Dear all,

When we use a PETSc solver for a linear system **Ax=b**, such as

```
set(A, sparams = "-ksp_monitor -ksp_type fgmres -ksp_converged_reason -ksp_atol 1.e-20 -ksp_rtol 1.0e-8 -ksp_max_it 500 -pc_type fieldsplit -pc_fieldsplit_type schur "
+ "-fieldsplit_velocity_pc_type gamg -fieldsplit_velocity_pc_gamg_sym_graph true -fieldsplit_pressure_ksp_max_it 5 "
+ "-fieldsplit_pressure_pc_type jacobi -fieldsplit_velocity_ksp_type preonly -pc_fieldsplit_schur_fact_type full",
fields = ux[], names = names);
```

How can we **specify an initial guess**? such as using the solution of the previous time step rather than always iterating from 0.

In addition, how can we define the **stop criterion** such as

`||Ax_n- b||/||b|| < tol`

.

If the tinitial guess is 0, this is equivalent to define

`-ksp_rtol<tol`

However, if the initial guess in not 0, how shall we define this?

Best,

Yongxing