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How do I insert multiple vectors as diagonals in the same matrix?

Asked by Nicholas Boccella on 16 Apr 2019
Latest activity Answered by Kelly Kearney
on 16 Apr 2019
I'm trying to use a forward-difference method to solve a parabolic PDE. I need a matrix with the main diagonal and both diagonals one spot off from the main diagonal to be defined and the rest 0. My current code is this:
v = lambda*ones(1,m-2);
v1 = (1-2*lambda)*ones(1,m-1);
A = diag(v,1);
disp(A);
A = diag(v1,0);
disp(A);
A = diag(v,-1);
disp(A);
lambda is just a scalar. This is what my intuition told me to do, but after each time I do A=diag, it overwrites the previous matrix. I need the vector v to be on the off diagonals -1 and 1, and the vector v1 to be on the main diagonal. This is probably pretty easy but I am just missing it. Any help would be appreciated.

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1 Answer

Answer by Kelly Kearney
on 16 Apr 2019
 Accepted Answer

You're close... you just need to add the matrices together:
A = diag(v,1) + diag(v,-1) + diag(v1,0)

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