[petsc-users] stokes and fieldsplit

[Anton Popov]

Hi petsc team,
I have a question about using nested matrices and vectors for coupled 
solution of Stokes system with filedsplit preconditioner.

Is there a simple example explaining how I can create nested matrix and 
vector using
"MatCreateNest" and "VecCreateNest" functions, provided that I already 
have all constituting blocks correctly assembled?
I could not find these example online, and if I do it straightforwardly 
according to documentation, petsc complains that some vector blocks are 
not setup.

Do I need to define velocity and pressure splits using 
"PCFieldSplitSetIS", because if I don't, petsc says "Unhandled case, 
must have at least two fields, not 1!"?

Do I need to explicitly specify the blocksize, if I want to use the 
filedsplit preconditioner for the velocity block?

A small example would be extremely helpful

Thanks a lot,

Anton