[petsc-users] TSSetIFunction

[Jed Brown]

"Jin, Shuangshuang" <Shuangshuang.Jin at pnnl.gov> writes:

> As a test, I changed the line “  f[1] = (x[1]*x[1]*x[1]/3 - x[1])-x[0];” to “f[1] = (PetscRealPart(x[1])*PetscRealPart(x[1])*PetscRealPart(x[1])/3 - PetscRealPart(x[1]))-PetscRealPart(x[0]);”, and it runs with the same results as I run ex19.c alone.
>
> However, I don’t understand why I only need to change this line but leave nothing else such as the line before it “f[0] = xdot[0] + x[1];” unchanged?

You were just missing an overload.

> And can anyone please tell me in ex19.c what does this part of code do?
>
>   PetscErrorCode ierr;
>
>   PetscFunctionBegin;
>   {
>     const PetscReal
>       A[3][3] = {{0,0,0},
>                  {0.41421356237309504880,0,0},
>                  {0.75,0.25,0}},
>       At[3][3] = {{0,0,0},
>                   {0.12132034355964257320,0.29289321881345247560,0},
>                   {0.20710678118654752440,0.50000000000000000000,0.29289321881345247560}};
>       ierr = TSARKIMEXRegister("myark2",2,3,&At[0][0],PETSC_NULL,PETSC_NULL,&A[0][0],PETSC_NULL,PETSC_NULL,PETSC_NULL,PETSC_NULL,0,PETSC_NULL,PETSC_NULL);CHKERRQ(ierr);
>   }
>   PetscFunctionReturn(0);
>
> It seems to me an initialization of the A matrix. But I looked into TSARKIMEXRegister, still couldn’t understand how it works.

This registers an additive Runge-Kutta IMEX method by providing its
"Butcher table". Don't worry about this unless you are designing
integration methods. This method, and many others are already in the
library.