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# DMComputeL2GradientDiff
This function computes the L_2 difference between the gradient of a function u and an FEM interpolant solution grad u_h. 
## Synopsis
```
#include "petscdm.h"          
#include "petscdmlabel.h"     
#include "petscds.h"     
PetscErrorCode DMComputeL2GradientDiff(DM dm, PetscReal time, PetscErrorCode (**funcs)(PetscInt, PetscReal, const PetscReal[], const PetscReal[], PetscInt, PetscScalar *, void *), void **ctxs, Vec X, const PetscReal n[], PetscReal *diff)
```
Collective


## Input Parameters

- ***dm    -*** The `DM`
, time  - The time
- ***funcs -*** The gradient functions to evaluate for each field component
- ***ctxs  -*** Optional array of contexts to pass to each function, or NULL.
- ***X     -*** The coefficient vector u_h, a global vector
- ***n     -*** The vector to project along



## Output Parameter

- ***diff -*** The diff ||(grad u - grad u_h) . n||_2





## Developer Notes
This API is specific to only particular usage of `DM`

The notes need to provide some information about what has to be provided to the `DM` to be able to perform the computation.


## See Also
 [](ch_dmbase), `DM`, `DMProjectFunction()`, `DMComputeL2Diff()`, `DMComputeL2FieldDiff()`

## Level
developer

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/dm/interface/dm.c.html#DMComputeL2GradientDiff">src/dm/interface/dm.c</A>

## Implementations

<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/dm/impls/plex/plexfem.c.html#DMComputeL2GradientDiff_Plex(DM dm, PetscReal time, PetscErrorCode (**funcs)">DMComputeL2GradientDiff_Plex(DM dm, PetscReal time, PetscErrorCode (**funcs) in src/dm/impls/plex/plexfem.c</A><BR>


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