# stev - Man Page

stev: eig, QR iteration

## Synopsis

### Functions

subroutine dstev (jobz, n, d, e, z, ldz, work, info)
DSTEV computes the eigenvalues and, optionally, the left and/or right eigenvectors for OTHER matrices
subroutine sstev (jobz, n, d, e, z, ldz, work, info)
SSTEV computes the eigenvalues and, optionally, the left and/or right eigenvectors for OTHER matrices

## Function Documentation

### subroutine dstev (character jobz, integer n, double precision, dimension( * ) d, double precision, dimension( * ) e, double precision, dimension( ldz, * ) z, integer ldz, double precision, dimension( * ) work, integer info)

DSTEV computes the eigenvalues and, optionally, the left and/or right eigenvectors for OTHER matrices

Purpose:

``` DSTEV computes all eigenvalues and, optionally, eigenvectors of a
real symmetric tridiagonal matrix A.```
Parameters

JOBZ

```          JOBZ is CHARACTER*1
= 'N':  Compute eigenvalues only;
= 'V':  Compute eigenvalues and eigenvectors.```

N

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

D

```          D is DOUBLE PRECISION array, dimension (N)
On entry, the n diagonal elements of the tridiagonal matrix
A.
On exit, if INFO = 0, the eigenvalues in ascending order.```

E

```          E is DOUBLE PRECISION array, dimension (N-1)
On entry, the (n-1) subdiagonal elements of the tridiagonal
matrix A, stored in elements 1 to N-1 of E.
On exit, the contents of E are destroyed.```

Z

```          Z is DOUBLE PRECISION array, dimension (LDZ, N)
If JOBZ = 'V', then if INFO = 0, Z contains the orthonormal
eigenvectors of the matrix A, with the i-th column of Z
holding the eigenvector associated with D(i).
If JOBZ = 'N', then Z is not referenced.```

LDZ

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

WORK

```          WORK is DOUBLE PRECISION array, dimension (max(1,2*N-2))
If JOBZ = 'N', WORK is not referenced.```

INFO

```          INFO is INTEGER
= 0:  successful exit
< 0:  if INFO = -i, the i-th argument had an illegal value
> 0:  if INFO = i, the algorithm failed to converge; i
off-diagonal elements of E did not converge to zero.```
Author

Univ. of Tennessee

Univ. of California Berkeley

NAG Ltd.

Definition at line 115 of file dstev.f.

### subroutine sstev (character jobz, integer n, real, dimension( * ) d, real, dimension( * ) e, real, dimension( ldz, * ) z, integer ldz, real, dimension( * ) work, integer info)

SSTEV computes the eigenvalues and, optionally, the left and/or right eigenvectors for OTHER matrices

Purpose:

``` SSTEV computes all eigenvalues and, optionally, eigenvectors of a
real symmetric tridiagonal matrix A.```
Parameters

JOBZ

```          JOBZ is CHARACTER*1
= 'N':  Compute eigenvalues only;
= 'V':  Compute eigenvalues and eigenvectors.```

N

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

D

```          D is REAL array, dimension (N)
On entry, the n diagonal elements of the tridiagonal matrix
A.
On exit, if INFO = 0, the eigenvalues in ascending order.```

E

```          E is REAL array, dimension (N-1)
On entry, the (n-1) subdiagonal elements of the tridiagonal
matrix A, stored in elements 1 to N-1 of E.
On exit, the contents of E are destroyed.```

Z

```          Z is REAL array, dimension (LDZ, N)
If JOBZ = 'V', then if INFO = 0, Z contains the orthonormal
eigenvectors of the matrix A, with the i-th column of Z
holding the eigenvector associated with D(i).
If JOBZ = 'N', then Z is not referenced.```

LDZ

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

WORK

```          WORK is REAL array, dimension (max(1,2*N-2))
If JOBZ = 'N', WORK is not referenced.```

INFO

```          INFO is INTEGER
= 0:  successful exit
< 0:  if INFO = -i, the i-th argument had an illegal value
> 0:  if INFO = i, the algorithm failed to converge; i
off-diagonal elements of E did not converge to zero.```
Author

Univ. of Tennessee

Univ. of California Berkeley