---
name: "sstech.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/sstech.f"
---

# sstech.f(3)

## NAME

TESTING/EIG/sstech.f

## SYNOPSIS

### Functions/Subroutines

subroutine **sstech** (n, a, b, eig, tol, work, info)\
 **SSTECH**

## Function/Subroutine Documentation

### subroutine sstech (integer n, real, dimension( \* ) a, real, dimension( \* ) b, real, dimension( \* ) eig, real tol, real, dimension( \* ) work, integer info)

**SSTECH**

**Purpose:**

```
!>
!>    Let T be the tridiagonal matrix with diagonal entries A(1) ,...,
!>    A(N) and offdiagonal entries B(1) ,..., B(N-1)).  SSTECH checks to
!>    see if EIG(1) ,..., EIG(N) are indeed accurate eigenvalues of T.
!>    It does this by expanding each EIG(I) into an interval
!>    [SVD(I) - EPS, SVD(I) + EPS], merging overlapping intervals if
!>    any, and using Sturm sequences to count and verify whether each
!>    resulting interval has the correct number of eigenvalues (using
!>    SSTECT).  Here EPS = TOL*MACHEPS*MAXEIG, where MACHEPS is the
!>    machine precision and MAXEIG is the absolute value of the largest
!>    eigenvalue. If each interval contains the correct number of
!>    eigenvalues, INFO = 0 is returned, otherwise INFO is the index of
!>    the first eigenvalue in the first bad interval.
!>
```

**Parameters**

*N*

```
!>          N is INTEGER
!>          The dimension of the tridiagonal matrix T.
!>
```

*A*

```
!>          A is REAL array, dimension (N)
!>          The diagonal entries of the tridiagonal matrix T.
!>
```

*B*

```
!>          B is REAL array, dimension (N-1)
!>          The offdiagonal entries of the tridiagonal matrix T.
!>
```

*EIG*

```
!>          EIG is REAL array, dimension (N)
!>          The purported eigenvalues to be checked.
!>
```

*TOL*

```
!>          TOL is REAL
!>          Error tolerance for checking, a multiple of the
!>          machine precision.
!>
```

*WORK*

```
!>          WORK is REAL array, dimension (N)
!>
```

*INFO*

```
!>          INFO is INTEGER
!>          0  if the eigenvalues are all correct (to within
!>             1 +- TOL*MACHEPS*MAXEIG)
!>          >0 if the interval containing the INFO-th eigenvalue
!>             contains the incorrect number of eigenvalues.
!>
```

**Author**

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

Definition at line **100** of file **sstech.f**.

## Author

Generated automatically by Doxygen for LAPACK from the source code.