exact = cos(pi * X_ / 2) .* exp( (-pi * pi * nu / 4) * T_);
    figure; mesh(X_, T_, u);
    xlabel(<span class="string">'x'</span>); ylabel(<span class="string">'t'</span>); zlabel(<span class="string">'u(numerical)'</span>);
    figure; mesh(X_, T_, u_exact);
    xlabel(<span class="string">'x'</span>); ylabel(<span class="string">'t'</span>); zlabel(<span class="string">'u(exact)'</span>);
    delta = u_exact - u;
    err = max(abs(delta(:)));
    figure; mesh(X_, T_, u_exact - u);
    xlabel(<span class