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# PCILU
Incomplete factorization preconditioners. 
## Options Database Keys

- ***-pc_factor_levels <k> -*** number of levels of fill for ILU(k)
- ***-pc_factor_in_place -*** only for ILU(0) with natural ordering, reuses the space of the matrix for
its factorization (overwrites original matrix)
- ***-pc_factor_diagonal_fill -*** fill in a zero diagonal even if levels of fill indicate it wouldn't be fill
- ***-pc_factor_reuse_ordering -*** reuse ordering of factorized matrix from previous factorization
- ***-pc_factor_fill <nfill> -*** expected amount of fill in factored matrix compared to original matrix, nfill > 1
- ***-pc_factor_nonzeros_along_diagonal -*** reorder the matrix before factorization to remove zeros from the diagonal,
this decreases the chance of getting a zero pivot
- ***-pc_factor_mat_ordering_type <natural,nd,1wd,rcm,qmd> -*** set the row/column ordering of the factored matrix
- ***-pc_factor_pivot_in_blocks -*** for block ILU(k) factorization, i.e. with BAIJ matrices with block size larger
than 1 the diagonal blocks are factored with partial pivoting (this increases the
stability of the ILU factorization





## Notes
Only implemented for some matrix format and sequential. For parallel see `PCHYPRE` for hypre's ILU

For `MATSEQBAIJ` matrices this implements a point block ILU

The "symmetric" application of this preconditioner is not actually symmetric since L is not transpose(U)
even when the matrix is not symmetric since the U stores the diagonals of the factorization.

If you are using `MATSEQAIJCUSPARSE` matrices (or `MATMPIAIJCUSPARSE` matrices with block Jacobi), factorization
is never done on the GPU).


## References

- **** -*** T. Dupont, R. Kendall, and H. Rachford. An approximate factorization procedure for solving
self adjoint elliptic difference equations. SIAM J. Numer. Anal., 5, 1968.
- **** -*** T.A. Oliphant. An implicit numerical method for solving two dimensional timedependent diffusion problems. Quart. Appl. Math., 19, 1961.
- **** -*** TONY F. CHAN AND HENK A. VAN DER VORST, APPROXIMATE AND INCOMPLETE FACTORIZATIONS,
Chapter in Parallel Numerical
Algorithms, edited by D. Keyes, A. Semah, V. Venkatakrishnan, ICASE/LaRC Interdisciplinary Series in
Science and Engineering, Kluwer.



## See Also
 `PCCreate()`, `PCSetType()`, `PCType`, `PC`, `PCSOR`, `MatOrderingType`, `PCLU`, `PCICC`, `PCCHOLESKY`,
`PCFactorSetZeroPivot()`, `PCFactorSetShiftSetType()`, `PCFactorSetAmount()`,
`PCFactorSetDropTolerance()`, `PCFactorSetFill()`, `PCFactorSetMatOrderingType()`, `PCFactorSetReuseOrdering()`,
`PCFactorSetLevels()`, `PCFactorSetUseInPlace()`, `PCFactorSetAllowDiagonalFill()`, `PCFactorSetPivotInBlocks()`,
`PCFactorGetAllowDiagonalFill()`, `PCFactorGetUseInPlace()`

## Level
beginner

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/pc/impls/factor/ilu/ilu.c.html#PCILU">src/ksp/pc/impls/factor/ilu/ilu.c</A>

## Examples
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/tutorials/ex52.c.html">src/ksp/ksp/tutorials/ex52.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/tutorials/ex62.c.html">src/ksp/ksp/tutorials/ex62.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/tutorials/ex7.c.html">src/ksp/ksp/tutorials/ex7.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/tutorials/ex7f.F90.html">src/ksp/ksp/tutorials/ex7f.F90</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/tutorials/ex8.c.html">src/ksp/ksp/tutorials/ex8.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/pc/tutorials/ex1.c.html">src/ksp/pc/tutorials/ex1.c</A><BR>


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[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/ksp/pc/impls/factor/ilu/ilu.c)


[Index of all PC routines](index.md)  
[Table of Contents for all manual pages](/manualpages/index.md)  
[Index of all manual pages](/manualpages/singleindex.md)