In MATLAB, array indexing starts at 1(not zero).
Inside the function Duffusion() you have a number of for loops in each if-else cases.
for i = 1:(length(x)-1) % Iterate over tri diagonal
L(i,i) = 2/(delta^2)+SigmaA/D; % Diagonal Terms
L(i,i+1) = -1/(delta^2); % Above diagonal term
L(i,i-1) = -1/(delta^2); % Below diagonal term
end
In each of these for loops, you iterate the value of i from 1 to something. But when i=1, you have L(i,i-1) as L(1,0) which is invalid in MATLAB. And thus the error.
