New subject: Solver error -- Provided residual is not a linear form

15 May 2017

      Hi Anna,

The proximate cause of your error is line 24 of solvers.py which contains
multiple subexpressions of the form:

Z0 - H - tan(a)*(x-L))

Here, Z0 is a Function but x is a numpy array containing coordinate values.
You are therefore subtracting a numpy array from a Function. This makes no
mathematical sense and, unsurprisingly, causes UFL to choke.

Regards,

David

On Mon, 15 May 2017 at 13:25 Homolya, Miklós <m.homolya14@imperial.ac.uk>
wrote:
...
If you want to preserve the symbolic structure of maximum and its
dependence on u, then you need neither interpolate nor assign.
maximum = 0.5*(u + abs(u))
F = v*(u + maximum)*dx
On 15/05/17 10:06, Anna Kalogirou wrote:
Hi David,
What I would like to do is to define a residual that includes some
nonlinear function depending on the unknown, similar to the example with
the maximum below (ultimately, I need to use functions abs, exp, tan). I do
want to dependence of maximum on u to appear in the definition of the
residual somehow; how can I do that then?
Thanks,
Anna.
On 15 May 2017, at 09:44, David Ham <David.Ham@imperial.ac.uk> wrote:
On Sun, 14 May 2017 at 21:31 Anna Kalogirou <A.Kalogirou@leeds.ac.uk>
wrote:
...
Following up on my previous email, I have found a workaround to those
issues, but now I have a problem with the solvers:
Traceback (most recent call last):
  File
"/Users/matak/Downloads/Inequality-constraint-Rockafellar/solvers.py", line
32, in solver_F
    F_problem = NonlinearVariationalProblem(F_solve, w)#, aP=Ap)
  File
"/Users/matak/firedrake/src/firedrake/firedrake/variational_solver.py",
line 47, in __init__
    raise ValueError("Provided residual is not a linear form")
ValueError: Provided residual is not a linear form
There two issues here:
1. I cannot give a preconditioning matrix aP -- is this not possible for
nonlinear problems?
As the documentation for NonlinearVariationalProblem suggests, you can
provide Jp, a UFL form for the preconditioning Jacobian. This is the
equivalent of aP.
...
2. I suspect the error above is because I have functions that depend on
the solution, and I don’t know if that is acceptable in firedrake. I have
something like the following:
u = Function(V)
v = TestFunction(V)
maximum = interpolate(0.5*(u + abs(u)), V)
F = v*(u + maximum)*dx
Actually, as Colin sort of says, as far as Firedrake is concerned, maximum
does not depend on u. This is because doing the interpolation breaks the
symbolic link between u and maximum. In the code you have written, maximum
does not get updated inside the variational solver as the guess for u gets
updated. Whether that is what you wanted to do or not depends on the maths
of your problem.
The error you are seeing, however, is caused by the Form having two
arguments. This almost certainly means you have a TrialFunction in your
nonlinear problem, which makes no sense.
Regards,
David
...
Best wishes,
Anna.
--
Dr David Ham
Department of Mathematics
Imperial College London
_______________________________________________
firedrake mailing list
firedrake@imperial.ac.uk
https://mailman.ic.ac.uk/mailman/listinfo/firedrake
_______________________________________________
firedrake mailing listfiredrake@imperial.ac.ukhttps://mailman.ic.ac.uk/mailman/listinfo/firedrake
--
Dr David Ham
Department of Mathematics
Imperial College London

Re: [firedrake] Solver error -- Provided residual is not a linear form

David Ham

Anna Kalogirou

tags

participants (2)