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# SNESNASM

Nonlinear Additive Schwarz solver [CK02], [BKST15]

## 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_nasm_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

[BKST15]

Peter R. Brune, Matthew G. Knepley, Barry F. Smith, and Xuemin Tu. Composing scalable nonlinear algebraic solvers. SIAM Review, 57(4):535–565, 2015. http://www.mcs.anl.gov/papers/P2010-0112.pdf. URL: http://www.mcs.anl.gov/papers/P2010-0112.pdf, doi:10.1137/130936725.

[CK02]

X.-C. Cai and D. E. Keyes. Nonlinearly preconditioned inexact Newton algorithms. SIAM J. Sci. Comput., 24:183–200, 2002. URL: http://www.cs.colorado.edu/homes/cai/public_html/papers/aspin.ps.

## See Also

SNES: Nonlinear Solvers, `SNESCreate()`, `SNES`, `SNESSetType()`, `SNESType`, `SNESNASMSetType()`, `SNESNASMGetType()`, `SNESNASMSetSubdomains()`, `SNESNASMGetSubdomains()`, `SNESNASMGetSubdomainVecs()`, `SNESNASMSetComputeFinalJacobian()`, `SNESNASMSetDamping()`, `SNESNASMGetDamping()`, `SNESNASMSetWeight()`, `SNESNASMGetSNES()`, `SNESNASMGetNumber()`

## Level

advanced

## Location

src/snes/impls/nasm/nasm.c

Index of all SNES routines
Table of Contents for all manual pages
Index of all manual pages