NAG CL Interface
f11dac (real_gen_precon_ilu)
1
Purpose
f11dac computes an incomplete
$LU$ factorization of a real sparse nonsymmetric matrix, represented in coordinate storage format. This factorization may be used as a preconditioner in combination with
f11dcc.
2
Specification
void 
f11dac (Integer n,
Integer nnz,
double *a[],
Integer *la,
Integer *irow[],
Integer *icol[],
Integer lfill,
double dtol,
Nag_SparseNsym_Piv pstrat,
Nag_SparseNsym_Fact milu,
Integer ipivp[],
Integer ipivq[],
Integer istr[],
Integer idiag[],
Integer *nnzc,
Integer *npivm,
NagError *fail) 

The function may be called by the names: f11dac, nag_sparse_real_gen_precon_ilu or nag_sparse_nsym_fac.
3
Description
f11dac computes an incomplete
$LU$ factorization (
Meijerink and Van der Vorst (1977) and
Meijerink and Van der Vorst (1981)) of a real sparse nonsymmetric
$n$ by
$n$ matrix
$A$. The factorization is intended primarily for use as a preconditioner with the iterative solver
f11dcc.
The decomposition is written in the form
where
and
$L$ is lower triangular with unit diagonal elements,
$D$ is diagonal,
$U$ is upper triangular with unit diagonals,
$P$ and
$Q$ are permutation matrices, and
$R$ is a remainder matrix.
The amount of fillin occurring in the factorization can vary from zero to complete fill, and can be controlled by specifying either the maximum level of fill
lfill, or the drop tolerance
dtol.
The argument
pstrat defines the pivoting strategy to be used. The options currently available are no pivoting, userdefined pivoting, partial pivoting by columns for stability, and complete pivoting by rows for sparsity and by columns for stability. The factorization may optionally be modified to preserve the rowsums of the original matrix.
The sparse matrix
$A$ is represented in coordinate storage (CS) format (see
Section 2.1.2 in the
F11 Chapter Introduction). The array
a stores all the nonzero elements of the matrix
$A$, while arrays
irow and
icol store the corresponding row and column indices respectively. Multiple nonzero elements may not be specified for the same row and column index.
The preconditioning matrix
$M$ is returned in terms of the CS representation of the matrix
Further algorithmic details are given in
Section 9.3.
4
References
Meijerink J and Van der Vorst H (1977) An iterative solution method for linear systems of which the coefficient matrix is a symmetric Mmatrix Math. Comput. 31 148–162
Meijerink J and Van der Vorst H (1981) Guidelines for the usage of incomplete decompositions in solving sets of linear equations as they occur in practical problems J. Comput. Phys. 44 134–155
Salvini S A and Shaw G J (1996) An evaluation of new NAG Library solvers for large sparse unsymmetric linear systems NAG Technical Report TR2/96
5
Arguments

1:
$\mathbf{n}$ – Integer
Input

On entry: the order of the matrix $A$.
Constraint:
${\mathbf{n}}\ge 1$.

2:
$\mathbf{nnz}$ – Integer
Input

On entry: the number of nonzero elements in the matrix $A$.
Constraint:
$1\le {\mathbf{nnz}}\le {{\mathbf{n}}}^{2}$.

3:
$\mathbf{a}\left[{\mathbf{la}}\right]$ – double *
Input/Output

On entry: the nonzero elements in the matrix
$A$, 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
f11zac may be used to order the elements in this way.
On exit: the first
nnz entries of
a contain the nonzero elements of
$A$ and the next
nnzc entries contain the elements of the matrix
$C$. Matrix elements are ordered by increasing row index, and by increasing column index within each row.

4:
$\mathbf{la}$ – Integer *
Input/Output

On entry: the
second
dimension of the arrays
a,
irow and
icol.
These arrays must be of sufficient size to store both
$A$ (
nnz elements) and
$C$ (
nnzc elements); for this reason the length of the arrays may be changed internally by calls to
realloc. It is therefore
imperative that these arrays are
allocated using
NAG_ALLOC and
not declared as automatic arrays
On exit: if internal allocation has taken place then
la is set to
${\mathbf{nnz}}+{\mathbf{nnzc}}$, otherwise it remains unchanged.
Constraint:
${\mathbf{la}}\ge 2\times {\mathbf{nnz}}$.

