# dlasy2.f man page

dlasy2.f —

## Synopsis

### Functions/Subroutines

subroutinedlasy2(LTRANL, LTRANR, ISGN, N1, N2, TL, LDTL, TR, LDTR, B, LDB, SCALE, X, LDX, XNORM, INFO)DLASY2solves the Sylvester matrix equation where the matrices are of order 1 or 2.

## Function/Subroutine Documentation

### subroutine dlasy2 (logicalLTRANL, logicalLTRANR, integerISGN, integerN1, integerN2, double precision, dimension( ldtl, * )TL, integerLDTL, double precision, dimension( ldtr, * )TR, integerLDTR, double precision, dimension( ldb, * )B, integerLDB, double precisionSCALE, double precision, dimension( ldx, * )X, integerLDX, double precisionXNORM, integerINFO)

**DLASY2** solves the Sylvester matrix equation where the matrices are of order 1 or 2.

**Purpose:**

```
DLASY2 solves for the N1 by N2 matrix X, 1 <= N1,N2 <= 2, in
op(TL)*X + ISGN*X*op(TR) = SCALE*B,
where TL is N1 by N1, TR is N2 by N2, B is N1 by N2, and ISGN = 1 or
-1. op(T) = T or T**T, where T**T denotes the transpose of T.
```

**Parameters:**

*LTRANL*

```
LTRANL is LOGICAL
On entry, LTRANL specifies the op(TL):
= .FALSE., op(TL) = TL,
= .TRUE., op(TL) = TL**T.
```

*LTRANR*

```
LTRANR is LOGICAL
On entry, LTRANR specifies the op(TR):
= .FALSE., op(TR) = TR,
= .TRUE., op(TR) = TR**T.
```

*ISGN*

```
ISGN is INTEGER
On entry, ISGN specifies the sign of the equation
as described before. ISGN may only be 1 or -1.
```

*N1*

```
N1 is INTEGER
On entry, N1 specifies the order of matrix TL.
N1 may only be 0, 1 or 2.
```

*N2*

```
N2 is INTEGER
On entry, N2 specifies the order of matrix TR.
N2 may only be 0, 1 or 2.
```

*TL*

```
TL is DOUBLE PRECISION array, dimension (LDTL,2)
On entry, TL contains an N1 by N1 matrix.
```

*LDTL*

```
LDTL is INTEGER
The leading dimension of the matrix TL. LDTL >= max(1,N1).
```

*TR*

```
TR is DOUBLE PRECISION array, dimension (LDTR,2)
On entry, TR contains an N2 by N2 matrix.
```

*LDTR*

```
LDTR is INTEGER
The leading dimension of the matrix TR. LDTR >= max(1,N2).
```

*B*

```
B is DOUBLE PRECISION array, dimension (LDB,2)
On entry, the N1 by N2 matrix B contains the right-hand
side of the equation.
```

*LDB*

```
LDB is INTEGER
The leading dimension of the matrix B. LDB >= max(1,N1).
```

*SCALE*

```
SCALE is DOUBLE PRECISION
On exit, SCALE contains the scale factor. SCALE is chosen
less than or equal to 1 to prevent the solution overflowing.
```

*X*

```
X is DOUBLE PRECISION array, dimension (LDX,2)
On exit, X contains the N1 by N2 solution.
```

*LDX*

```
LDX is INTEGER
The leading dimension of the matrix X. LDX >= max(1,N1).
```

*XNORM*

```
XNORM is DOUBLE PRECISION
On exit, XNORM is the infinity-norm of the solution.
```

*INFO*

```
INFO is INTEGER
On exit, INFO is set to
0: successful exit.
1: TL and TR have too close eigenvalues, so TL or
TR is perturbed to get a nonsingular equation.
NOTE: In the interests of speed, this routine does not
check the inputs for errors.
```

**Author:**

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

**Date:**

September 2012

Definition at line 174 of file dlasy2.f.

## Author

Generated automatically by Doxygen for LAPACK from the source code.

## Referenced By

dlasy2(3) is an alias of dlasy2.f(3).

Sat Nov 16 2013 Version 3.4.2 LAPACK