NAG Library Routine Document

F11BTF

1  Purpose

2  Specification

3  Description

4  References

5  Parameters

1:     ITN – INTEGEROutput

On exit: the number of iterations carried out by F11BSF.

2:     STPLHS – REAL (KIND=nag_wp)Output

On exit: the current value of the left-hand side of the termination criterion used by F11BSF.

3:     STPRHS – REAL (KIND=nag_wp)Output

On exit: the current value of the right-hand side of the termination criterion used by F11BSF.

4:     ANORM – REAL (KIND=nag_wp)Output

On exit: if $\mathbf{ITERM}=1$ in the previous call to F11BRF, then ANORM contains ${‖A‖}_p$, where $p=1$, $2$ or $\infty$, either supplied or, in the case of $1$ or $\infty$, estimated by F11BSF; otherwise $\mathbf{ANORM}=0.0$.

5:     SIGMAX – REAL (KIND=nag_wp)Output

On exit: if $\mathbf{ITERM}=2$ in the previous call to F11BRF, the current estimate of the largest singular value ${\sigma }_1\left(\stackrel{-}{A}\right)$ of the preconditioned iteration matrix when it is used by the termination criterion in F11BSF, either when it has been supplied to F11BRF or it has been estimated by F11BSF (see also Sections 3 and 5 in F11BRF); otherwise, $\mathbf{SIGMAX}=0.0$ is returned.

6:     WORK(LWORK) – COMPLEX (KIND=nag_wp) arrayCommunication Array

On entry: the array WORK as returned by F11BSF (see also Sections 3 and 5 in F11BSF).

7:     LWORK – INTEGERInput

On entry: the dimension of the array WORK as declared in the (sub)program from which F11BTF is called (see also F11BRF).

Constraint: $\mathbf{LWORK}\ge 120$.

Note:  although the minimum value of LWORK ensures the correct functioning of F11BTF, a larger value is required by the iterative solver F11BSF (see also F11BRF).

8:     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

$\mathbf{IFAIL}=-i$

On entry, the $i$th argument had an illegal value.

$\mathbf{IFAIL}=1$

F11BTF has been called out of sequence. For example, the last call to F11BSF did not return $\mathbf{IREVCM}=3$ or $4$.

7  Accuracy

8  Further Comments

9  Example