How can I enter a source term which is a vector, to solve -div(cgradu)=F(F a vector)?

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Hello, I want to solve the pde (-div(cgrad(u))= F) with F given as a column vector of length N. I used the assempde to solve directly the pde with that F but it always answers me
??? Error using ==> plus Matrix dimensions must agree.
Error in ==> assempde at 245 KK=K+M+Q;
Error in ==> myprogram at 119 u=assempde(b,p,e,t,c,a,F);
Any help, please? Thank you in advance
  1 comentario
Walter Roberson
Walter Roberson el 17 de Dic. de 2013
At the MATLAB command line, command
dbstop if error
and then run the program. When it stops, what does size(K), size(M), size(Q) indicate? Then, give the command
dbup
and show size(b), size(p), size(e), size(t), size(c), size(a), size(F)

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Respuestas (2)

Bill Greene
Bill Greene el 12 de Dic. de 2013
I assume N is > 1? How did you define the first argument to assempde, b? I'm guessing that the problem at line 245 of assempde is caused by K and M having a different size from Q where Q is calculated based on your boundary condition definition, b.
Bill
  3 comentarios
Bill Greene
Bill Greene el 13 de Dic. de 2013
OK, well I suspect there is a problem in your pdebound function. There is no way to say more with only the information you have provided.
Bill
Batoul
Batoul el 17 de Dic. de 2013
Editada: Walter Roberson el 17 de Dic. de 2013
Yes but when i give F as a constant(i.e. N=1), it works... and for the pdebound i used this
function [qmatrix,gmatrix,hmatrix,rmatrix] = pdebound(p,e,~,~)
ne = size(e,2); % number of edges
qmatrix = zeros(1,ne);
gmatrix = qmatrix;
hmatrix = zeros(1,2*ne);
rmatrix = hmatrix;
for k = 1:ne
hmatrix(k) = 1;
hmatrix(k+ne) = 1;
rmatrix(k) =0;
rmatrix(k+ne) =0;
end
since i have u=0 on the boundary
So, i can't find the mistake :(

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Bill Greene
Bill Greene el 17 de Dic. de 2013
Yes, your pdebound function is incorrect for a system of PDE (N>1). Specifically, as this documentation page shows,
the returned matrices must have these dimensions (for this example, N=3):
N = 3; % Set N = the number of equations
ne = size(e,2); % number of edges
qmatrix = zeros(N^2,ne);
gmatrix = zeros(N,ne);
hmatrix = zeros(N^2,2*ne);
rmatrix = zeros(N,2*ne);
I recommend taking a close look at the www page I list above so you can see exactly how those matrices must be defined.
Bill

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