% Runge-Kutta av orden 4.
function x = rk4(T,N)
    f = @(t,x) 2*x/(t+1)-x^2/(t+1)^3;
    h = T/N;
    x = 1;
    for n = 0:N-1
            k1 = h*f(n*h, x);
            k2 = h*f(n*h+ h/2, x+k1/2);
            k3 = h*f(n*h+ h/2, x+k2/2);
            k4 = h*f(n*h+ h, x+k3);
        x= x+1/6*(k1+2*k2+2*k3+k4);
    end
end