NAG Library Routine Document

F08KGF (DORMBR)

1  Purpose

2  Specification

3  Description

4  References

5  Parameters

1:     VECT – CHARACTER(1)Input

On entry: indicates whether $Q$ or $Q^{\mathrm{T}}$ or $P$ or $P^{\mathrm{T}}$ is to be applied to $C$.

$\mathbf{VECT}=\text{'Q'}$

$Q$ or $Q^{\mathrm{T}}$ is applied to $C$.

$\mathbf{VECT}=\text{'P'}$

$P$ or $P^{\mathrm{T}}$ is applied to $C$.

Constraint: $\mathbf{VECT}=\text{'Q'}$ or $\text{'P'}$.

2:     SIDE – CHARACTER(1)Input

On entry: indicates how $Q$ or $Q^{\mathrm{T}}$ or $P$ or $P^{\mathrm{T}}$ is to be applied to $C$.

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

$Q$ or $Q^{\mathrm{T}}$ or $P$ or $P^{\mathrm{T}}$ is applied to $C$ from the left.

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

$Q$ or $Q^{\mathrm{T}}$ or $P$ or $P^{\mathrm{T}}$ is applied to $C$ from the right.

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

3:     TRANS – CHARACTER(1)Input

On entry: indicates whether $Q$ or $P$ or $Q^{\mathrm{T}}$ or $P^{\mathrm{T}}$ is to be applied to $C$.

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

$Q$ or $P$ is applied to $C$.

$\mathbf{TRANS}=\text{'T'}$

$Q^{\mathrm{T}}$ or $P^{\mathrm{T}}$ is applied to $C$.

Constraint: $\mathbf{TRANS}=\text{'N'}$ or $\text{'T'}$.

4:     M – INTEGERInput

On entry: $m$, the number of rows of the matrix $C$.

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

5:     N – INTEGERInput

On entry: $n$, the number of columns of the matrix $C$.

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

6:     K – INTEGERInput

On entry: if $\mathbf{VECT}=\text{'Q'}$, the number of columns in the original matrix $A$.

If $\mathbf{VECT}=\text{'P'}$, the number of rows in the original matrix $A$.

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

7:     A(LDA,$*$) – REAL (KIND=nag_wp) arrayInput

Note: the second dimension of the array A must be at least $\max \left(1,\min \left(\mathit{r},\mathbf{K}\right)\right)$ if $\mathbf{VECT}=\text{'Q'}$ and at least $\max \left(1,\mathit{r}\right)$ if $\mathbf{VECT}=\text{'P'}$.

On entry: details of the vectors which define the elementary reflectors, as returned by F08KEF (DGEBRD).

8:     LDA – INTEGERInput

On entry: the first dimension of the array A as declared in the (sub)program from which F08KGF (DORMBR) is called.

Constraints:

• if $\mathbf{VECT}=\text{'Q'}$, $\mathbf{LDA}\ge \max \left(1,\mathit{r}\right)$;
• if $\mathbf{VECT}=\text{'P'}$, $\mathbf{LDA}\ge \max \left(1,\min \left(\mathit{r},\mathbf{K}\right)\right)$.

9:     TAU($*$) – REAL (KIND=nag_wp) arrayInput

Note: the dimension of the array TAU must be at least $\max \left(1,\min \left(\mathit{r},\mathbf{K}\right)\right)$.

On entry: further details of the elementary reflectors, as returned by F08KEF (DGEBRD) in its parameter TAUQ if $\mathbf{VECT}=\text{'Q'}$, or in its parameter TAUP if $\mathbf{VECT}=\text{'P'}$.

10:   C(LDC,$*$) – REAL (KIND=nag_wp) arrayInput/Output

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

On entry: the matrix $C$.

On exit: C is overwritten by $QC$ or $Q^{\mathrm{T}}C$ or $CQ$ or $C^{\mathrm{T}}Q$ or $PC$ or $P^{\mathrm{T}}C$ or $CP$ or $C^{\mathrm{T}}P$ as specified by VECT, SIDE and TRANS.

11:   LDC – INTEGERInput

On entry: the first dimension of the array C as declared in the (sub)program from which F08KGF (DORMBR) is called.

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

12:   WORK($\max \left(1,\mathbf{LWORK}\right)$) – REAL (KIND=nag_wp) arrayWorkspace

On exit: if $\mathbf{INFO}=\mathbf{0}$, $\mathbf{WORK}\left(1\right)$ contains the minimum value of LWORK required for optimal performance.

13:   LWORK – INTEGERInput

On entry: the dimension of the array WORK as declared in the (sub)program from which F08KGF (DORMBR) is called.

If $\mathbf{LWORK}=-1$, a workspace query is assumed; the routine only calculates the optimal size of the WORK array, returns this value as the first entry of the WORK array, and no error message related to LWORK is issued.

Suggested value: for optimal performance, $\mathbf{LWORK}\ge \mathbf{N}\times \mathit{nb}$ if $\mathbf{SIDE}=\text{'L'}$ and at least $\mathbf{M}\times \mathit{nb}$ if $\mathbf{SIDE}=\text{'R'}$, where $\mathit{nb}$ is the optimal block size.

Constraints:

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

14:   INFO – INTEGEROutput

On exit: $\mathbf{INFO}=0$ unless the routine detects an error (see Section 6).

6  Error Indicators and Warnings

$\mathbf{INFO}<0$

If $\mathbf{INFO}=-i$, argument $i$ had an illegal value. An explanatory message is output, and execution of the program is terminated.

7  Accuracy

8  Further Comments

9  Example

9.1  Program Text

Program Text (f08kgfe.f90)

9.2  Program Data

Program Data (f08kgfe.d)

9.3  Program Results

Program Results (f08kgfe.r)