# Issue fem.create\_form with DG0 function and ufl.derivative **URL:** \ **Category:** dolfinx\ **Created:** [January 12, 2025, 6:00pm UTC](https://fenicsproject.discourse.group/t/issue-fem-create-form-with-dg0-function-and-ufl-derivative/16718 "2025-01-12T18:00:33Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![ordinary-slim](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/ordinary-slim/32/4821_2.png) [@ordinary-slim](https://fenicsproject.discourse.group/u/ordinary-slim)\ **Post date:** [January 12, 2025, 6:00pm UTC](https://fenicsproject.discourse.group/t/issue-fem-create-form-with-dg0-function-and-ufl-derivative/16718/1 "2025-01-12T18:00:33Z")
I am unsure whether this is a bug or I have a mistake. Consider this MWE: ```python from dolfinx import fem, mesh import ufl from mpi4py import MPI import numpy as np def main(): els_side = 4 domain = mesh.create_unit_square(MPI.COMM_WORLD, els_side, els_side) V = fem.functionspace(domain, ("Lagrange", 1),) DG0 = fem.functionspace(domain, ("Discontinuous Lagrange", 0)) solution = fem.Function(V) dg0_func = True ufl_derivative = True if dg0_func: conductivity = fem.Function(DG0) conductivity.x.array[:] = 1.0 else: conductivity = fem.Constant(domain, 1.0) v = ufl.TestFunction(V) dx = ufl.dx if ufl_derivative: a_ufl = conductivity * ufl.dot(ufl.grad(solution), ufl.grad(v)) * dx form = ufl.derivative(a_ufl, solution) else: u = ufl.TrialFunction(V) form = conductivity * ufl.dot(ufl.grad(u), ufl.grad(v)) * dx j_compiled = fem.compile_form( domain.comm, form, form_compiler_options={"scalar_type": np.float64}) coefficient_map = {} for coeff in form.coefficients(): coefficient_map[coeff] = coeff constant_map = {} for const in form.constants(): constant_map[const] = const j_instance = fem.create_form(j_compiled, [V, V], msh=domain, subdomains={}, coefficient_map = coefficient_map, constant_map = constant_map) A = fem.assemble_matrix(j_instance) print(A.data) if __name__ ==" __main__": main() ``` When using `ufl.derivative` together with a DG0 function in the form, the matrix assembled from compiling and creating the bilinear form in two steps is wrong. Here I expect to see a stiffness matrix but I obtain a null matrix. In this MWE, both the flags `ufl_derivative` and `dg0_func` are set to `True`. If either of them is `False`, I obtain the expected stiffness matrix. Moreover, if both of them are `True`, the `coefficient_map` argument of `create_form` doesn’t even need the conductivity to run, when it normally complains about missing constants or coefficients. ---
**Author:** ![Stein](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/stein/32/8751_2.png) [@Stein](https://fenicsproject.discourse.group/u/Stein)\ **Post date:** [January 13, 2025, 9:59am UTC](https://fenicsproject.discourse.group/t/issue-fem-create-form-with-dg0-function-and-ufl-derivative/16718/2 "2025-01-13T09:59:04Z")
The issue is the coefficient map. It seems to me that this must map to the `fem.function`, not back to the `ufl.coefficient`. This runs: `coefficient_map = {form.coefficients()[0]:conductivity}` There are two `coefficients` in your form; the one associated with `conductivity` and the one associated to `solution`. Conductivity happens to be the first, hence the `[0]`. Not sure how to do that procedurally… ---
**Author:** ![dokken](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/dokken/32/1560_2.png) [@dokken](https://fenicsproject.discourse.group/u/dokken)\ **Post date:** [January 13, 2025, 10:31am UTC](https://fenicsproject.discourse.group/t/issue-fem-create-form-with-dg0-function-and-ufl-derivative/16718/3 "2025-01-13T10:31:06Z")
There is a bug in the packing of constants and coefficients in DOLFINx, see: > > > Initially reported at: https://fenicsproject.discourse.group/t/issue-fem-create-…form-with-dg0-function-and-ufl-derivative/16718?u=dokken > > When constants and/or coefficients are eliminated from a ufl-form due to differentiation, this was not taken into account when creating the maps for coefficients. > > This PR corrects this behavior, ensuring that all remaining coefficients in the form are provided, as well as mapping them to the correct index. However, I don’t quite understand your need for `compile_form` and `create_form` in the example above, as the problem is not domain independent (similarly constants and coefficients are defined on the actual grid, not in a symbolical fashion). Anyhow, with the proposed PR above, your code should run correctly. ---
**Author:** ![ordinary-slim](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/ordinary-slim/32/4821_2.png) [@ordinary-slim](https://fenicsproject.discourse.group/u/ordinary-slim)\ **Post date:** [January 13, 2025, 10:46am UTC](https://fenicsproject.discourse.group/t/issue-fem-create-form-with-dg0-function-and-ufl-derivative/16718/4 "2025-01-13T10:46:20Z")
@Stein many thanks for having a look. `form.coefficients()[0]` is actually `solution` so the coefficient map you propose is `{solution : conductivity}`, which works but doesn’t make sense. @dokken thanks a lot as always. The example above is a MWE replicating the bug. I want to run this with 3D printing and domain decomposition examples which rely on changing the `subdomain_data` for both boundary and domain integrals. If I can get this to work it will avoid me recompiling my forms. ---
**Author:** ![Stein](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/stein/32/8751_2.png) [@Stein](https://fenicsproject.discourse.group/u/Stein)\ **Post date:** [January 13, 2025, 11:39am UTC](https://fenicsproject.discourse.group/t/issue-fem-create-form-with-dg0-function-and-ufl-derivative/16718/5 "2025-01-13T11:39:45Z")
Ah Ok, I got weirdly lucky then… I now wonder why my modification gave the expected result 😅