How to write derivative of a function

[jjuma1992]

I am looking for help on how to implement these as the linear functional on variational formulation.

[mapengfei-nwpu]

Let me show you an example on 2D mesh. Here are the expressions of \nabla f,f_x,f_y.

from fenics import *
mesh = UnitSquareMesh(10,10)
V = FunctionSpace(mesh,"P",1)
f = Function(V)
grad_f = grad(f)
f_x = grad(f)[0]
f_y = grad(f)[1]