 Clearly, no matter what masses are chosen to stand in place of the electron, muon, and tau, The upper bound follows from the fact that the square roots are necessarily positive.The lower bound follows from the Cauchy–Bunyakovsky–Schwarz inequality.It can also be shown by interpreting the value formula_3 as the squared cosine of the angle between the vector formula_4 and the vector formula_5 (see dot product).