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# PCFactorSetDropTolerance
The preconditioner will use an `PCILU` based on a drop tolerance. 
## Synopsis
```
#include "petscpc.h" 
PetscErrorCode PCFactorSetDropTolerance(PC pc, PetscReal dt, PetscReal dtcol, PetscInt maxrowcount)
```
Logically Collective


## Input Parameters

- ***pc -*** the preconditioner context
- ***dt -*** the drop tolerance, try from 1.e-10 to .1
- ***dtcol -*** tolerance for column pivot, good values [0.1 to 0.01]
- ***maxrowcount -*** the max number of nonzeros allowed in a row, best value
depends on the number of nonzeros in row of original matrix



## Options Database Key

- ***-pc_factor_drop_tolerance <dt,dtcol,maxrowcount> -*** Sets drop tolerance





## Note
There are NO default values for the 3 parameters, you must set them with reasonable values for your
matrix. We don't know how to compute reasonable values.


## See Also
 `PCILU`

## Level
intermediate

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

## Implementations

<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/pc/impls/factor/factimpl.c.html#PCFactorSetDropTolerance_Factor">PCFactorSetDropTolerance_Factor in src/ksp/pc/impls/factor/factimpl.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/pc/impls/factor/ilu/ilu.c.html#PCFactorSetDropTolerance_ILU">PCFactorSetDropTolerance_ILU in src/ksp/pc/impls/factor/ilu/ilu.c</A><BR>


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


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