%% Method of Lines with D = diffusivity: Fick's 2nd Law of Diffusion
% by Prof. Roche C. de Guzman
%% Custom fx
function Y1 = F(~,Y,D,nx,dx)
c = Y; % assign concentration as Y
Y1 = zeros(nx,1); % temporary derivatives
for i = 1:nx-2 % position counter
Y1(i+1) = D*(c(i+2)-2*c(i+1)+c(i))/dx^2; % solve for derivatives
end
Y1 = [Y1(2); Y1(2:nx-1); Y1(nx-1)]; % derivatives with zero-flux boundary
end