Re: [Getfem-users] Warning: Inefficient addition of element in rsvector

[Yves Renard]

Dear David,

You add a global term, so that the stiffness matrix is no longer sparse at all. This is why gmm complains. And the linear system solve will only be possible for a small number of degrees of freedom. This is avoidable with a specific solve of the system, writing explicitely the constraint matrix as a rank one matrix. Of course, a simpler alternative is to prescribe the value of the unknown at any point of the domain (with a pointwise constraint brick, this is very easy) and if there is a specific need for solution to be of zero average, make a post-translation of the solution.

Yves.


Le 23/02/2017 à 11:17, David Danan a écrit :

Dear Getfem users,

i wanted to solve a simple laplacian with pure Neumann boundary condition on a cube.
In order to do so, since the problem is ill-posed, i added the following constraint on the unknown u
 int_omega u dx=0
via a Lagrangian multiplier.

In the code, the left-hand side is defined by
  Model.add_fem_variable("u", mf_u);
  getfem::add_linear_generic_assembly_brick(model, mim, "(Grad_u.Grad_Test_u)");
  model.add_fixed_size_variable("lambda", 1);
  getfem::add_linear_generic_assembly_brick(model, mim, "(lambda*Test_u)");
  getfem::add_linear_generic_assembly_brick(model, mim, "(u*Test_lambda)");

If i am not mistaken, it should correspond to the left hand side described in
https://fenicsproject.org/olddocs/dolfin/1.3.0/python/demo/documented/neumann-poisson/python/documentation.html
for instance
 
As a result, the mean value is relatively small in comparison with u (while not exactly 0).

However, when i run the program, i get the following warning:
Level 2 Warning in ../../src/gmm/gmm_vector.h, line 592: Inefficient addition of element in rsvector with **** non-zero entries

Did i do something wrong? If i didn't, is it possible to disable this warning?

Thanks in advance,
David.

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  Yves Renard (address@hidden)       tel : (33) 04.72.43.87.08
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