Exercise 10. Groups and Covering spaces. Lima

120 Views Asked by At

Let $X$ be the space obtained from the sphere $S^2$ by gluing the north pole to the south pole, let $Y=\mathbb{R}^3-S^1$, where $S^1=\left\{(x,y,0)\in\mathbb{R}^3:x^2+y^2=1\right\}$ and let $Z$ be the union of a torus of revolution with a disk whose boundary is the smallest of the parallels of the torus. Prove that $X$,$Y$ and $Z$ have the same homotopy type.

How can I prove that $Z$ have the same homotopy type with to $X$ or $Y$?

1

There are 1 best solutions below

0
On

Goal

Provide a plan along with some guidance for the reader to complete the details.

Notation: $\sigma=\sqrt{x^2+y^2}$. $Y=\mathbb R^3\setminus\{\sigma=1,\;z=0\}$.

Plan

Step 1. Define $B_2=\{(x,y,z)\mid x^2+y^2+z^2\le4\}$. Replace $Y$ with $\bar Y=B_2\setminus\{\sigma=1,\;z=0\}$.

Step 2. Define $(0,0,z)\sim(0,0,0)$. Replace $\bar Y$ with $\bar Y/{\sim}$.

Step 3. Replace $\bar Y/{\sim}$ with $T^*=\{(\sigma-1)^2+z^2\le1\}\setminus\{\sigma=1,\;z=0\}$.

Step 4. Replace $T^*$ with $T=\{(\sigma-1)^2+z^2=1\}$.

Guidance

Step 1. Put $\rho=\sqrt{\sigma^2+z^2}$. $$ f(x,y,z)=\begin{cases} \tfrac{2}{\rho}(x,y,z) &\rho\ge 2,\\ (x,y,z) &\rm otherwise. \end{cases} $$

Step 2. Define $g\colon\bar Y/{\sim}\to\bar Y$ as $g\colon[x,y,z]\mapsto(x,y,\frac{\sigma}{2}z)$.

Step 3. Write $\zeta=\sqrt{1-(\sigma-1)^2}$. Define $h\colon\bar Y/{\sim}\to T^*$ as $$ h([x,y,z]) = \begin{cases} (x,y,\zeta) &z\ge\zeta,\\ (x,y,z) &-\zeta\le z\le\zeta,\\ (x,y,-\zeta) &z\le-\zeta. \end{cases} $$

Step 4. Write $\xi=\sqrt{(\sigma-1)^2+z^2}$. Define $r\colon T^*\to T$ as $$ r(x,y,z) = \begin{cases} \tfrac{1}{\xi}(x-x/\sigma,y-y/\sigma,z)+(x/\sigma,y/\sigma,0) &\sigma\ne0,\\ (0,0,0) &\sigma=0. \end{cases} $$

To do

  1. Draw pictures to understand the idea.
  2. Prove the continuity of the functions defined above.
  3. Use the functions to build homotopies between domain and codomain.
  4. Complete the solution to the problem. (see also How can I prove that the horn torus and $\mathbb{T} \cup D_1$ have the same homotopy type?)