Hello,

Given the mixed space defined as follows

```
P_z = FiniteElement('P', triangle, 1)
P_omega = VectorElement('P', triangle, 1)
element = MixedElement([P_z, P_omega])
Q_z_omega = FunctionSpace(mesh, element)
```

and the function

```
z_omega = Function(Q_z_omega)
z, omega = split(z_omega)
```

I want to assign to the `z`

part of `z_omega`

a given function profile, derived from the analytical expression x^2/2\, y^2/2. I tried

```
class z_Expression(UserExpression):
def eval(self, values, x):
values[0] = ((x[0])**2)/2.0 * ((x[1])**2)/2.0
def value_shape(self):
return (1,)
z = interpolate(z_Expression(element=Q_z.ufl_element()), Q_z)
```

But the assignement `z = interpolate ... `

seems to have no effect. How may I assign to the `z`

part of `z_omega`

the function x^2/2\, y^2/2 ?

(please not that I am *not* using dolfinx)

Thanks