NAG FL Interface
f08pnf (zgees)

1 Purpose

2 Specification

3 Description

4 References

5 Arguments

1: $\mathbf{jobvs}$ – Character(1) Input

On entry: if $\mathbf{jobvs}=\text{'N'}$, Schur vectors are not computed.

If $\mathbf{jobvs}=\text{'V'}$, Schur vectors are computed.

Constraint: $\mathbf{jobvs}=\text{'N'}$ or $\text{'V'}$.

2: $\mathbf{sort}$ – Character(1) Input

On entry: specifies whether or not to order the eigenvalues on the diagonal of the Schur form.

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

Eigenvalues are not ordered.

$\mathbf{sort}=\text{'S'}$

Eigenvalues are ordered (see select).

Constraint: $\mathbf{sort}=\text{'N'}$ or $\text{'S'}$.

3: $\mathbf{select}$ – Logical Function, supplied by the user. External Procedure

If $\mathbf{sort}=\text{'S'}$, select is used to select eigenvalues to sort to the top left of the Schur form.

If $\mathbf{sort}=\text{'N'}$, select is not referenced and f08pnf may be called with the dummy function f08pnz.

An eigenvalue $\mathbf{w}\left(j\right)$ is selected if $\mathbf{select}\left(\mathbf{w}\left(j\right)\right)$ is .TRUE..

The specification of select is:

Fortran Interface

Function select ( w) Logical :: select Complex (Kind=nag_wp), Intent (In) :: w

C Header Interface

Nag_Boolean  select_ (const Complex *w)

C++ Header Interface

#include <nag.h> extern "C" { Nag_Boolean  select_ (const Complex &w) }

1: $\mathbf{w}$ – Complex (Kind=nag_wp) Input

On entry: the real and imaginary parts of the eigenvalue.

select must either be a module subprogram USEd by, or declared as EXTERNAL in, the (sub)program from which f08pnf is called. Arguments denoted as Input must not be changed by this procedure.

4: $\mathbf{n}$ – Integer Input

On entry: $n$, the order of the matrix $A$.

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

5: $\mathbf{a}\left(\mathbf{lda},*\right)$ – Complex (Kind=nag_wp) array Input/Output

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

On entry: the $n$ by $n$ matrix $A$.

On exit: a is overwritten by its Schur form $T$.

6: $\mathbf{lda}$ – Integer Input

On entry: the first dimension of the array a as declared in the (sub)program from which f08pnf is called.

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

7: $\mathbf{sdim}$ – Integer Output

On exit: if $\mathbf{sort}=\text{'N'}$, $\mathbf{sdim}=0$.

If $\mathbf{sort}=\text{'S'}$, $\mathbf{sdim}=$ number of eigenvalues for which select is .TRUE..

8: $\mathbf{w}\left(*\right)$ – Complex (Kind=nag_wp) array Output

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

On exit: contains the computed eigenvalues, in the same order that they appear on the diagonal of the output Schur form $T$.

9: $\mathbf{vs}\left(\mathbf{ldvs},*\right)$ – Complex (Kind=nag_wp) array Output

Note: the second dimension of the array vs must be at least $\max \left(1,\mathbf{n}\right)$ if $\mathbf{jobvs}=\text{'V'}$, and at least $1$ otherwise.

On exit: if $\mathbf{jobvs}=\text{'V'}$, vs contains the unitary matrix $Z$ of Schur vectors.

If $\mathbf{jobvs}=\text{'N'}$, vs is not referenced.

10: $\mathbf{ldvs}$ – Integer Input

On entry: the first dimension of the array vs as declared in the (sub)program from which f08pnf is called.

Constraints:

• if $\mathbf{jobvs}=\text{'V'}$, $\mathbf{ldvs}\ge \max \left(1,\mathbf{n}\right)$;
• otherwise $\mathbf{ldvs}\ge 1$.

11: $\mathbf{work}\left(\max \left(1,\mathbf{lwork}\right)\right)$ – Complex (Kind=nag_wp) array Workspace

On exit: if $\mathbf{info}=\mathbf{0}$, the real part of $\mathbf{work}\left(1\right)$ contains the minimum value of lwork required for optimal performance.

12: $\mathbf{lwork}$ – Integer Input

On entry: the dimension of the array work as declared in the (sub)program from which f08pnf 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, lwork must generally be larger than the minimum, say $2\times \mathbf{n}+\mathit{nb}\times \mathbf{n}$, where $\mathit{nb}$ is the optimal block size for f08nsf.

Constraint: $\mathbf{lwork}\ge \max \left(1,2\times \mathbf{n}\right)$.

13: $\mathbf{rwork}\left(*\right)$ – Real (Kind=nag_wp) array Workspace

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

14: $\mathbf{bwork}\left(*\right)$ – Logical array Workspace

Note: the dimension of the array bwork must be at least $1$ if $\mathbf{sort}=\text{'N'}$, and at least $\max \left(1,\mathbf{n}\right)$ otherwise.

If $\mathbf{sort}=\text{'N'}$, bwork is not referenced.

15: $\mathbf{info}$ – Integer Output

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.

$\mathbf{info}=1,\dots ,\mathbf{n}$

The $QR$ algorithm failed to compute all the eigenvalues.

$\mathbf{info}=\mathbf{n}+1$

The eigenvalues could not be reordered because some eigenvalues were too close to separate (the problem is very ill-conditioned).

$\mathbf{info}=\mathbf{n}+2$

After reordering, roundoff changed values of some complex eigenvalues so that leading eigenvalues in the Schur form no longer satisfy $\mathbf{select}=\mathrm{.TRUE.}$. This could also be caused by underflow due to scaling.

7 Accuracy

8 Parallelism and Performance

9 Further Comments

10 Example

10.1 Program Text

10.2 Program Data

10.3 Program Results