[petsc-users] Question about TSSetRHSJacobian for linear time dependent problem

[Smith, Barry F.]

> On Jan 25, 2019, at 4:51 PM, Sajid Ali via petsc-users <petsc-users at mcs.anl.gov> wrote:
> 
> Hi, 
> 
> If I have a linear time dependent equation I'm trying to solve using TS, I can use :
> TSSetProblemType(ts,TS_LINEAR);
> TSSetRHSFunction(ts,NULL,TSComputeRHSFunctionLinear,NULL);
> TSSetRHSJacobian(ts,A,A,YourComputeRHSJacobian, &appctx); 
> 
> If the matrix that's being evaluated by YourComputeRHSJacobian is such that the non-zero structure stays the same and only the diagonal changes with time, is there a way to optimize the function so that it doesn't create the whole matrix from scratch each time ? 

If it is a linear PDE u_t = A u  then how can A change with time? It sounds like it really isn't a linear problem?

   Barry

> 
> Naively I can make a dummy matrix and store the copy from t=0 and change the diagonal at each iteration but that unnecessarily doubles the memory consumption, is there a better way? 
> 
> 
> Thank You,
> Sajid Ali
> Applied Physics
> Northwestern University