Wendel's theorem explained

In geometric probability theory, Wendel's theorem, named after James G. Wendel, gives the probability that N points distributed uniformly at random on an

(n-1)

-dimensional hypersphere all lie on the same "half" of the hypersphere. In other words, one seeks the probability that there is some half-space with the origin on its boundary that contains all N points. Wendel's theorem says that the probability is

pn,N=2-N+1

n-1
\sum
k=0

\binom{N-1}{k}.

The statement is equivalent to

pn,N

being the probability that the origin is not contained in the convex hull of the N points and holds for any probability distribution on that is symmetric around the origin. In particular this includes all distribution which are rotationally invariant around the origin.

This is essentially a probabilistic restatement of Schläfli's theorem that

N

hyperplanes in general position in

\Rn

divides it into
n-1
2\sum
k=0

\binom{N-1}{k}

regions.[1]

Notes and References

  1. Cover . Thomas M. . Efron . Bradley . February 1967 . Geometrical Probability and Random Points on a Hypersphere . The Annals of Mathematical Statistics . 38 . 1 . 213–220 . 10.1214/aoms/1177699073 . 0003-4851. free .