F01 Chapter Contents
F01 Chapter Introduction
NAG Library Manual

# NAG Library Routine DocumentF01KBF

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.

## 1  Purpose

F01KBF computes an estimate of the absolute condition number of a matrix function $f$ at a complex $n$ by $n$ matrix $A$ in the $1$-norm. Numerical differentiation is used to evaluate the derivatives of $f$ when they are required.

## 2  Specification

 SUBROUTINE F01KBF ( N, A, LDA, F, IUSER, RUSER, IFLAG, CONDA, NORMA, NORMFA, IFAIL)
 INTEGER N, LDA, IUSER(*), IFLAG, IFAIL REAL (KIND=nag_wp) RUSER(*), CONDA, NORMA, NORMFA COMPLEX (KIND=nag_wp) A(LDA,*) EXTERNAL F

## 3  Description

The absolute condition number of $f$ at $A$, ${\mathrm{cond}}_{\mathrm{abs}}\left(f,A\right)$ is given by the norm of the Fréchet derivative of $f$, $L\left(A,E\right)$, which is defined by
 $LX := maxE≠0 LX,E E .$
The Fréchet derivative in the direction $E$, $L\left(X,E\right)$ is linear in $E$ and can therefore be written as
 $vec LX,E = KX vecE ,$
where the $\mathrm{vec}$ operator stacks the columns of a matrix into one vector, so that $K\left(X\right)$ is ${n}^{2}×{n}^{2}$. F01KBF computes an estimate $\gamma$ such that $\gamma \le {‖K\left(X\right)‖}_{1}$, where ${‖K\left(X\right)‖}_{1}\in \left[{n}^{-1}{‖L\left(X\right)‖}_{1},n{‖L\left(X\right)‖}_{1}\right]$. 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 $\gamma$ is detailed in Section 3.4 of Higham (2008).
The function $f$ is supplied via subroutine F which evaluates $f\left({z}_{i}\right)$ at a number of points ${z}_{i}$.

## 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: ${\mathbf{N}}\ge 0$.
2:     A(LDA,$*$) – COMPLEX (KIND=nag_wp) arrayInput/Output
Note: the second dimension of the array A must be at least ${\mathbf{N}}$.
On entry: the $n$ by $n$ matrix $A$.
On exit: the $n$ by $n$ matrix, $f\left(A\right)$.
3:     LDA – INTEGERInput
On entry: the first dimension of the array A as declared in the (sub)program from which F01KBF is called.
Constraint: ${\mathbf{LDA}}\ge \mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(1,{\mathbf{N}}\right)$.
4:     F – SUBROUTINE, supplied by the user.External Procedure
The subroutine F evaluates $f\left({z}_{i}\right)$ at a number of points ${z}_{i}$.
The specification of F is:
 SUBROUTINE F ( IFLAG, NZ, Z, FZ, IUSER, RUSER)
 INTEGER IFLAG, NZ, IUSER(*) REAL (KIND=nag_wp) RUSER(*) COMPLEX (KIND=nag_wp) Z(NZ), FZ(NZ)
1:     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 $f\left(z\right)$; for instance $f\left(z\right)$ may not be defined. If IFLAG is returned as nonzero then F01KBF will terminate the computation, with ${\mathbf{IFAIL}}={\mathbf{3}}$.
2:     NZ – INTEGERInput
On entry: ${n}_{z}$, the number of function values required.
3:     Z(NZ) – COMPLEX (KIND=nag_wp) arrayInput
On entry: the ${n}_{z}$ points ${z}_{1},{z}_{2},\dots ,{z}_{{n}_{z}}$ at which the function $f$ is to be evaluated.
4:     FZ(NZ) – COMPLEX (KIND=nag_wp) arrayOutput
On exit: the ${n}_{z}$ function values. ${\mathbf{FZ}}\left(\mathit{i}\right)$ should return the value $f\left({z}_{\mathit{i}}\right)$, for $\mathit{i}=1,2,\dots ,{n}_{z}$.
5:     IUSER($*$) – INTEGER arrayUser Workspace
6:     RUSER($*$) – REAL (KIND=nag_wp) arrayUser Workspace
F is called with the parameters IUSER and RUSER as supplied to F01KBF. 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 F01KBF 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 F01KBF, 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: ${\mathbf{IFLAG}}=0$, unless IFLAG has been set nonzero inside F, in which case IFLAG will be the value set and IFAIL will be set to ${\mathbf{IFAIL}}={\mathbf{3}}$.
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 $f\left(A\right)$.
11:   IFAIL – INTEGERInput/Output
On entry: IFAIL must be set to $0$, $-1\text{​ or ​}1$. 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 $-1\text{​ or ​}1$ 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 $-\mathbf{1}\text{​ or ​}\mathbf{1}$ is used it is essential to test the value of IFAIL on exit.
On exit: ${\mathbf{IFAIL}}={\mathbf{0}}$ unless the routine detects an error or a warning has been flagged (see Section 6).

## 6  Error Indicators and Warnings

If on entry ${\mathbf{IFAIL}}={\mathbf{0}}$ or $-{\mathbf{1}}$, explanatory error messages are output on the current error message unit (as defined by X04AAF).
Errors or warnings detected by the routine:
${\mathbf{IFAIL}}=1$
An internal error occurred when estimating the norm of the Fréchet derivative of $f$ at $A$. Please contact NAG.
${\mathbf{IFAIL}}=2$
An internal error occurred while evaluating the matrix function $f\left(A\right)$. You can investigate further by calling F01FLF with the matrix $A$ and the function $f$.
${\mathbf{IFAIL}}=3$
IFLAG has been set nonzero by the user-supplied subroutine.
${\mathbf{IFAIL}}=-1$
On entry, ${\mathbf{N}}<0$.
${\mathbf{IFAIL}}=-3$
On entry, parameter LDA is invalid.
Constraint: ${\mathbf{LDA}}\ge {\mathbf{N}}$.
${\mathbf{IFAIL}}=-999$
Allocation of memory failed.

## 7  Accuracy

F01KBF uses the norm estimation routine F04YDF to estimate a quantity $\gamma$, where $\gamma \le {‖K\left(X\right)‖}_{1}$ and ${‖K\left(X\right)‖}_{1}\in \left[{n}^{-1}{‖L\left(X\right)‖}_{1},n{‖L\left(X\right)‖}_{1}\right]$. For further details on the accuracy of norm estimation, see the documentation for F04ZDF.

Approximately $6{n}^{2}$ of complex allocatable memory is required by the routine, in addition to the memory used by the underlying matrix function routine F01FLF.
F01KBF returns the matrix function $f\left(A\right)$. This is computed using F01FLF. If only $f\left(A\right)$ is required, without an estimate of the condition number, then it is far more efficient to use F01FLF directly.
The real analogue of this routine is F01JBF.

## 9  Example

This example estimates the absolute and relative condition numbers of the matrix function $\mathrm{sin}2A$ where
 $A = 2.0+0.0i 0.0+1.0i 1.0+1.0i 0.0+3.0i 1.0+1.0i 0.0+2.0i 2.0+2.0i 0.0+0.0i 0.0+0.0i 2.0+0.0i 1.0+2.0i 1.0+0.0i 1.0+1.0i 3.0+0.0i 0.0+0.0i 1.0+2.0i .$

### 9.1  Program Text

Program Text (f01kbfe.f90)

### 9.2  Program Data

Program Data (f01kbfe.d)

### 9.3  Program Results

Program Results (f01kbfe.r)