---
name: "ssteqr.f"
section: "3"
platform: "linux"
package: "lapack-doc"
version: "3.12.1-2"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/ssteqr.f"
---

# ssteqr.f(3)

## NAME

SRC/ssteqr.f

## SYNOPSIS

### Functions/Subroutines

subroutine **ssteqr** (compz, n, d, e, z, ldz, work, info)\
 **SSTEQR**

## Function/Subroutine Documentation

### subroutine ssteqr (character compz, integer n, real, dimension( \* ) d, real, dimension( \* ) e, real, dimension( ldz, \* ) z, integer ldz, real, dimension( \* ) work, integer info)

**SSTEQR**

**Purpose:**

```
!>
!> SSTEQR computes all eigenvalues and, optionally, eigenvectors of a
!> symmetric tridiagonal matrix using the implicit QL or QR method.
!> The eigenvectors of a full or band symmetric matrix can also be found
!> if SSYTRD or SSPTRD or SSBTRD has been used to reduce this matrix to
!> tridiagonal form.
!>
```

**Parameters**

*COMPZ*

```
!>          COMPZ is CHARACTER*1
!>          = 'N':  Compute eigenvalues only.
!>          = 'V':  Compute eigenvalues and eigenvectors of the original
!>                  symmetric matrix.  On entry, Z must contain the
!>                  orthogonal matrix used to reduce the original matrix
!>                  to tridiagonal form.
!>          = 'I':  Compute eigenvalues and eigenvectors of the
!>                  tridiagonal matrix.  Z is initialized to the identity
!>                  matrix.
!>
```

*N*

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

*D*

```
!>          D is REAL array, dimension (N)
!>          On entry, the diagonal elements of the tridiagonal matrix.
!>          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.
!>          On exit, E has been destroyed.
!>
```

*Z*

```
!>          Z is REAL array, dimension (LDZ, N)
!>          On entry, if  COMPZ = 'V', then Z contains the orthogonal
!>          matrix used in the reduction to tridiagonal form.
!>          On exit, if INFO = 0, then if  COMPZ = 'V', Z contains the
!>          orthonormal eigenvectors of the original symmetric matrix,
!>          and if COMPZ = 'I', Z contains the orthonormal eigenvectors
!>          of the symmetric tridiagonal matrix.
!>          If COMPZ = 'N', then Z is not referenced.
!>
```

*LDZ*

```
!>          LDZ is INTEGER
!>          The leading dimension of the array Z.  LDZ >= 1, and if
!>          eigenvectors are desired, then  LDZ >= max(1,N).
!>
```

*WORK*

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

*INFO*

```
!>          INFO is INTEGER
!>          = 0:  successful exit
!>          < 0:  if INFO = -i, the i-th argument had an illegal value
!>          > 0:  the algorithm has failed to find all the eigenvalues in
!>                a total of 30*N iterations; if INFO = i, then i
!>                elements of E have not converged to zero; on exit, D
!>                and E contain the elements of a symmetric tridiagonal
!>                matrix which is orthogonally similar to the original
!>                matrix.
!>
```

**Author**

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

Definition at line **130** of file **ssteqr.f**.

## Author

Generated automatically by Doxygen for LAPACK from the source code.