### Simple question about divergence of a vector field

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3 months ago by
I am new to FEniCS and I am trying to verify a small portion of my code that is giving me unexpected results. I am trying to verify  $\Delta F=4$ΔF=4 where  $F=x^2+y^2$F=x2+y2 . My simple code snippet is as follows:

cells_per_side = 32
mesh = UnitSquareMesh(cells_per_side,cells_per_side)
V = FunctionSpace(mesh, 'P', 1)
F = project(Expression('x[0]*x[0] + x[1]*x[1]',degree=2), V)
​

$F$F  and its gradients seem to compute just fine but I get a function identical to zero for  $\Delta F$ΔF . Is there something obvious that I am missing here?

Community: FEniCS Project
3
Is it because you project degree=2 to FunctionSpace P1, which seems too low in this case?
written 3 months ago by Corwinpro
1
You might also want to use interpolate(...) instead of project(...), since projection only results in approximate values.
written 3 months ago by Adam Janecka
Too right, too right.
written 3 months ago by pf4d