# Preconditioning

Many iterative solvers have the option to provide left and right preconditioners (`Pl`

and `Pr`

resp.) in order to speed up convergence or prevent stagnation. They transform a problem $Ax = b$ into a better conditioned system $(P_l^{-1}AP_r^{-1})y = P_l^{-1}b$, where $x = P_r^{-1}y$.

These preconditioners should support the operations

`ldiv!(y, P, x)`

computes`P \ x`

in-place of`y`

;`ldiv!(P, x)`

computes`P \ x`

in-place of`x`

;- and
`P \ x`

.

If no preconditioners are passed to the solver, the method will default to

`Pl = Pr = IterativeSolvers.Identity()`

## Available preconditioners

IterativeSolvers.jl itself does not provide any other preconditioners besides `Identity()`

, but recommends the following external packages:

- IncompleteLU.jl for incomplete LU decompositions (using drop tolerance).
- IncompleteSelectedInversion.jl for incomplete LDLt decompositions.
- AlgebraicMultigrid.jl for some algebraic multigrid (AMG) preconditioners.
- Preconditioners.jl which wraps a bunch of preconditioners from other packages. If you are a beginner or want to try different ones quickly, this is good starting place.