Re: [Getfem-users] assembling Matrix

[julien pommier]

Hello Mario,
The simplest way to set the Dirichlet boundary condition is, (if you use 
Lagrange finite element such as PK or QK fem), to simply eliminate the 
columns and rows of the matrix that correspound to the imposed dof, for 
example

ddof=gf_mesh_fem_get(mf, 'dof on boundary', ibound);
M(ddof, ddof) = []
K(ddof, ddof) = []

A "better" (more general) way would be to eliminate the Dirichlet 
unknowns this way
(taken from the demo_tripod_alt.m of getfem-interface-2.0):
% handle Dirichlet condition
[H,R]=gf_asm('dirichlet', 2, mim, mfu, mfd, repmat(eye(3),[1,1,nbd]), 
zeros(3, nbd));
[N,U0]=gf_spmat_get(H, 'dirichlet_nullspace', R);
% build the reduced system
KK=N'*K*N;
FF=N'*F;
UU=KK\FF;
% get back to the unreduced solution
U=(N*UU).'+U0;

--
julien