% create a rectangular function
u0 = zeros(1,100);
u0(40:60) =1;

% run the diffusion K = 1, mu = 0, dt = 0.1, nt = 10
[x,u] = diff1d_implicit(0,100,u0,1,0,2,11);

% plot the evolution of the rectangle
figure
hold
for k = 1:10
    plot(x,u(:,k));
end