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The question I have is:

Let $f: S^1 \rightarrow S^1$ be a continuous function, where $S^1$ is the unit circle. Prove that if $f$ is not onto, then $f$ must have a fixed point.

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    A more algebro topological answer would be: if $f$ is not onto, then $\text{deg} f = 0$, but any map $f: S^n \to S^n$ without fixed points must satisfy $\text{deg} f = (-1)^{n+1}$.2012-08-10

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Let $x\in S^1$ be not in the range of $f:S^1\to S^1$, and let us introduce a (constant-speed) parameterization $\phi:[0,1)\to S^1$ such that $\phi(0)=x$. Then we consider $F=\phi^{-1}\circ f\circ \phi:[0,1)\to[0,1)$. We can continuously extend $F$ to $F:[0,1]\to[0,1]$, and obviously, we have $F(0)>0$ and $F(1)<1$. Hence $F$ has a fixed point.

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    @neelp: I think you can say that. My explanation would be that $x$ is irrelevant because it is not in the range. In other words, you can safely remove the point $x$ from the circle, so it becomes an interval.2012-08-21
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$f$ is not ONTO,this is nul homotopic, so it will have fixed point.

let $f:S^1\rightarrow S^1$ be nulhomotopic, then it extends to a map $F$ from $B^2$ to $S^1$ which can be thought of as a map from $B^2$ to $B^2$, and then apply Brouwer's Fixed Point Theorem, $F$ must have a fixed point,. Since the image of $F$ is contained in $S^1$ so this fixed point must lie in $S^1$ so it is also a fixed point of $f$.

Also we see as this is null homotopic so degree of $f=0$, but there is a result saying $f:S^1\rightarrow S^1$ with no fixed point has degree $(-1)^{n+1}$

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    @AlexanderThumm Why can't we just indentify continuous functions on a cirlce with $2\pi$ periodic continuous functions on a real line? This obviously gives the desired point.2012-08-12
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Pick a point $x_0$ so that $x_0\not\in f(S^1)$. Then the map $g:S^1\mapsto T_{x_0}$ by $ g(x)=4\frac{x-x_0}{|x-x_0|^2}+x_0\tag{1} $ is the inversion map with respect to the circle of radius $2$ centered at $x_0$, where $T_{x_0}$ is the tangent to $S^1$ at $-x_0$. $g$ is continuous and injective on $f(S^1)$.

$g\circ f(S^1)$ is a compact, connected subset of $T_{x_0}$ (a line segment). Thus, there must be two points, $x_1$ and $x_2$, so that $g\circ f(x_1)$ and $g\circ f(x_2)$ are the endpoints of $g\circ f(S^1)$.

Note that if $g\circ f(x_1)=g\circ f(x_2)$, then $f(S^1)$ is a single point which must be a fixed point. Thus, we can assume that $g\circ f(x_1)\not=g\circ f(x_2)$

Consider $h:S^1\mapsto\mathbb{R}$ defined by $ h(x)=(g\circ f\circ f(x)-g\circ f(x))\cdot(g\circ f(x_2)-g\circ f(x_1))\tag{2} $ $h$ is continuous and since $g\circ f(S^1)$ is between or equal to $g\circ f(x_1)$ and $g\circ f(x_2)$, we have $ h(x_1)\ge0\quad\text{and}\quad h(x_2)\le0\tag{3} $ Since $h(S^1)$ is connected, there must be some $x_3\in S^1$ so that $ h(x_3)=0\tag{4} $ Since the range of $g$ is contained in $T_{x_0}$, $(2)$ and $(4)$ imply that $ f\circ f(x_3)=f(x_3)\tag{5} $ and $f(x_3)$ is a fixed point of $f$.

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    Would the downvoter care to comment?2012-08-11