The linear correlation between the logarithm of the total density and the energy index with a negative coefficient is proven as the necessary condition for the cross of the power law distribution lines or the common cutoff energy of the non-thermal electrons responsible for synchrotron and bremsstrahlung radiations. The statistical results calculated from a typical solar microwave burst shows a positive correlation coefficient, which is just opposite to the necessary condition. Therefore, the common cutoff energy does not exist in this event.
