# 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:

$cat test.py #!/usr/bin/env python from __future__ import division, print_function, absolute_import import dolfin as df ORDER=1 def u0_boundary(x, on_boundary): return on_boundary mesh = df.UnitSphere(30) V = df.FunctionSpace(mesh, 'Lagrange', ORDER) u = df.TrialFunction(V) v = df.TestFunction(V) bc = df.DirichletBC(V, df.Constant('42'), u0_boundary) a = df.inner(df.nabla_grad(u), df.nabla_grad(v)) * df.dx L = df.Constant(0) * v * df.dx psi = df.Function(V) df.set_log_level(df.PROGRESS) df.solve(a == L, psi, bc, solver_parameters = {'linear_solver': 'cg', 'preconditioner': 'ilu'}) However, this fails with$ 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>
'preconditioner': 'ilu'})
File "/usr/lib/python2.7/dist-packages/dolfin/fem/solving.py", line 250, in solve
_solve_varproblem(*args, **kwargs)
File "/usr/lib/python2.7/dist-packages/dolfin/fem/solving.py", line 273, in _solve_varproblem
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
***
***
*** 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_INDEFINITE_PC, norm 2.779098e+03).
*** Where: This error was encountered inside PETScKrylovSolver.cpp.
*** -------------------------------------------------------------------------

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!

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 Revision history for this message Johannes Ring (johannr) said on 2014-01-14: #1