In mathematics, an extremepoint of a convex set S in a real vector space is a point in S which does not lie in any open line segment joining two points of S. Intuitively, an extremepoint is a "vertex" of S.
The Krein–Milman theorem states that if S is convex and compact in a locally convex space, then S is the closed convex hull of its extremepoints: In particular, such a set has extremepoints.
The Krein–Milman theorem is stated for locally convex topological vector spaces. The next theorems are stated for Banach spaces with the Radon–Nikodym property :
A theorem of Joram Lindenstrauss states that, in a Banach space with the Radon–Nikodym property, a nonempty closed and bounded set has an extremepoint. (In infinite-dimensional spaces, the property of compactness is stronger than the joint properties of being closed and being bounded).
A theorem of Gerald Edgar states that, in a Banach space with the Radon–Nikodym property, a closed and bounded set is the closed convex hull of its extremepoints:
Let E be a Banach space with the Radon-Nikodym property, let C be a separable, closed, bounded, convex subset of E, and let a be a point in C. Then there is a probability measure p on the universally measurable sets in C such that a is the barycenter of p, and the set of extremepoints of C has p-measure 1.