# 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: