NAG Library Function Document
nag_sparse_herm_sol (f11jsc)
1
Purpose
nag_sparse_herm_sol (f11jsc) solves a complex sparse Hermitian system of linear equations, represented in symmetric coordinate storage format, using a conjugate gradient or Lanczos method, without preconditioning, with Jacobi or with SSOR preconditioning.
2
Specification
#include <nag.h> |
#include <nagf11.h> |
void |
nag_sparse_herm_sol (Nag_SparseSym_Method method,
Nag_SparseSym_PrecType precon,
Integer n,
Integer nnz,
const Complex a[],
const Integer irow[],
const Integer icol[],
double omega,
const Complex b[],
double tol,
Integer maxitn,
Complex x[],
double *rnorm,
Integer *itn,
double rdiag[],
NagError *fail) |
|
3
Description
nag_sparse_herm_sol (f11jsc) solves a complex sparse Hermitian linear system of equations
using a preconditioned conjugate gradient method (see
Barrett et al. (1994)), or a preconditioned Lanczos method based on the algorithm SYMMLQ (see
Paige and Saunders (1975)). The conjugate gradient method is more efficient if
is positive definite, but may fail to converge for indefinite matrices. In this case the Lanczos method should be used instead. For further details see
Barrett et al. (1994).
nag_sparse_herm_sol (f11jsc) allows the following choices for the preconditioner:
- – no preconditioning;
- – Jacobi preconditioning (see Young (1971));
- – symmetric successive-over-relaxation (SSOR) preconditioning (see Young (1971)).
For incomplete Cholesky (IC) preconditioning see
nag_sparse_herm_chol_sol (f11jqc).
The matrix
is represented in symmetric coordinate storage (SCS) format (see
Section 2.1.2 in the f11 Chapter Introduction) in the arrays
a,
irow and
icol. The array
a holds the nonzero entries in the lower triangular part of the matrix, while
irow and
icol hold the corresponding row and column indices.
4
References
Barrett R, Berry M, Chan T F, Demmel J, Donato J, Dongarra J, Eijkhout V, Pozo R, Romine C and Van der Vorst H (1994) Templates for the Solution of Linear Systems: Building Blocks for Iterative Methods SIAM, Philadelphia
Paige C C and Saunders M A (1975) Solution of sparse indefinite systems of linear equations SIAM J. Numer. Anal. 12 617–629
Young D (1971) Iterative Solution of Large Linear Systems Academic Press, New York
5
Arguments
- 1:
– Nag_SparseSym_MethodInput
-
On entry: specifies the iterative method to be used.
- Conjugate gradient method.
- Lanczos method (SYMMLQ).
Constraint:
or .
- 2:
– Nag_SparseSym_PrecTypeInput
-
On entry: specifies the type of preconditioning to be used.
- No preconditioning.
- Jacobi.
- Symmetric successive-over-relaxation (SSOR).
Constraint:
, or .
- 3:
– IntegerInput
-
On entry: , the order of the matrix .
Constraint:
.
- 4:
– IntegerInput
-
On entry: the number of nonzero elements in the lower triangular part of the matrix .
Constraint:
.
- 5:
– const ComplexInput
-
On entry: the nonzero elements of the lower triangular part of the matrix
, ordered by increasing row index, and by increasing column index within each row. Multiple entries for the same row and column indices are not permitted. The function
nag_sparse_herm_sort (f11zpc) may be used to order the elements in this way.
- 6:
– const IntegerInput
- 7:
– const IntegerInput
-
On entry: the row and column indices of the nonzero elements supplied in array
a.
Constraints:
irow and
icol must satisfy these constraints (which may be imposed by a call to
nag_sparse_herm_sort (f11zpc)):
- and , for ;
- or and , for .
- 8:
– doubleInput
-
On entry: if
,
omega is the relaxation parameter
to be used in the SSOR method. Otherwise
omega need not be initialized.
Constraint:
.
- 9:
– const ComplexInput
-
On entry: the right-hand side vector .
