# Weakform of nonlinear equation

Dear all

How can I represent this pde using ufl.

-d/dx(du/dx) - 2u*du/dx = 0

I'm having trouble particularly with 2u*du/dx.

Thanks

Jack

## Question information

- Language:
- English Edit question

- Status:
- Solved

- For:
- DOLFIN Edit question

- Assignee:
- No assignee Edit question

- Solved by:
- Jack

- Solved:
- 2010-03-27

- Last query:
- 2010-03-27

- Last reply:
- 2010-03-27

## This question was reopened

- 2010-03-27 by Jack

Anders Logg (logg) said : | #1 |

On Fri, Mar 26, 2010 at 10:11:24PM -0000, Jack wrote:

> New question #105644 on DOLFIN:

> https:/

>

> Dear all

>

> How can I represent this pde using ufl.

>

> -d/dx(du/dx) - 2u*du/dx = 0

>

> I'm having trouble particularly with 2u*du/dx.

>

> Thanks

>

> Jack

Does it not work if you write 2*u*u.dx(0)*dx?

--

Anders

Jack (attacking-chess) said : | #2 |

Thanks Anders Logg, that solved my question.

Jack (attacking-chess) said : | #3 |

Thanks Anders.

Jack (attacking-chess) said : | #4 |

I just spotted that ffc -l dolfin Nonlinear.ufl gives this error at the end:

Compiler stage 2: Computing intermediate representation

-------

Computing representation of 1 elements

Computing representation of 1 dofmaps

Computing representation of integrals

Computing tensor representation

Extracting monomial form representation from UFL form

Transforming monomial form to reference element

Precomputing integrals on reference element

Precomputing integrals on reference element

Unable to extract all indices.

*** FFC: Unable to extract all indices.

*** FFC: To get more information about this error, rerun FFC with --debug.

This is what I have in my UFL form file

element = FiniteElement(

v = TestFunction(

du = TrialFunction(

u = Coefficient(

L = inner(grad(v), grad(u))*dx - 2*u*u.dx(0)*dx

a = derivative(L, u, du)

Thanks

Jack

Jack (attacking-chess) said : | #5 |

I forgot to multiply the second term by the TestFunction.