A mathematical object X has the fixed-point property if every suitably well-behaved mapping from X to itself has a fixed point. The term is most commonly used to describe topological spaces on which every continuous mapping has a fixed point. But another use is in order theory, where a partially ordered set P is said to have the fixed point property if every increasing function on P has a fixed point.

Definition

Let A be an object in the concrete category C. Then A has the fixed-point property if every morphism (i.e., every function) ${\displaystyle f:A\to A}$ has a fixed point.

The most common usage is when C = Top is the category of topological spaces. Then a topological space X has the fixed-point property if every continuous map ${\displaystyle f:X\to X}$ has a fixed point.

Examples

Topology

A retract A of a space X with the fixed-point property also has the fixed-point property. This is because if ${\displaystyle r:X\to A}$ is a retraction and ${\displaystyle f:A\to A}$ is any continuous function, then the composition ${\displaystyle i\circ f\circ r:X\to X}$ (where ${\displaystyle i:A\to X}$ is inclusion) has a fixed point. That is, there is ${\displaystyle x\in A}$ such that ${\displaystyle f\circ r(x)=x}$. Since ${\displaystyle x\in A}$ we have that ${\displaystyle r(x)=x}$ and therefore ${\displaystyle f(x)=x.}$

A topological space has the fixed-point property if and only if its identity map is universal.

A product of spaces with the fixed-point property in general fails to have the fixed-point property even if one of the spaces is the closed real interval.

The FPP is a topological invariant, i.e. is preserved by any homeomorphism. The FPP is also preserved by any retraction.

According to Brouwer fixed point theorem every compact and convex subset of a Euclidean space has the FPP. More generally, according to the Schauder-Tychonoff fixed point theorem every compact and convex subset of a locally convex topological vector space has the FPP. Compactness alone does not imply the FPP and convexity is not even a topological property so it makes sense to ask how to topologically characterize the FPP. In 1932 Borsuk asked whether compactness together with contractibility could be a sufficient condition for the FPP to hold. The problem was open for 20 years until the conjecture was disproved by Kinoshita who found an example of a compact contractible space without the FPP.^[1]

References

• Samuel Eilenberg, Norman Steenrod (1952). Foundations of Algebraic Topology. Princeton University Press.
• Schröder, Bernd (2002). Ordered Sets. Birkhäuser Boston.