Higham N J and Lin L (2013) An improved Schur–Padé algorithm for fractional powers of a matrix and their Fréchet derivatives SIAM J. Matrix Anal. Appl. 34(3) 1341–1360

5 Arguments

1: $n$ – Integer Input: On entry: $n$ , the order of the matrix $A$ .

Constraint: $n \geq 0$ .
2: $a (lda, *)$ – Complex (Kind=nag_wp) array Input/Output: Note: the second dimension of the array a must be at least $n$ .

On entry: the $n \times n$ matrix $A$ .

On exit: if $ifail = 0$ , the $n \times n$ matrix $p$ th power, $A^{p}$ . Alternatively, if $ifail = 1$ , contains an $n \times n$ non-principal power of $A$ .
3: $lda$ – Integer Input: On entry: the first dimension of the array a as declared in the (sub)program from which f01fqf is called.

Constraint: $lda \geq n$ .
4: $p$ – Real (Kind=nag_wp) Input: On entry: the required power of $A$ .
5: $ifail$ – Integer Input/Output: On entry: ifail must be set to $0$ , $−1$ or $1$ to set behaviour on detection of an error; these values have no effect when no error is detected.
A value of $0$ causes the printing of an error message and program execution will be halted; otherwise program execution continues. A value of $−1$ means that an error message is printed while a value of $1$ means that it is not.

If halting is not appropriate, the value $−1$ or $1$ is recommended. If message printing is undesirable, then the value $1$ is recommended. Otherwise, the value $0$ is recommended. When the value $- 1$ or $1$ is used it is essential to test the value of ifail on exit.

On exit: $ifail = 0$ unless the routine detects an error or a warning has been flagged (see Section 6).

6 Error Indicators and Warnings

If on entry

ifail = 0

−1

, explanatory error messages are output on the current error message unit (as defined by x04aaf).

Errors or warnings detected by the routine:

$ifail = 1$: $A$ has eigenvalues on the negative real line. The principal $p$ th power is not defined so a non-principal power is returned.

$ifail = 2$: $A$ is singular so the $p$ th power cannot be computed.

$ifail = 3$: $A^{p}$ has been computed using an IEEE double precision Padé approximant, although the arithmetic precision is higher than IEEE double precision.

$ifail = 4$: An unexpected internal error occurred. This failure should not occur and suggests that the routine has been called incorrectly.

$ifail = - 1$: On entry, $n = ⟨ value ⟩$ .
Constraint: $n \geq 0$ .

$ifail = - 3$: On entry, $lda = ⟨ value ⟩$ and $n = ⟨ value ⟩$ .
Constraint: $lda \geq n$ .

$ifail = - 99$: An unexpected error has been triggered by this routine. Please contact NAG.
See Section 7 in the Introduction to the NAG Library FL Interface for further information.

$ifail = - 399$: Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library FL Interface for further information.

$ifail = - 999$: Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.

7 Accuracy

For positive integer

p

, the algorithm reduces to a sequence of matrix multiplications. For negative integer

p

, the algorithm consists of a combination of matrix inversion and matrix multiplications.

For a normal matrix

A

(for which

A^{H} A = A A^{H}

) and non-integer

p

, the Schur decomposition is diagonal and the algorithm reduces to evaluating powers of the eigenvalues of

A

and then constructing

A^{p}

using the Schur vectors. This should give a very accurate result. In general however, no error bounds are available for the algorithm.

8 Parallelism and Performance

f01fqf is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.

f01fqf makes calls to BLAS and/or LAPACK routines, which may be threaded within the vendor library used by this implementation. Consult the documentation for the vendor library for further information.

Please consult the X06 Chapter Introduction for information on how to control and interrogate the OpenMP environment used within this routine. Please also consult the Users' Note for your implementation for any additional implementation-specific information.

9 Further Comments

The cost of the algorithm is

O (n^{3})

. The exact cost depends on the matrix

A

but if

p \in (−1, 1)

then the cost is independent of

p

O (4 \times n^{2})

complex allocatable memory is required by the routine.

If estimates of the condition number of

A^{p}

are required then f01kef should be used.

10 Example

This example finds

A^{p}

where

p = 0.2

and

A = (\begin{array}{r} 2 & 3 & 2 & 1 + 3 i \\ 2 + i & 1 & 1 & 2 + 2 i \\ 2 + i & 2 + 2 i & 2 i & 2 + 4 i \\ 3 & 2 + 2 i & 3 & 1 \end{array}) .

f01fq: FL CL CPP AD

NAG FL Interfacef01fqf (complex_​gen_​matrix_​pow)

▸▿ Contents

1 Purpose

2 Specification

3 Description

4 References

5 Arguments

6 Error Indicators and Warnings

7 Accuracy

8 Parallelism and Performance

9 Further Comments

10 Example

10.1 Program Text

10.2 Program Data

10.3 Program Results

NAG FL Interface
f01fqf (complex_gen_matrix_pow)