# SNESSetLagJacobian#

Set when the Jacobian is rebuilt in the nonlinear solve. See SNESSetLagPreconditioner() for determining how often the preconditioner is rebuilt.

## Synopsis#

#include "petscsnes.h"
PetscErrorCode SNESSetLagJacobian(SNES snes, PetscInt lag)


Logically Collective

## Input Parameters#

• snes - the SNES context

• lag - -1 indicates NEVER rebuild, 1 means rebuild every time the Jacobian is computed within a single nonlinear solve, 2 means every second time the Jacobian is built etc. -2 means rebuild at next chance but then never again

## Options Database Keys#

• -snes_lag_jacobian_persists <true,false> - sets the persistence

• -snes_lag_jacobian <- 2,1,2,…> - sets the lag

• -snes_lag_preconditioner_persists <true,false> - sets the persistence

• -snes_lag_preconditioner <- 2,1,2,…> - sets the lag.

## Notes#

The default is 1

The Jacobian is ALWAYS built in the first iteration of a nonlinear solve unless lag is -1

If -1 is used before the very first nonlinear solve the CODE WILL FAIL! because no Jacobian is used, use -2 to indicate you want it recomputed at the next Newton step but never again (unless it is reset to another value)

SNES, SNESSetTrustRegionTolerance(), SNESGetLagPreconditioner(), SNESSetLagPreconditioner(), SNESGetLagJacobianPersists(), SNESSetLagPreconditionerPersists()