# No convergence for Laplace equation with constant boundary?

Hello,

I'm just beginning to use FEniCS, and I think I'm struggling with some basics. As a trivial test problem, I have tried to solve Laplace's equation in a sphere with constant boundary conditions:

$ cat test.py

#!/usr/bin/env python

from __future__ import division, print_function, absolute_import

import dolfin as df

ORDER=1

def u0_boundary(x, on_boundary):

return on_boundary

mesh = df.UnitSphere(30)

V = df.FunctionSpac

u = df.TrialFunction(V)

v = df.TestFunction(V)

bc = df.DirichletBC(V, df.Constant('42'), u0_boundary)

a = df.inner(

L = df.Constant(0) * v * df.dx

psi = df.Function(V)

df.set_

df.solve(a == L, psi, bc,

However, this fails with

$ python test.py

Solving linear variational problem.

Applying boundary conditions to linear system.

Solving linear system of size 29791 x 29791 (PETSc Krylov solver).

PETSc Krylov solver starting to solve 29791 x 29791 system.

Traceback (most recent call last):

File "test.py", line 21, in <module>

'preconditi

File "/usr/lib/

_solve_

File "/usr/lib/

solver.solve()

RuntimeError:

*** -------

*** DOLFIN encountered an error. If you are not able to resolve this issue

*** using the information listed below, you can ask for help at

***

*** https:/

***

*** Remember to include the error message listed below and, if possible,

*** include a *minimal* running example to reproduce the error.

***

*** -------

*** Error: Unable to solve linear system using PETSc Krylov solver.

*** Reason: Solution failed to converge in 3 iterations (PETSc reason DIVERGED_

*** Where: This error was encountered inside PETScKrylovSolv

*** -------

However, if I replace the boundary condition with 0 instead of 42, things seem to work.

Am I doing something wrong? Why isn't FEniCS finding the trivial solution with psi=42 everywhere?

Thanks!

## Question information

- Language:
- English Edit question

- Status:
- Answered

- For:
- DOLFIN Edit question

- Assignee:
- No assignee Edit question

- Last query:
- 2014-01-08

- Last reply:
- 2014-01-14

Johannes Ring (johannr) said : | #1 |

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