Initialization of u, b, x

110: /*

Create parallel vectors.

– We form 1 vector from scratch and then duplicate as needed.

– When using VecCreate(), VecSetSizes and VecSetFromOptions()

in this example, we specify only the

vector's global dimension; the parallel partitioning is determined at runtime.

– When solving a linear system, the vectors and matrices MUST

be partitioned accordingly. PETSc automatically generates

appropriately partitioned matrices and vectors when MatCreate()

and VecCreate() are used with the same communicator.

– The user can alternatively specify the local vector and matrix

dimensions when more sophisticated partitioning is needed

(replacing the PETSC_DECIDE argument in the VecSetSizes() statement

below).

126: VecCreate(PETSC_COMM_WORLD,&u);

127: VecSetSizes(u,PETSC_DECIDE,m*n);

128: VecSetFromOptions(u);

129: VecDuplicate(u,&b);

130: VecDuplicate(b,&x);