# dlarra.f man page

dlarra.f —

## Synopsis

### Functions/Subroutines

subroutinedlarra(N, D, E, E2, SPLTOL, TNRM, NSPLIT, ISPLIT, INFO)DLARRAcomputes the splitting points with the specified threshold.

## Function/Subroutine Documentation

### subroutine dlarra (integerN, double precision, dimension( * )D, double precision, dimension( * )E, double precision, dimension( * )E2, double precisionSPLTOL, double precisionTNRM, integerNSPLIT, integer, dimension( * )ISPLIT, integerINFO)

**DLARRA** computes the splitting points with the specified threshold.

**Purpose:**

```
Compute the splitting points with threshold SPLTOL.
DLARRA sets any "small" off-diagonal elements to zero.
```

**Parameters:**

*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 T.
```

*E*

```
E is DOUBLE PRECISION array, dimension (N)
On entry, the first (N-1) entries contain the subdiagonal
elements of the tridiagonal matrix T; E(N) need not be set.
On exit, the entries E( ISPLIT( I ) ), 1 <= I <= NSPLIT,
are set to zero, the other entries of E are untouched.
```

*E2*

```
E2 is DOUBLE PRECISION array, dimension (N)
On entry, the first (N-1) entries contain the SQUARES of the
subdiagonal elements of the tridiagonal matrix T;
E2(N) need not be set.
On exit, the entries E2( ISPLIT( I ) ),
1 <= I <= NSPLIT, have been set to zero
```

*SPLTOL*

```
SPLTOL is DOUBLE PRECISION
The threshold for splitting. Two criteria can be used:
SPLTOL<0 : criterion based on absolute off-diagonal value
SPLTOL>0 : criterion that preserves relative accuracy
```

*TNRM*

```
TNRM is DOUBLE PRECISION
The norm of the matrix.
```

*NSPLIT*

```
NSPLIT is INTEGER
The number of blocks T splits into. 1 <= NSPLIT <= N.
```

*ISPLIT*

```
ISPLIT is INTEGER array, dimension (N)
The splitting points, at which T breaks up into blocks.
The first block consists of rows/columns 1 to ISPLIT(1),
the second of rows/columns ISPLIT(1)+1 through ISPLIT(2),
etc., and the NSPLIT-th consists of rows/columns
ISPLIT(NSPLIT-1)+1 through ISPLIT(NSPLIT)=N.
```

*INFO*

```
INFO is INTEGER
= 0: successful exit
```

**Author:**

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

**Date:**

September 2012

**Contributors:**

Beresford Parlett, University of California, Berkeley, USA

Jim Demmel, University of California, Berkeley, USA

Inderjit Dhillon, University of Texas, Austin, USA

Osni Marques, LBNL/NERSC, USA

Christof Voemel, University of California, Berkeley, USA

Definition at line 136 of file dlarra.f.

## Author

Generated automatically by Doxygen for LAPACK from the source code.

## Referenced By

dlarra(3) is an alias of dlarra.f(3).