Hi everyone,

I have fit an exponential function to some data, and now I want to use this exponential function with T, the solution to my finite element simulation, as an input. However, I am encountering the following error:

```
File "target-spotsize-fixedP.py", line 47, in exponenial_func
return a*exp(-b/x)
ufl.log.UFLValueError: Invalid type conversion: [...] can not be converted to any UFL type
```

However I cannot see where my problem is, is there something wrong with using Project() in this case to output the value of this exponential function across my whole mesh?

Below is my simplified code:

```
from __future__ import print_function
from fenics import *
import numpy as np
import matplotlib.pyplot as plt
from dolfin import *
from ufl import as_tensor
from ufl import Index
import math
from scipy.optimize import curve_fit
from mpi4py import MPI
mesh = UnitSquareMesh(16,16)
x = [0.01,10,20,30,40,50,60,80,100,1000,10000] #T range
y = [0,0.1,0.36,0.61,0.72,0.78,0.82,0.84,0.87,0.99,0.99999] # y to fit
def exponenial_func(x, a, b):
return a*np.exp(-b/x)
popt1, pcov1 = curve_fit(exponenial_func, x, y ,p0 = [2, 10])
Space = FunctionSpace(mesh,'P', 1)
T = project(Expression("x[0]+0.1*x[1]", degree=2), Space)
vp_space = FunctionSpace(mesh, 'CG',1)
vp_out = Function(vp_space, name = '$T$')
vp_ = exponenial_func(T,*popt1)
vp_out = project(vp_, vp_space,solver_type='cg')
```