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# SNESNGS
Either calls the user-provided solution routine provided with `SNESSetNGS()` or does a finite difference secant approximation using coloring. 


## Options Database Keys

- ***-snes_ngs_sweeps <n> -*** Number of sweeps of nonlinear GS to apply
- ***-snes_ngs_atol <atol> -*** Absolute residual tolerance for nonlinear GS iteration
- ***-snes_ngs_rtol <rtol> -*** Relative residual tolerance for nonlinear GS iteration
- ***-snes_ngs_stol <stol> -*** Absolute update tolerance for nonlinear GS iteration
- ***-snes_ngs_max_it <maxit> -*** Maximum number of sweeps of nonlinea GS to apply
- ***-snes_ngs_secant -*** Use pointwise secant local Jacobian approximation with coloring instead of user provided Gauss-Seidel routine, this is
used by default if no user provided Gauss-Seidel routine is available. Requires either that a `DM` that can compute a coloring
is available or a Jacobian sparse matrix is provided (from which to get the coloring).
- ***-snes_ngs_secant_h <h> -*** Differencing parameter for secant approximation
- ***-snes_ngs_secant_mat_coloring -*** Use the graph coloring of the Jacobian for the secant GS even if a DM is available.
- ***-snes_norm_schedule <none, always, initialonly, finalonly, initialfinalonly> -*** how often the residual norms are computed



## Notes
the Gauss-Seidel smoother is inherited through composition.  If a solver has been created with `SNESGetNPC()`, it will have
its parent's Gauss-Seidel routine associated with it.

By default this routine computes the solution norm at each iteration, this can be time consuming, you can turn this off with `SNESSetNormSchedule()`
or -snes_norm_schedule none


## References

- **** -*** Peter R. Brune, Matthew G. Knepley, Barry F. Smith, and Xuemin Tu, "Composing Scalable Nonlinear Algebraic Solvers",
SIAM Review, 57(4), 2015



## See Also
 `SNESNCG`, `SNESCreate()`, `SNES`, `SNESSetType()`, `SNESSetNGS()`, `SNESType`, `SNESNGSSetSweeps()`, `SNESNGSSetTolerances()`,
`SNESSetNormSchedule()`

## Level
advanced

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/snes/impls/gs/snesgs.c.html#SNESNGS">src/snes/impls/gs/snesgs.c</A>


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[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/snes/impls/gs/snesgs.c)


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