NAG Library Routine Document

F08NGF (DORMHR)

1  Purpose

2  Specification

3  Description

4  References

5  Parameters

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

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

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

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

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

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

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

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

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

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

$Q$ is applied to $C$.

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

$Q^{\mathrm{T}}$ is applied to $C$.

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

3:     $\mathrm{M}$ – INTEGERInput

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

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

4:     $\mathrm{N}$ – INTEGERInput

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

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

5:     $\mathrm{ILO}$ – INTEGERInput

6:     $\mathrm{IHI}$ – INTEGERInput

On entry: these must be the same parameters ILO and IHI, respectively, as supplied to F08NEF (DGEHRD).

Constraints:

• if $\mathbf{SIDE}=\text{'L'}$ and $\mathbf{M}>0$, $1\le \mathbf{ILO}\le \mathbf{IHI}\le \mathbf{M}$;
• if $\mathbf{SIDE}=\text{'L'}$ and $\mathbf{M}=0$, $\mathbf{ILO}=1$ and $\mathbf{IHI}=0$;
• if $\mathbf{SIDE}=\text{'R'}$ and $\mathbf{N}>0$, $1\le \mathbf{ILO}\le \mathbf{IHI}\le \mathbf{N}$;
• if $\mathbf{SIDE}=\text{'R'}$ and $\mathbf{N}=0$, $\mathbf{ILO}=1$ and $\mathbf{IHI}=0$.

7:     $\mathrm{A}\left(\mathbf{LDA},*\right)$ – 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: details of the vectors which define the elementary reflectors, as returned by F08NEF (DGEHRD).

8:     $\mathrm{LDA}$ – INTEGERInput

On entry: the first dimension of the array A as declared in the (sub)program from which F08NGF (DORMHR) is called.

Constraints:

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

9:     $\mathrm{TAU}\left(*\right)$ – REAL (KIND=nag_wp) arrayInput

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

On entry: further details of the elementary reflectors, as returned by F08NEF (DGEHRD).

10:   $\mathrm{C}\left(\mathbf{LDC},*\right)$ – 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 $m$ by $n$ matrix $C$.

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

11:   $\mathrm{LDC}$ – INTEGERInput

On entry: the first dimension of the array C as declared in the (sub)program from which F08NGF (DORMHR) is called.

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

12:   $\mathrm{WORK}\left(\max \left(1,\mathbf{LWORK}\right)\right)$ – 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:   $\mathrm{LWORK}$ – INTEGERInput

On entry: the dimension of the array WORK as declared in the (sub)program from which F08NGF (DORMHR) 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:   $\mathrm{INFO}$ – 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  Parallelism and Performance

9  Further Comments

10  Example

10.1  Program Text

Program Text (f08ngfe.f90)

10.2  Program Data

Program Data (f08ngfe.d)

10.3  Program Results

Program Results (f08ngfe.r)