### Interaction of MixedFunction's sub(), [], derivative and diff

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I would expect all the vectors assembled in the attached code snippet to be different from zero (ideally the same result, possibly with different permutations and an additional zero for the case with the MixedElement). However, the result varies with different permutations of

File attached: Test.py (2 KB)

`.sub()`

, `as_vector`

, `diff`

and `derivative`

. What I'm missing, and why some of the results are equal to zero? I'm particularly bothered by the apparent need to use `as_vector`

when dealing with a `MixedElement`

.File attached: Test.py (2 KB)

Community: FEniCS Project

### 1 Answer

0

Not all the vectors are supposed to be zero. This can be seen from the following 1D example:

\[ f(u) = \frac{\partial u}{\partial x}, \]

you have in the first case

\[ F = \delta [ f (u) v ] [v] = \frac{\partial}{\partial k} \left. \int \frac{\partial}{\partial x} (u + k v) v \,\mathrm{d} x \right|_{k=0} = \int \frac{\partial v}{\partial x} v \,\mathrm{d} x, \]

which is non-zero, as can be seen from the shape functions. In the second case, you have

\[ F = \frac{\partial^2 u}{\partial x^2} v, \]

which leads to zero values, since \( u \equiv 0 \).
I'm not sure but it looks like

```
from fenics import *
mesh = UnitIntervalMesh(1)
V = FunctionSpace(mesh, 'P', 1)
u = Function(V)
v = TestFunction(V)
f = u.dx(0)
F = derivative(f, u, v)
print(as_backend_type(assemble(F * dx)).vec().view())
F = diff(f, u) * v
print(as_backend_type(assemble(F * dx)).vec().view())
```

For\[ f(u) = \frac{\partial u}{\partial x}, \]

you have in the first case

\[ F = \delta [ f (u) v ] [v] = \frac{\partial}{\partial k} \left. \int \frac{\partial}{\partial x} (u + k v) v \,\mathrm{d} x \right|_{k=0} = \int \frac{\partial v}{\partial x} v \,\mathrm{d} x, \]

which is non-zero, as can be seen from the shape functions. In the second case, you have

\[ F = \frac{\partial^2 u}{\partial x^2} v, \]

which leads to zero values, since \( u \equiv 0 \).

`F`

is somehow not a function of `UF3_6`

anymore. The following seems to work```
from fenics import *
mesh = UnitSquareMesh(1, 1)
P1 = VectorElement('P', mesh.ufl_cell(), 1)
R0 = FiniteElement('R', mesh.ufl_cell(), 0)
mixed_element = MixedElement(P1, R0)
V = FunctionSpace(mesh, mixed_element)
u = Function(V)
(v0, _) = TestFunctions(V)
u0 = u.sub(0)
f = grad(u0)
F = derivative(f, u0, v0)
l = inner(F, Identity(2)) * dx
print(as_backend_type(assemble(l)).vec().view())
```

written
7 months ago by
Adam Janecka

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from fenics import *

mesh = UnitSquareMesh(1, 1)

UF3_ELEMENT = VectorElement("CG", mesh.ufl_cell(), 1, 2)

`R3_ELEMENT = VectorElement("R", mesh.ufl_cell(), 0, 1)`

UF3_6_ELEMENT = MixedElement(UF3_ELEMENT, R3_ELEMENT)

`UF3_6_SPACE = FunctionSpace(mesh, UF3_6_ELEMENT)`

UF3_6 = Function(UF3_6_SPACE)

`VU3_6 = TestFunction(UF3_6_SPACE)`

F = grad(UF3_6.sub(0))

delta_F = derivative(F, UF3_6, VU3_6)

`LinearForm = inner(delta_F, Identity(2)) * dx`

`print(as_backend_type(assemble(LinearForm)).vec().view())`