MATLAB: Adding Matrices Diagonally for FEM

Question

Hello.

I am trying to make a diagnoal matrix in which the three matrices overlap.

I am attaching a picture which will make things more clear.

I can do it for 2×2, but for 3×3 or more like 12×12. I am having confusion.

Can somebody help me expand the logic of 2×2, that would be very nice

here is the code:

B = [ 1 -1;

-1 1];

C = [ 1 -1;

-1 1];

D = [ 1 -1;

-1 1];

T = zeros(4,4)

d=zeros(2,2,3);

d(:,:,1) = B;

d(:,:,2) = C;

d(:,:,3) = D;

for k = 1:1:3

for i = 1:1:2

for j = 1:1:2

T(i+k-1,j+k-1) = T(i+k-1,j+k-1) + d(i,j,k);

end

end

end

Answer 1

A(:,:,1) = [1 -1 1;-1 2 -1; 1 -1 1];

A(:,:,3) = [1 -1 1;-1 2 -1; 1 -1 1];

A(:,:,5) = [1 -1 1;-1 2 -1; 1 -1 1];

R=zeros(7,7)

for n=1:2:5

i=n+[0 1 2]

j=i

R(i,j)=R(i,j)+A(:,:,n)

end