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# SNESNASM
Nonlinear Additive Schwarz solver 
## Options Database Keys

- ***-snes_nasm_log -*** enable logging events for the communication and solve stages
- ***-snes_nasm_type <basic,restrict> -*** type of subdomain update used
- ***-snes_asm_damping <dmp> -*** the new solution is obtained as old solution plus dmp times (sum of the solutions on the subdomains)
- ***-snes_nasm_finaljacobian -*** compute the local and global jacobians of the final iterate
- ***-snes_nasm_finaljacobian_type <finalinner,finalouter,initial> -*** pick state the jacobian is calculated at
- ***-sub_snes_ -*** options prefix of the subdomain nonlinear solves
- ***-sub_ksp_ -*** options prefix of the subdomain Krylov solver
- ***-sub_pc_ -*** options prefix of the subdomain preconditioner





## Note
This is not often used directly as a solver, it converges too slowly. However it works well as a nonlinear preconditioner for
the `SNESASPIN` solver


## Developer Note
This is a non-Newton based nonlinear solver that does not directly require a Jacobian; hence the flag snes->usesksp is set to
false and `SNESView()` and -snes_view do not display a `KSP` object. However, if the flag nasm->finaljacobian is set (for example, if
`SNESNASM` is used as a nonlinear preconditioner for  `SNESASPIN`) then `SNESSetUpMatrices()` is called to generate the
Jacobian (needed by `SNESASPIN`)
and this utilizes the inner `KSP` object for storing the matrices, but the `KSP` is never used for solving a linear system. When `SNESNASM` is
used by `SNESASPIN` they share the same Jacobian matrices because `SNESSetUp()` (called on the outer `SNESASPIN`) causes the inner `SNES`
object (in this case `SNESNASM`) to inherit the outer Jacobian matrices.


## 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
 `SNESCreate()`, `SNES`, `SNESSetType()`, `SNESType`, `SNESNASMSetType()`, `SNESNASMGetType()`, `SNESNASMSetSubdomains()`, `SNESNASMGetSubdomains()`, `SNESNASMGetSubdomainVecs()`, `SNESNASMSetComputeFinalJacobian()`, `SNESNASMSetDamping()`, `SNESNASMGetDamping()`

## Level
advanced

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/snes/impls/nasm/nasm.c.html#SNESNASM">src/snes/impls/nasm/nasm.c</A>


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