NAG Library Routine Document

F06YJF (DTRSM)

1  Purpose

2  Specification

3  Description

4  References

5  Parameters

1:     $\mathrm{SIDE}$ – CHARACTER(1)Input

On entry: specifies whether $B$ is operated on from the left or the right.

$\mathbf{SIDE}=\text{'L'}$

$B$ is pre-multiplied from the left.

$\mathbf{SIDE}=\text{'R'}$

$B$ is post-multiplied from the right.

Constraint: $\mathbf{SIDE}=\text{'L'}$ or $\text{'R'}$.

2:     $\mathrm{UPLO}$ – CHARACTER(1)Input

On entry: specifies whether $A$ is upper or lower triangular.

$\mathbf{UPLO}=\text{'U'}$

$A$ is upper triangular.

$\mathbf{UPLO}=\text{'L'}$

$A$ is lower triangular.

Constraint: $\mathbf{UPLO}=\text{'U'}$ or $\text{'L'}$.

3:     $\mathrm{TRANSA}$ – CHARACTER(1)Input

On entry: specifies whether the operation involves $A^{-1}$ or $A^{-\mathrm{T}}$.

$\mathbf{TRANSA}=\text{'N'}$

The operation involves $A^{-1}$.

$\mathbf{TRANSA}=\text{'T'}$ or $\text{'C'}$

The operation involves $A^{-\mathrm{T}}$.

Constraint: $\mathbf{TRANSA}=\text{'N'}$, $\text{'T'}$ or $\text{'C'}$.

4:     $\mathrm{DIAG}$ – CHARACTER(1)Input

On entry: specifies whether $A$ has nonunit or unit diagonal elements.

$\mathbf{DIAG}=\text{'N'}$

The diagonal elements are stored explicitly.

$\mathbf{DIAG}=\text{'U'}$

The diagonal elements are assumed to be $1$, and are not referenced.

Constraint: $\mathbf{DIAG}=\text{'N'}$ or $\text{'U'}$.

5:     $\mathrm{M}$ – INTEGERInput

On entry: $m$, the number of rows of the matrix $B$; the order of $A$ if $\mathbf{SIDE}=\text{'L'}$.

Constraint: $\mathbf{M}\ge 0$.

6:     $\mathrm{N}$ – INTEGERInput

On entry: $n$, the number of columns of the matrix $B$; the order of $A$ if $\mathbf{SIDE}=\text{'R'}$.

Constraint: $\mathbf{N}\ge 0$.

7:     $\mathrm{ALPHA}$ – REAL (KIND=nag_wp)Input

On entry: the scalar $\alpha$.

8:     $\mathrm{A}\left(\mathbf{LDA},*\right)$ – REAL (KIND=nag_wp) arrayInput

Note: the second dimension of the array A must be at least $\max \left(1,\mathbf{M}\right)$ if $\mathbf{SIDE}=\text{'L'}$ and at least $\max \left(1,\mathbf{N}\right)$ if $\mathbf{SIDE}=\text{'R'}$.

On entry: the triangular matrix $A$; $A$ is $m$ by $m$ if $\mathbf{SIDE}=\text{'L'}$, or $n$ by $n$ if $\mathbf{SIDE}=\text{'R'}$.

• If $\mathbf{UPLO}=\text{'U'}$, $A$ is upper triangular and the elements of the array below the diagonal are not referenced.
• If $\mathbf{UPLO}=\text{'L'}$, $A$ is lower triangular and the elements of the array above the diagonal are not referenced.
• If $\mathbf{DIAG}=\text{'U'}$, the diagonal elements of $A$ are assumed to be $1$, and are not referenced.

9:     $\mathrm{LDA}$ – INTEGERInput

On entry: the first dimension of the array A as declared in the (sub)program from which F06YJF (DTRSM) is called.

Constraints:

• if $\mathbf{SIDE}=\text{'L'}$, $\mathbf{LDA}\ge \max \left(1,\mathbf{M}\right)$;
• if $\mathbf{SIDE}=\text{'R'}$, $\mathbf{LDA}\ge \max \left(1,\mathbf{N}\right)$.

10:   $\mathrm{B}\left(\mathbf{LDB},*\right)$ – REAL (KIND=nag_wp) arrayInput/Output

Note: the second dimension of the array B must be at least $\max \left(1,\mathbf{N}\right)$.

On entry: the $m$ by $n$ matrix $B$.

If $\mathbf{ALPHA}=0$, B need not be set.

On exit: the updated matrix $B$.

11:   $\mathrm{LDB}$ – INTEGERInput

On entry: the first dimension of the array B as declared in the (sub)program from which F06YJF (DTRSM) is called.

Constraint: $\mathbf{LDB}\ge \max \left(1,\mathbf{M}\right)$.

6  Error Indicators and Warnings

7  Accuracy

8  Parallelism and Performance

9  Further Comments

10  Example