5:
$\mathbf{irow}\left[{\mathbf{la}}\right]$ – Integer *
Input/Output

6:
$\mathbf{icol}\left[{\mathbf{la}}\right]$ – Integer *
Input/Output

On entry: the row and column indices of the nonzero elements supplied in
a.
Constraints:
 irow and icol must satisfy the following constraints (which may be imposed by a call to f11zac):;
 $1\le {\mathbf{irow}}\left[\mathit{i}\right]\le {\mathbf{n}}$ and $1\le {\mathbf{icol}}\left[\mathit{i}\right]\le {\mathbf{n}}$, for $\mathit{i}=0,1,\dots ,{\mathbf{nnz}}1$;
 ${\mathbf{irow}}\left[\mathit{i}1\right]<{\mathbf{irow}}\left[\mathit{i}\right]$ or ${\mathbf{irow}}\left[\mathit{i}1\right]={\mathbf{irow}}\left[\mathit{i}\right]$ and ${\mathbf{icol}}\left[\mathit{i}1\right]<{\mathbf{icol}}\left[\mathit{i}\right]$, for $\mathit{i}=1,2,\dots ,{\mathbf{nnz}}1$.
On exit: the row and column indices of the nonzero elements returned in
a.

7:
$\mathbf{lfill}$ – Integer
Input

On entry: if
${\mathbf{lfill}}\ge 0$ its value is the maximum level of fill allowed in the decomposition (see
Section 9.2). A negative value of
lfill indicates that
dtol will be used to control the fill instead.

8:
$\mathbf{dtol}$ – double
Input

On entry: if
${\mathbf{lfill}}<0$ then
dtol is used as a drop tolerance to control the fillin (see
Section 9.2); otherwise
dtol is not referenced.
Constraint:
if ${\mathbf{lfill}}<0$, ${\mathbf{dtol}}\ge 0.0$.

9:
$\mathbf{pstrat}$ – Nag_SparseNsym_Piv
Input

On entry: specifies the pivoting strategy to be adopted as follows:
 if ${\mathbf{pstrat}}=\mathrm{Nag\_SparseNsym\_NoPiv}$, no pivoting is carried out;
 if ${\mathbf{pstrat}}=\mathrm{Nag\_SparseNsym\_UserPiv}$, pivoting is carried out according to the userdefined input value of ipivp and ipivq;
 if ${\mathbf{pstrat}}=\mathrm{Nag\_SparseNsym\_PartialPiv}$, partial pivoting by columns for stability is carried out;
 if ${\mathbf{pstrat}}=\mathrm{Nag\_SparseNsym\_CompletePiv}$, complete pivoting by rows for sparsity, and by columns for stability, is carried out.
Suggested value:
${\mathbf{pstrat}}=\mathrm{Nag\_SparseNsym\_CompletePiv}$.
Constraint:
${\mathbf{pstrat}}=\mathrm{Nag\_SparseNsym\_NoPiv}$, $\mathrm{Nag\_SparseNsym\_UserPiv}$, $\mathrm{Nag\_SparseNsym\_PartialPiv}$ or $\mathrm{Nag\_SparseNsym\_CompletePiv}$.

10:
$\mathbf{milu}$ – Nag_SparseNsym_Fact
Input

On entry: indicates whether or not the factorization should be modified to preserve row sums (see
Section 9.4):
 if ${\mathbf{milu}}=\mathrm{Nag\_SparseNsym\_ModFact}$, the factorization is modified (milu);
 if ${\mathbf{milu}}=\mathrm{Nag\_SparseNsym\_UnModFact}$, the factorization is not modified.
Constraint:
${\mathbf{milu}}=\mathrm{Nag\_SparseNsym\_ModFact}$ or $\mathrm{Nag\_SparseNsym\_UnModFact}$.

