# Fitzsimmons-Fristedt-Shepp theorem

A theorem asserting that two particular random sets obtained in quite different ways have the same distribution law [a1]. A first version of this theorem was obtained by B. Mandelbrot in 1972 [a2]. It is a key fact for understanding the link between random coverings and potential theory (see also Dvoretzky problem; Billard method).

The first random set is defined as Here, the random cutouts are associated with points that are randomly distributed in in such a way that their number in any given rectangle is a Poisson random variable (cf. Poisson process) with parameter , where is the Lebesgue measure and is a given measure on that is locally bounded except at . The set of points is called a point Poisson process with intensity , and can be viewed as the set of points in that are never in the shadow of the point Poisson process when light comes from the directions , .

The second random set is the closure of the range of a positive Lévy process with drift and Lévy measure . By definition, this process, ( ), has independent and stationary increments, and  The theorem asserts that for any given with (a1)

one can explicitly define and in such a way that The drift vanishes precisely when For example, when , , then and (this is the case of a stable Lévy process of index ).

When the integral in (a1) is infinite, a formal computation gives and concentrated at . This is the case when is empty. Therefore, the Fitzsimmons–Fristedt–Shepp theorem is an extension of Shepp's theorem, which states that (a1) is a necessary and sufficient condition for almost surely [a3].

Now, given a compact subset of , the probabilities of the events and are the same; in other words: almost surely if and only if is a polar set for the Lévy process . Since compact polar sets are precisely the compact sets of vanishing capacity with respect to a potential kernel (associated with and and therefore with ), the link between Poisson covering of and potential theory is manifest (see also Billard method).

How to Cite This Entry:
Fitzsimmons-Fristedt-Shepp theorem. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Fitzsimmons-Fristedt-Shepp_theorem&oldid=22429
This article was adapted from an original article by J.-P. Kahane (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article