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In mathematics, a topological space X has the fixed point property if all continuous mappings from X to X have a fixed point.

Examples

The closed interval

The closed interval has the fixed point property: Let f: → be a mapping. If f(0) = 0 or f(1) = 1, then our mapping has a fixed point at 0 or 1. If not, then f(0) > 0 and f(1) - 1 < 0. Thus the function g(x) = f(x) - x is a continuous real valued function which is positive at x=0 and negative at x=1. By the intermediate value theorem, there is some point x₀ with g(x₀) = 0, which is to say that f(x₀) - x₀ = 0, and so x₀ is a fixed point.

The open interval does not have the fixed point property. The mapping f(x) = x has no fixed point on the interval (0,1).

The closed disc

The closed interval is a special case of the closed disc, which in any dimension has the fixed point property by the Brouwer fixed point theorem.

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