diff --git a/content/Math4202/Math4202_L29.md b/content/Math4202/Math4202_L29.md new file mode 100644 index 0000000..debdb11 --- /dev/null +++ b/content/Math4202/Math4202_L29.md @@ -0,0 +1,57 @@ +# Math4202 Topology II (Lecture 29) + +## Algebraic Topology + +### Fundamental Groups of Some Surfaces + +Recall from previous lecture, we talked about figure 8 shape. + +#### Lemma The fundamental group of figure-8 is not abelian + +The fundamental group of figure-8 is not abelian. + +
+Proof + +Consider $U,V$ be two "fish shape" where $U\cup V$ is the figure-8 shape, and $U\cap V$ is $x$ shape. + +The $x$ shape is path connected, + +$\pi_1(U,x_0)$ is isomorphic to $\pi_1(S^1,x_0)$, and $\pi_1(V,x_0)$ is isomorphic to $\pi_1(S^1,x_0)$. + +To show that is not abelian, we need to show that $\alpha*\beta\neq \beta*\alpha$. + +We will use covering map to do this. + +[Universal covering of figure-8](https://notenexta.trance-0.com/Math4202/universal-covering-of-figure-8.png) + +However, for proving our result, it is sufficient to use xy axis with loops on each integer lattice. + +And $\tilde{\alpha*\beta}(1)=(1,0)$ and $\tilde{\beta*\alpha}(1)=(0,1)$. By path lifting correspondence, the two loops are not homotopic. + +
+ +#### Theorem for fundamental groups of double torus (Torus with genus 2) + +The fundamental group of Torus with genus 2 is not abelian. + +
+Proof + +If we cut the torus in the middle, we can have $U,V$ is two "punctured torus", which is homotopic to the figure-8 shape. + +But the is trick is not enough to show that the fundamental group is not abelian. + +--- + +First we use quotient map $q_1$ to map double torus to two torus connected at one point. + +Then we use quotient map $q_2$ to map two torus connected at one point to figure-8 shape. + +So $q=q_2\circ q_1$ is a quotient map from double torus to figure-8 shape. + +Then consider the inclusion map $i$ and let the double torus be $X$, we claim that $i_*:\pi_1(\infty,x_0)\to \pi_1(X,x_0)$ is injective. + +If $\pi_1(X,x_0)$ is abelian, then the figure 8 shape is abelian, that is contradiction. + +
\ No newline at end of file diff --git a/content/Math4202/_meta.js b/content/Math4202/_meta.js index 945d47a..8cb73ab 100644 --- a/content/Math4202/_meta.js +++ b/content/Math4202/_meta.js @@ -34,4 +34,5 @@ export default { Math4202_L26: "Topology II (Lecture 26)", Math4202_L27: "Topology II (Lecture 27)", Math4202_L28: "Topology II (Lecture 28)", + Math4202_L29: "Topology II (Lecture 29)", } diff --git a/content/Math4302/_meta.js b/content/Math4302/_meta.js index 5e39aa6..efe02c5 100644 --- a/content/Math4302/_meta.js +++ b/content/Math4302/_meta.js @@ -1,5 +1,6 @@ export default { index: "Course Description", + Exam_reviews: "Exam reviews", "---":{ type: 'separator' }, diff --git a/public/Math4202/universal-covering-of-figure-8.png b/public/Math4202/universal-covering-of-figure-8.png new file mode 100644 index 0000000..3baf1da Binary files /dev/null and b/public/Math4202/universal-covering-of-figure-8.png differ