11:
$\mathbf{ipivp}\left[{\mathbf{n}}\right]$ – Integer
Input/Output

12:
$\mathbf{ipivq}\left[{\mathbf{n}}\right]$ – Integer
Input/Output

On entry: if
${\mathbf{pstrat}}=\mathrm{Nag\_SparseNsym\_UserPiv}$,
${\mathbf{ipivp}}\left[k1\right]$ and
${\mathbf{ipivq}}\left[k1\right]$ must specify the row and column indices of the element used as a pivot at elimination stage
$k$. Otherwise
ipivp and
ipivq need not be initialized.
Constraint:
if
${\mathbf{pstrat}}=\mathrm{Nag\_SparseNsym\_UserPiv}$,
ipivp and
ipivq must both hold valid permutations of the integers on
$\left[1,{\mathbf{n}}\right]$.
On exit: the pivot indices. If ${\mathbf{ipivp}}\left[k1\right]=i$ and ${\mathbf{ipivq}}\left[k1\right]=j$ then the element in row $i$ and column $j$ was used as the pivot at elimination stage $k$.

13:
$\mathbf{istr}\left[{\mathbf{n}}+1\right]$ – Integer
Output

On exit:
${\mathbf{istr}}\left[\mathit{i}1\right]1$, for
$\mathit{i}=1,2,\dots ,{\mathbf{n}}$ is the index of arrays
a,
irow and
icol where row
$i$ of the matrix
$C$ starts.
${\mathbf{istr}}\left[{\mathbf{n}}\right]1$ is the address of the last nonzero element in
$C$ plus one.

14:
$\mathbf{idiag}\left[{\mathbf{n}}\right]$ – Integer
Output

On exit:
${\mathbf{idiag}}\left[\mathit{i}1\right]$, for
$\mathit{i}=1,2,\dots ,{\mathbf{n}}$ holds the index in the arrays
a,
irow and
icol which holds the diagonal element in row
$\mathit{i}$ of the matrix
$C$.

15:
$\mathbf{nnzc}$ – Integer *
Output

On exit: the number of nonzero elements in the matrix $C$.

16:
$\mathbf{npivm}$ – Integer *
Output

On exit: if
${\mathbf{npivm}}>0$ it gives the number of pivots which were modified during the factorization to ensure that
$M$ exists.
If
${\mathbf{npivm}}=1$ no pivot modifications were required, but a local restart occurred (
Section 9.4). The quality of the preconditioner will generally depend on the returned value of
npivm. If
npivm is large the preconditioner may not be satisfactory. In this case it may be advantageous to call
f11dac again with an increased value of
lfill, a reduced value of
dtol, or
${\mathbf{pstrat}}=\mathrm{Nag\_SparseNsym\_CompletePiv}$.

17:
$\mathbf{fail}$ – NagError *
Input/Output

The NAG error argument (see
Section 7 in the Introduction to the NAG Library CL Interface).
6
Error Indicators and Warnings
 NE_2_INT_ARG_LT

On entry, ${\mathbf{la}}=\u2329\mathit{\text{value}}\u232a$ while ${\mathbf{nnz}}=\u2329\mathit{\text{value}}\u232a$. These arguments must satisfy ${\mathbf{la}}\ge 2\times {\mathbf{nnz}}$.
 NE_ALLOC_FAIL

Dynamic memory allocation failed.
 NE_BAD_PARAM

On entry, argument
milu had an illegal value.
On entry, argument
pstrat had an illegal value.
 NE_INT_2

On entry, ${\mathbf{nnz}}=\u2329\mathit{\text{value}}\u232a$, ${\mathbf{n}}=\u2329\mathit{\text{value}}\u232a$.
Constraint: $1\le {\mathbf{nnz}}\le {{\mathbf{n}}}^{2}\text{.}$.
 NE_INT_ARG_LT

