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Nik Weaver
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Edit: I'm retaining my original answer below, but here is a simpler formula which builds on Goldstern's modification of my answer.

Work in the complex plane. Let $$A = \{z \in \mathbb{C}: 1 \leq |z| \leq 2\mbox{ and }0 \leq {\rm arg}(z) \leq \pi\}.$$ This set is homeomorphic to the unit ball, so for topological questions like the existence of fixed points the change is inessential.

Define $f: A \to \mathbb{C}$ by $f(z) = iz^2$. Then the image under $f$ of the part of $A$ with argument between $0$ and $\pi/4$ is $\{z: 1 \leq |z| \leq 4$ and $\pi/2 \leq {\rm arg}(z) \leq \pi\}$, and the image of the part of $A$ with argument between $3\pi/4$ and $\pi$ is $\{z: 1 \leq |z| \leq 4$ and $0 \leq {\rm arg}(z) \leq \pi/2\}$. The rest of $A$ maps into the lower half-plane. So the image of $A$ contains $A$, and it is obvious from looking at arguments that there are no fixed points.

(The alternative formula $f(z) = iz^2/|z|$ would eliminate the radial stretching, if one prefers this.)

Original answer below.


This fails for $n = 2$. For simplicity consider the unit square $C = [0,1]^2$ instead of $B$. Then define $f$ by first setting $f(x,y) = (x + 1/2, y)$ for $(x,y) \in C$ with $x \leq 1/2$ --- this shifts the left half of the square onto the right half, no fixed points there. Define $f(x,y) = (2x - 3/2, y)$ for $(x,y) \in C$ with $x \geq 3/4$ --- this takes the right one-fourth of $C$ onto the left half, again no fixed points. Then the middle strip $[1/2, 3/4] \times [0,1]$ can be stretched around over the top of the square to complete the definition of a continuous map. I have a hard time describing that last step without drawing a picture, is it clear?

This fails for $n = 2$. For simplicity consider the unit square $C = [0,1]^2$ instead of $B$. Then define $f$ by first setting $f(x,y) = (x + 1/2, y)$ for $(x,y) \in C$ with $x \leq 1/2$ --- this shifts the left half of the square onto the right half, no fixed points there. Define $f(x,y) = (2x - 3/2, y)$ for $(x,y) \in C$ with $x \geq 3/4$ --- this takes the right one-fourth of $C$ onto the left half, again no fixed points. Then the middle strip $[1/2, 3/4] \times [0,1]$ can be stretched around over the top of the square to complete the definition of a continuous map. I have a hard time describing that last step without drawing a picture, is it clear?

Edit: I'm retaining my original answer below, but here is a simpler formula which builds on Goldstern's modification of my answer.

Work in the complex plane. Let $$A = \{z \in \mathbb{C}: 1 \leq |z| \leq 2\mbox{ and }0 \leq {\rm arg}(z) \leq \pi\}.$$ This set is homeomorphic to the unit ball, so for topological questions like the existence of fixed points the change is inessential.

Define $f: A \to \mathbb{C}$ by $f(z) = iz^2$. Then the image under $f$ of the part of $A$ with argument between $0$ and $\pi/4$ is $\{z: 1 \leq |z| \leq 4$ and $\pi/2 \leq {\rm arg}(z) \leq \pi\}$, and the image of the part of $A$ with argument between $3\pi/4$ and $\pi$ is $\{z: 1 \leq |z| \leq 4$ and $0 \leq {\rm arg}(z) \leq \pi/2\}$. The rest of $A$ maps into the lower half-plane. So the image of $A$ contains $A$, and it is obvious from looking at arguments that there are no fixed points.

(The alternative formula $f(z) = iz^2/|z|$ would eliminate the radial stretching, if one prefers this.)

Original answer below.


This fails for $n = 2$. For simplicity consider the unit square $C = [0,1]^2$ instead of $B$. Then define $f$ by first setting $f(x,y) = (x + 1/2, y)$ for $(x,y) \in C$ with $x \leq 1/2$ --- this shifts the left half of the square onto the right half, no fixed points there. Define $f(x,y) = (2x - 3/2, y)$ for $(x,y) \in C$ with $x \geq 3/4$ --- this takes the right one-fourth of $C$ onto the left half, again no fixed points. Then the middle strip $[1/2, 3/4] \times [0,1]$ can be stretched around over the top of the square to complete the definition of a continuous map. I have a hard time describing that last step without drawing a picture, is it clear?

Source Link
Nik Weaver
  • 42.8k
  • 3
  • 112
  • 213

This fails for $n = 2$. For simplicity consider the unit square $C = [0,1]^2$ instead of $B$. Then define $f$ by first setting $f(x,y) = (x + 1/2, y)$ for $(x,y) \in C$ with $x \leq 1/2$ --- this shifts the left half of the square onto the right half, no fixed points there. Define $f(x,y) = (2x - 3/2, y)$ for $(x,y) \in C$ with $x \geq 3/4$ --- this takes the right one-fourth of $C$ onto the left half, again no fixed points. Then the middle strip $[1/2, 3/4] \times [0,1]$ can be stretched around over the top of the square to complete the definition of a continuous map. I have a hard time describing that last step without drawing a picture, is it clear?