etotheipi
AFAIK there are no problems up to here: $$\frac{d}{dx} \lim_{x \to \infty} \int_{0}^{x} \left[f(t) - g(t) \right] = 0$$ but yes I don't think you can generally interchange those limits to get ##\lim_{x \to \infty} \frac{d}{dx} \int_{0}^{x} \left[f(t) - g(t) \right] = 0##