Knaster–Kuratowski–Mazurkiewicz lemma
The Knaster–Kuratowski–Mazurkiewicz lemma is a basic result in mathematical fixed-point theory published in 1929 by Knaster, Kuratowski and Mazurkiewicz.[1]
The KKM lemma can be proved from Sperner's lemma and can be used to prove the Brouwer fixed-point theorem.
Statement
Suppose that a simplex is covered by the closed sets
for
and that for all
the face of
that is spanned by
for
is covered by
for
then all the
have a common intersection point.
Example
The two-dimensional case may serve as an illustration. In this case the simplex is a triangle, whose vertices we can label 1, 2 and 3. We are given three closed sets
which collectively cover the triangle; also we are told that
covers vertex 1,
covers vertex 2,
covers vertex 3, and that the edge 12 (from vertex 1 to vertex 2) is covered by the sets
and
, the edge 23 is covered by the sets
and
, the edge 31 is covered by the sets
and
. The KKM lemma states that the sets
have at least one point in common.
References
- ↑ Knaster, B.; Kuratowski, C.; Mazurkiewicz, S. (1929), "Ein Beweis des Fixpunktsatzes für n-dimensionale Simplexe", Fundamenta Mathematicae (in German) 14 (1): 132–137.
External links
- See the proof of KKM Lemma in Planet Math.