Exponential of a Function gives negative values.

Question

Hi,

I have a Function variable u on a FunctionSpace V ("Lagrange",1).

If I construct f, where

f = project(exp(u),V)

or

f = project(2.7182818284**u,V)

all values of f are greater than zero.

However, if I construct

f = project(exp(u/Constant(0.1)),V)

or

f = project(2.7182818284**(u/Constant(0.1)),V)

f will be less then zero at certain points.

I don't see how this could be. Has this happened to anyone else?

Unfortunately it's proving difficult constructing a toy model to reproduce the error.

Thanks

[edit] - Here is a toy script that gives the strange result

from dolfin import *

mesh = UnitSquareMesh(10,10)
V = FunctionSpace(mesh,"Lagrange",1)

q = project(Expression("10*sin(x[0]*6)"),V)
f = project(exp(q),V)

print min(f.vector())

I am solving a nonlinear poisson equation and it seems this error really messes with the result.

Comments

Can you use interpolate instead of project ?

---

This worked! Thanks

Answer 1

chris_richardson's suggestion worked.

Here is how I managed to implement interpolate in the toy script.

from dolfin import *

mesh = UnitSquareMesh(10,10)
V = FunctionSpace(mesh,"Lagrange",1)

q = interpolate(Expression("10*sin(x[0]*6)"),V)

f = interpolate(Expression("exp(f)",f=q),V)

print min(f.vector())