split-off block and ordered from
smallest to largest within the block.  ( The output array
W from DSTEBZ with ORDER = 'B' is expected here. )
```

IBLOCK

```          IBLOCK is INTEGER array, dimension (N)
The submatrix indices associated with the corresponding
eigenvalues in W; IBLOCK(i)=1 if eigenvalue W(i) belongs to
the first submatrix from the top, =2 if W(i) belongs to
the second submatrix, etc.  ( The output array IBLOCK
from DSTEBZ is expected here. )
```

ISPLIT

```          ISPLIT is INTEGER array, dimension (N)
The splitting points, at which T breaks up into submatrices.
The first submatrix consists of rows/columns 1 to
ISPLIT( 1 ), the second of rows/columns ISPLIT( 1 )+1
through ISPLIT( 2 ), etc.
( The output array ISPLIT from DSTEBZ is expected here. )
```

Z

```          Z is DOUBLE PRECISION array, dimension (LDZ, M)
The computed eigenvectors.  The eigenvector associated
with the eigenvalue W(i) is stored in the i-th column of
Z.  Any vector which fails to converge is set to its current
iterate after MAXITS iterations.
```

LDZ

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

WORK

```          WORK is DOUBLE PRECISION array, dimension (5*N)
```

IWORK

```          IWORK is INTEGER array, dimension (N)
```

IFAIL

```          IFAIL is INTEGER array, dimension (M)
On normal exit, all elements of IFAIL are zero.
If one or more eigenvectors fail to converge after
MAXITS iterations, then their indices are stored in
array IFAIL.
```

INFO

```          INFO is INTEGER
= 0: successful exit.
< 0: if INFO = -i, the i-th argument had an illegal value
> 0: if INFO = i, then i eigenvectors failed to converge
in MAXITS iterations.  Their indices are stored in
array IFAIL.
```

Internal Parameters:

```  MAXITS  INTEGER, default = 5
The maximum number of iterations performed.

EXTRA   INTEGER, default = 2
The number of iterations performed after norm growth
criterion is satisfied, should be at least 1.
```

Author:

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

Date:

December 2016

Definition at line 176 of file dstein.f.

## Author

Generated automatically by Doxygen for LAPACK from the source code.