Let $K$ be the "knotted" $x$-axis. I have been able to show that $K$ is a retract of $\mathbb{R}^3 $ using the fact that $K$ and the real line $\mathbb{R}$ are homeomorphic, $\mathbb{R}^3$ is a normal space, and then applying the Tietze Extension Theorem. But then what would be an explicit retraction $r: \mathbb{R}^3 \rightarrow K$? Any ideas?
In regard to a retraction $r: \mathbb{R}^3 \rightarrow K$
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1Can you explain what a "knotted" $x$-axis is? – 2012-12-04
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0A Googling turns up: http://www.quora.com/How-is-the-knotted-x-axis-a-retract-of-R-3, which sounds very much like your question. – 2012-12-04
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0My point is to come up with an explicit retraction; I have already been able to prove the existence of such a retraction, but need to find one. – 2012-12-04
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2My question, though, is whether you have knotted the $x$-axis by a trefoil, a figure-eight, Knot $10_{165}$, etc... I believe any specific retraction would depend on how your $x$-axis is actually knotted. – 2012-12-04
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0I believe it is a trefoil. It can be found in $\textit{Topology}$ by Munkres, on p. 224 (Exercise 7(b)). – 2012-12-04
1 Answers
Let $f : K \to \Bbb R$ and pick a point $x \in K$.
Pick a infinite sheet of paper on the left side of the knot and imagine pinching and pushing it inside the knot all the way right to $x$, and use this to define $g$ on the space spanned by the sheet of paper into $( - \infty ; f(x))$, such that if $y
Do the same thing with another infinite sheet of paper from the right side of the knot moving all the way left to $x$.
Finally define $g(y) = f(x)$ for all $y$ in the remaining space between the two sheets of paper. Then $g : \Bbb R^3 \to \Bbb R$ is continuous, extends $f$, and except at $f(x)$ where the fiber is big, $g^{-1}(\{y\})$ is homeomorphic to $\Bbb R^2 $
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0What exactly is a fiber? I haven't yet learned that in topology. – 2012-12-08
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0@ Sachin : it is the set of the preimages of a point : The fiber at $y$ is $g^{-1}(\{y\}) = \{x \in \Bbb R^2 / g(x) = y\}$ – 2012-12-08