- 10:
– doubleInput
-
On entry: the required tolerance. Let
denote the approximate solution at iteration
, and
the corresponding residual. The algorithm is considered to have converged at iteration
if
If
,
is used, where
is the
machine precision. Otherwise
is used.
Constraint:
.
- 11:
– IntegerInput
-
On entry: the maximum number of iterations allowed.
Constraint:
.
- 12:
– ComplexInput/Output
-
On entry: an initial approximation to the solution vector .
On exit: an improved approximation to the solution vector .
- 13:
– double *Output
-
On exit: the final value of the residual norm
, where
is the output value of
itn.
- 14:
– Integer *Output
-
On exit: the number of iterations carried out.
- 15:
– doubleOutput
-
On exit: the elements of the diagonal matrix , where is the diagonal part of . Note that since is Hermitian the elements of are necessarily real.
- 16:
– NagError *Input/Output
-
The NAG error argument (see
Section 3.7 in How to Use the NAG Library and its Documentation).
6
Error Indicators and Warnings
- NE_ACCURACY
-
The required accuracy could not be obtained. However, a reasonable accuracy has been achieved and further iterations could not improve the result.
- NE_ALLOC_FAIL
-
Dynamic memory allocation failed.
See
Section 2.3.1.2 in How to Use the NAG Library and its Documentation for further information.
- NE_BAD_PARAM
-
On entry, argument had an illegal value.
- NE_COEFF_NOT_POS_DEF
-
The matrix of the coefficients
a appears not to be positive definite. The computation cannot continue.
- NE_CONVERGENCE
-
The solution has not converged after iterations.
- NE_INT
-
On entry, .
Constraint: .
On entry, .
Constraint: .
On entry, .
Constraint: .
- NE_INT_2
-
On entry, and .
Constraint:
- NE_INTERNAL_ERROR
-
An internal error has occurred in this function. Check the function call and any array sizes. If the call is correct then please contact
NAG for assistance.
See
Section 2.7.6 in How to Use the NAG Library and its Documentation for further information.
A serious error, code , has occurred in an internal call. Check all function calls and array sizes. Seek expert help.
- NE_INVALID_SCS
-
On entry, , and .
Constraint: and .
On entry, , and .
Constraint: and .
- NE_NO_LICENCE
-
Your licence key may have expired or may not have been installed correctly.
See
Section 2.7.5 in How to Use the NAG Library and its Documentation for further information.
- NE_NOT_STRICTLY_INCREASING
-
On entry, is out of order: .
On entry, the location (
) is a duplicate:
. Consider calling
nag_sparse_herm_sort (f11zpc) to reorder and sum or remove duplicates.
- NE_PRECOND_NOT_POS_DEF
-
The preconditioner appears not to be positive definite. The computation cannot continue.
- NE_REAL
-
On entry, .
Constraint: .
On entry, .
Constraint: .
- NE_ZERO_DIAG_ELEM
-
The matrix has a non-real diagonal entry in row .
The matrix has a zero diagonal entry in row .
The matrix has no diagonal entry in row .
7
Accuracy
On successful termination, the final residual
, where
, satisfies the termination criterion
The value of the final residual norm is returned in
rnorm.
8
Parallelism and Performance
nag_sparse_herm_sol (f11jsc) is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
nag_sparse_herm_sol (f11jsc) 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 function. Please also consult the
Users' Note for your implementation for any additional implementation-specific information.
The time taken by
nag_sparse_herm_sol (f11jsc) for each iteration is roughly proportional to
nnz. One iteration with the Lanczos method (SYMMLQ) requires a slightly larger number of operations than one iteration with the conjugate gradient method.
The number of iterations required to achieve a prescribed accuracy cannot easily be determined a priori, as it can depend dramatically on the conditioning and spectrum of the preconditioned matrix of the coefficients .
10
Example
This example solves a complex sparse Hermitian positive definite system of equations using the conjugate gradient method, with SSOR preconditioning.
10.1
Program Text
Program Text (f11jsce.c)
10.2
Program Data
Program Data (f11jsce.d)
10.3
Program Results
Program Results (f11jsce.r)