NAG FL Interface
f11bff (real_​gen_​basic_​diag)

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1 Purpose

f11bff is the third in a suite of three routines for the iterative solution of a real general (nonsymmetric) system of simultaneous linear equations (see Golub and Van Loan (1996)). f11bff returns information about the computations during an iteration and/or after this has been completed. The first routine of the suite, f11bdf, is a setup routine; the second routine, f11bef, is the iterative solver itself.
These three routines are suitable for the solution of large sparse general (nonsymmetric) systems of equations.

2 Specification

Fortran Interface
Subroutine f11bff ( itn, stplhs, stprhs, anorm, sigmax, work, lwork, ifail)
Integer, Intent (In) :: lwork
Integer, Intent (Inout) :: ifail
Integer, Intent (Out) :: itn
Real (Kind=nag_wp), Intent (In) :: work(lwork)
Real (Kind=nag_wp), Intent (Out) :: stplhs, stprhs, anorm, sigmax
C Header Interface
#include <nag.h>
void  f11bff_ (Integer *itn, double *stplhs, double *stprhs, double *anorm, double *sigmax, const double work[], const Integer *lwork, Integer *ifail)
The routine may be called by the names f11bff or nagf_sparse_real_gen_basic_diag.

3 Description

f11bff returns information about the solution process. It can be called either during a monitoring step of f11bef or after f11bef has completed its tasks. Calling f11bff at any other time will result in an error condition being raised.
For further information you should read the documentation for f11bdf and f11bef.

4 References

Golub G H and Van Loan C F (1996) Matrix Computations (3rd Edition) Johns Hopkins University Press, Baltimore

5 Arguments

1: itn Integer Output
On exit: the number of iterations carried out by f11bef.
2: stplhs Real (Kind=nag_wp) Output
On exit: the current value of the left-hand side of the termination criterion used by f11bef.
3: stprhs Real (Kind=nag_wp) Output
On exit: the current value of the right-hand side of the termination criterion used by f11bef.
4: anorm Real (Kind=nag_wp) Output
On exit: if iterm=1 in the previous call to f11bdf, then anorm contains Ap, where p=1, 2 or , either supplied or, in the case of 1 or , estimated by f11bef; otherwise anorm=0.0.
5: sigmax Real (Kind=nag_wp) Output
On exit: if iterm=2 in the previous call to f11bdf, the current estimate of the largest singular value σ1(A¯) of the preconditioned iteration matrix, either when it has been supplied to f11bdf or it has been estimated by f11bef (see also Sections 3 and 5 in f11bdf); otherwise, sigmax=0.0 is returned.
6: work(lwork) Real (Kind=nag_wp) array Communication Array
On entry: the array work as returned by f11bef (see also Sections 3 and 5 in f11bef).
7: lwork Integer Input
On entry: the dimension of the array work as declared in the (sub)program from which f11bff is called (see also Section 5 in f11bdf).
Constraint: lwork100.
Note: although the minimum value of lwork ensures the correct functioning of f11bff, a larger value is required by the iterative solver f11bef (see also Section 5 in f11bdf).
8: 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 or −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
f11bff has been called out of sequence.
ifail=-7
On entry, lwork=value.
Constraint: lwork100.
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

Not applicable.

8 Parallelism and Performance

Background information to multithreading can be found in the Multithreading documentation.
f11bff is not threaded in any implementation.

9 Further Comments

None.

10 Example

See f11bdf.