On entry, ${\mathbf{n}}=\u2329\mathit{\text{value}}\u232a$.
Constraint: ${\mathbf{n}}\ge 1$.
 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.
 NE_INVALID_ROWCOL_PIVOT

On entry,
${\mathbf{pstrat}}=\mathrm{Nag\_SparseNsym\_UserPiv}$, but one or both of the arrays
ipivp and
ipivq does not represent a valid permutation of the integers in
$\left[1,{\mathbf{n}}\right]$. An input value of
ipivp or
ipivq is either out of range or repeated.
 NE_NONSYMM_MATRIX_DUP

A nonzero matrix element has been supplied which does not lie within the matrix
$A$, is out of order or has duplicate row and column indices, i.e., one or more of the following constraints has been violated:
 $1\le {\mathbf{irow}}\left[\mathit{i}\right]\le {\mathbf{n}}$, $1\le {\mathbf{icol}}\left[\mathit{i}\right]\le {\mathbf{n}}$, for $\mathit{i}=0,1,\dots ,{\mathbf{nnz}}1$.
 ${\mathbf{irow}}\left[\mathit{i}1\right]<{\mathbf{irow}}\left[\mathit{i}\right]$, or
 ${\mathbf{irow}}\left[\mathit{i}1\right]={\mathbf{irow}}\left[\mathit{i}\right]$ and ${\mathbf{icol}}\left[\mathit{i}1\right]<{\mathbf{icol}}\left[\mathit{i}\right]$, for $\mathit{i}=1,2,\dots ,{\mathbf{nnz}}1$.
Call
f11zac to reorder and sum or remove duplicates.
 NE_REAL_INT_ARG_CONS

