Sets the type of coarse grid interpolation to use
pc - the preconditioner context
type - either PC_EXOTIC_FACE or PC_EXOTIC_WIREBASKET (defaults to face)
The face based interpolation has 1 degree of freedom per face and ignores the edge and vertex values completely in the coarse problem. For any seven point stencil the interpolation of a constant on all faces into the interior is that constant.
The wirebasket interpolation has 1 degree of freedom per vertex, per edge and per face. A constant on the subdomain boundary is interpolated as that constant in the interior of the domain.
The coarse grid matrix is obtained via the Galerkin computation A_c = R A R^T, hence if A is nonsingular A_c is also nonsingular.
Both interpolations are suitable for only scalar problems.