Theorem T000445

CompactConnectedLOTS ∧ ¬EmptyFixed point property

Let f ⁣:XXf\colon X\to X be continuous. For x0Xx_0\in X, if f(x0)>x0f(x_0)>x_0, then let U,VU,V be disjoint open neighborhoods with x0Ux_0\in U, f(x0)Vf(x_0)\in V, and u<vu<v for each uUu\in U, vVv\in V. Then for xf1(V)Ux\in f^{-1}(V)\cap U, f(x)>xf(x)>x, so A:={xXf(x)>x}A:=\{x\in X\mid f(x)>x\} is open, as is B:={xXf(x)<x}B:=\{x\in X\mid f(x)<x\}.

If ff has no fixed point, then X=ABX=A\cup B, and since XX is Connected, AA or BB must be empty. But if X=AX=A, then XX has no maximum element, and likewise if X=BX=B, it has no minimum element.

This contradicts that if XX is LOTS and Compact and not Empty, then it has a maximum and minimum element, by the remark in Math StackExchange 4786659.

The converse ( Fixed point propertyCompactConnectedLOTS ∧ ¬Empty ) cannot be proven from other theorems or disproven from a counterexample.

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