On entry, ${\mathbf{dtol}}=\u2329\mathit{\text{value}}\u232a$ and ${\mathbf{lfill}}=\u2329\mathit{\text{value}}\u232a$. These arguments must satisfy ${\mathbf{dtol}}\ge 0.0$ if ${\mathbf{lfill}}<0$.
7
Accuracy
The accuracy of the factorization will be determined by the size of the elements that are dropped and the size of any modifications made to the pivot elements. If these sizes are small then the computed factors will correspond to a matrix close to
$A$. The factorization can generally be made more accurate by increasing
lfill, or by reducing
dtol with
${\mathbf{lfill}}<0$.
If
f11dac is used in combination with
f11dcc, the more accurate the factorization the fewer iterations will be required. However, the cost of the decomposition will also generally increase.
8
Parallelism and Performance
f11dac is not threaded in any implementation.
The time taken for a call to f11dac is roughly proportional to ${{\mathbf{nnzc}}}^{2}/{\mathbf{n}}$.
If
${\mathbf{lfill}}\ge 0$ the amount of fillin occurring in the incomplete factorization is controlled by limiting the maximum
level of fillin to
lfill. The original nonzero elements of
$A$ are defined to be of level
$0$. The fill level of a new nonzero location occurring during the factorization is defined as:
where
${k}_{e}$ is the level of fill of the element being eliminated, and
${k}_{c}$ is the level of fill of the element causing the fillin.
If
${\mathbf{lfill}}<0$ the fillin is controlled by means of the
drop tolerance dtol. A potential fillin element
${a}_{ij}$ occurring in row
$i$ and column
$j$ will not be included if:
where
$\alpha $ is the maximum absolute value element in the matrix
$A$.
For either method of control, any elements which are not included are discarded unless ${\mathbf{milu}}=\mathrm{Nag\_SparseNsym\_ModFact}$, in which case their contributions are subtracted from the pivot element in the relevant elimination row, to preserve the rowsums of the original matrix.
Should the factorization process break down a local restart process is implemented as described in
Section 9.4. This will affect the amount of fill present in the final factorization.
The factorization is constructed row by row. At each elimination stage a row index is chosen. In the case of complete pivoting this index is chosen in order to reduce fillin. Otherwise the rows are treated in the order given, or some userdefined order.
The chosen row is copied from the original matrix
$A$ and modified according to those previous elimination stages which affect it. During this process any fillin elements are either dropped or kept according to the values of
lfill or
dtol. In the case of a modified factorization (
${\mathbf{milu}}=\mathrm{Nag\_SparseNsym\_ModFact}$) the sum of the dropped terms for the given row is stored.
Finally the pivot element for the row is chosen and the multipliers are computed for this elimination stage. For partial or complete pivoting the pivot element is chosen in the interests of stability as the element of largest absolute value in the row. Otherwise the pivot element is chosen in the order given, or some userdefined order.
If the factorization breaks down because the chosen pivot element is zero, or there is no nonzero pivot available, a local restart recovery process is implemented. The modification of the given pivot row according to previous elimination stages is repeated, but this time keeping all fill. Note that in this case the final factorization will include more fill than originally specified by the usersupplied value of
lfill or
dtol. The local restart usually results in a suitable nonzero pivot arising. The original criteria for dropping fillin elements is then resumed for the next elimination stage (hence the
local nature of the restart process). Should this restart process also fail to produce a nonzero pivot element an arbitrary unit pivot is introduced in an arbitrarily chosen column.
f11dac returns an integer argument
npivm which gives the number of these arbitrary unit pivots introduced. If no pivots were modified but local restarts occurred
${\mathbf{npivm}}=1$ is returned.
There is unfortunately no choice of the various algorithmic arguments which is optimal for all types of matrix, and some experimentation will generally be required for each new type of matrix encountered.
If the matrix
$A$ is not known to have any particular special properties the following strategy is recommended. Start with
${\mathbf{lfill}}=0$ and
${\mathbf{pstrat}}=\mathrm{Nag\_SparseNsym\_CompletePiv}$. If the value returned for
npivm is significantly larger than zero, i.e., a large number of pivot modifications were required to ensure that
$M$ existed, the preconditioner is not likely to be satisfactory. In this case increase
lfill until
npivm falls to a value close to zero.
If
$A$ has nonpositive offdiagonal elements, is nonsingular, and has only nonnegative elements in its inverse, it is called an ‘Mmatrix’. It can be shown that no pivot modifications are required in the incomplete
$LU$ factorization of an Mmatrix (
Meijerink and Van der Vorst (1977)). In this case a good preconditioner can generally be expected by setting
${\mathbf{lfill}}=0$,
${\mathbf{pstrat}}=\mathrm{Nag\_SparseNsym\_NoPiv}$ and
${\mathbf{milu}}=\mathrm{Nag\_SparseNsym\_ModFact}$.
Some illustrations of the application of
f11dac to linear systems arising from the discretization of twodimensional elliptic partial differential equations, and to randomvalued randomly structured linear systems, can be found in
Salvini and Shaw (1996).
Although it is not their primary purpose
f11dac and
f11dbc may be used together to obtain a
direct solution to a nonsingular sparse linear system. To achieve this the call to
f11dbc should be preceded by a
complete $LU$ factorization
A complete factorization is obtained from a call to
f11dac with
${\mathbf{lfill}}<0$ and
${\mathbf{dtol}}=0.0$, provided
${\mathbf{npivm}}\le 0$ on exit. A positive value of
npivm indicates that
$A$ is singular, or illconditioned. A factorization with positive
npivm may serve as a preconditioner, but will not result in a direct solution. It is therefore
essential to check the output value of
npivm if a direct solution is required.
The use of
f11dac and
f11dbc as a direct method is illustrated in
Section 10 in
f11dbc.
10
Example
This example program reads in a sparse matrix $A$ and calls f11dac to compute an incomplete $LU$ factorization. It then outputs the nonzero elements of both $A$ and $C=L+{D}^{1}+U2I$.
The call to f11dac has ${\mathbf{lfill}}=0$, and ${\mathbf{pstrat}}=\mathrm{Nag\_SparseNsym\_CompletePiv}$, giving an unmodified zerofill $LU$ factorization, with row pivoting for sparsity and column pivoting for stability.
10.1
Program Text
10.2
Program Data
10.3
Program Results