F01JCF (PDF version)
F01 Chapter Contents
F01 Chapter Introduction
NAG Library Manual

NAG Library Routine Document


Note:  before using this routine, please read the Users' Note for your implementation to check the interpretation of bold italicised terms and other implementation-dependent details.

+ Contents

    1  Purpose
    7  Accuracy

1  Purpose

F01JCF computes an estimate of the absolute condition number of a matrix function f at a real n by n matrix A in the 1-norm, using analytical derivatives of f you have supplied.

2  Specification


3  Description

The absolute condition number of f at A, condabsf,A is given by the norm of the Fréchet derivative of f, LA,E, which is defined by
LX := maxE0 LX,E E .
The Fréchet derivative in the direction E, LX,E is linear in E and can therefore be written as
vec LX,E = KX vecE ,
where the vec operator stacks the columns of a matrix into one vector, so that KX is n2×n2. F01JCF computes an estimate γ such that γ KX 1 , where KX 1 n-1 LX 1 , n LX 1 . The relative condition number can then be computed via
cond rel f,A = cond abs f,A A1 fA 1 .
The algorithm used to find γ is detailed in Section 3.4 of Higham (2008).
The function f, and the derivatives of f, are returned by subroutine F which, given an integer m, evaluates fmzi at a number of (generally complex) points zi, for i=1,2,,nz. For any z on the real line, fz must also be real. F01JCF is therefore appropriate for routines that can be evaluated on the complex plane and whose derivatives, of arbitrary order, can also be evaluated on the complex plane.

4  References

Higham N J (2008) Functions of Matrices: Theory and Computation SIAM, Philadelphia, PA, USA

5  Parameters

1:     N – INTEGERInput
On entry: n, the order of the matrix A.
Constraint: N0.
2:     A(LDA,*) – REAL (KIND=nag_wp) arrayInput/Output
Note: the second dimension of the array A must be at least N.
On entry: the n by n matrix A.
On exit: the n by n matrix, fA.
3:     LDA – INTEGERInput
On entry: the first dimension of the array A as declared in the (sub)program from which F01JCF is called.
Constraint: LDAmax1,N.
4:     F – SUBROUTINE, supplied by the user.External Procedure
Given an integer m, the subroutine F evaluates fmzi at a number of points zi.
The specification of F is:
REAL (KIND=nag_wp)  RUSER(*)
COMPLEX (KIND=nag_wp)  Z(NZ), FZ(NZ)
1:     M – INTEGERInput
On entry: the order, m, of the derivative required.
If M=0, fzi should be returned. For M>0, fmzi should be returned.
2:     IFLAG – INTEGERInput/Output
On entry: IFLAG will be zero.
On exit: IFLAG should either be unchanged from its entry value of zero, or may be set nonzero to indicate that there is a problem in evaluating the function fz; for instance fz may not be defined. If IFLAG is returned as nonzero then F01JCF will terminate the computation, with IFAIL=3.
3:     NZ – INTEGERInput
On entry: nz, the number of function or derivative values required.
4:     Z(NZ) – COMPLEX (KIND=nag_wp) arrayInput
On entry: the nz points z1,z2,,znz at which the function f is to be evaluated.
5:     FZ(NZ) – COMPLEX (KIND=nag_wp) arrayOutput
On exit: the nz function or derivative values. FZi should return the value fmzi, for i=1,2,,nz. If zi lies on the real line, then so must fmzi.
6:     IUSER(*) – INTEGER arrayUser Workspace
7:     RUSER(*) – REAL (KIND=nag_wp) arrayUser Workspace
F is called with the parameters IUSER and RUSER as supplied to F01JCF. You are free to use the arrays IUSER and RUSER to supply information to F as an alternative to using COMMON global variables.
F must either be a module subprogram USEd by, or declared as EXTERNAL in, the (sub)program from which F01JCF is called. Parameters denoted as Input must not be changed by this procedure.
5:     IUSER(*) – INTEGER arrayUser Workspace
6:     RUSER(*) – REAL (KIND=nag_wp) arrayUser Workspace
IUSER and RUSER are not used by F01JCF, but are passed directly to F and may be used to pass information to this routine as an alternative to using COMMON global variables.
7:     IFLAG – INTEGEROutput
On exit: IFLAG=0, unless IFLAG has been set nonzero inside F, in which case IFLAG will be the value set and IFAIL will be set to IFAIL=3.
8:     CONDA – REAL (KIND=nag_wp)Output
On exit: an estimate of the absolute condition number of f at A.
9:     NORMA – REAL (KIND=nag_wp)Output
On exit: the 1-norm of A.
10:   NORMFA – REAL (KIND=nag_wp)Output
On exit: the 1-norm of fA.
11:   IFAIL – INTEGERInput/Output
On entry: IFAIL must be set to 0, -1​ or ​1. If you are unfamiliar with this parameter you should refer to Section 3.3 in the Essential Introduction for details.
For environments where it might be inappropriate to halt program execution when an error is detected, the value -1​ or ​1 is recommended. If the output of error messages is undesirable, then the value 1 is recommended. Otherwise, if you are not familiar with this parameter, the recommended value is 0. 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 or -1, explanatory error messages are output on the current error message unit (as defined by X04AAF).
Errors or warnings detected by the routine:
An internal error occurred when estimating the norm of the Fréchet derivative of f at A. Please contact NAG.
An internal error occurred when evaluating the matrix function fA. You can investigate further by calling F01EMF with the matrix A and the function f.
IFLAG has been set nonzero by the user-supplied subroutine.
On entry, N<0.
Input argument number value is invalid.
On entry, parameter LDA is invalid.
Constraint: LDAN.
Allocation of memory failed.

7  Accuracy

F01JCF uses the norm estimation routine F04YDF to estimate a quantity γ, where γ KX 1  and KX 1 n-1 LX 1 , n LX 1 . For further details on the accuracy of norm estimation, see the documentation for F04YDF.

8  Further Comments

Approximately 6n2 of real allocatable memory is required by the routine, in addition to the memory used by the underlying matrix function routine F01EMF.
F01JCF returns the matrix function fA. This is computed using F01EMF. If only fA is required, without an estimate of the condition number, then it is far more efficient to use F01EMF directly.
The complex analogue of this routine is F01KCF.

9  Example

This example estimates the absolute and relative condition numbers of the matrix function e2A where
A= 0 -1 -1 1 -2 0 1 -1 2 -1 2 -2 -1 -2 0 -1 .

9.1  Program Text

Program Text (f01jcfe.f90)

9.2  Program Data

Program Data (f01jcfe.d)

9.3  Program Results

Program Results (f01jcfe.r)

F01JCF (PDF version)
F01 Chapter Contents
F01 Chapter Introduction
NAG Library Manual

© The Numerical Algorithms Group Ltd, Oxford, UK. 2012