Tails of probability density for sums of random independent variables

الأصل · EN

The exact expression for the probability density pₙ(x) for sums of a finite number N of random independent terms is obtained. It is shown that the very tail of pₙ(x) has a Gaussian form if and only if all the random terms are distributed according to the Gauss Law. In all other cases the tail for pₙ(x) differs from the Gaussian. If the variances of random terms diverge the non-Gaussian tail is related to a Levy distribution for pₙ(x). However, the tail is not Gaussian even if the variances are finite. In the latter case pₙ(x) has two different asymptotics. At small and moderate values of x the distribution is Gaussian. At large x the non-Gaussian tail arises. The crossover between the two asymptotics occurs at x proportional to N. For this reason the non-Gaussian tail exists at finite N only. In the limit N tends to infinity the origin of the tail is shifted to infinity, i. e., the tail vanishes. Depending on the particular type of the distribution of the random terms the non-Gaussian tail may decay either slower than the Gaussian, or faster than it. A number of particular examples is discussed in detail.