Interpolation and Nonlinearities

Question

Suppose I have a nonlinearity f(x) of some sort that appears in my PDE. One way of handling the nonlinearity, that works fine, is to project it into an appropriate function space using project:

f = project(u**3, V)

However, when I try to approximate the nonlinearity by interpolation, I get an error:

f = interpolate(u**3, V)

gives

TypeError: in method 'Function_interpolate', argument 2 of type 'dolfin::GenericFunction const &'

If this is not the way to handle interpolation, is there a way to do it?

Answer 1

If u is a function in the space V, you may work with u.vector() to
get what you want. If u is not in V then it is not clear what interpolate in
general means (at least not to me). However project always has a clear meaning.