Dabblings with Differential Geometry

Holiday Reading

Posted on December 26, 2019

Much of this discussion was based on the text “Geometric Control of Mechanical Systems” by Francesco Bullo and Andrew D.Lewis. A more formal development of the following ideas can be found in that text, this is simply meant to be a page of informal notes.

Table of Contents

Equivalence Classes

Consider the equivalence class $\mathcal{R}\equiv\mathbb{Z}$. Then $\mathbb{R}/\mathbb{Z}$ can be visualized as Alt text Consider $\mathbb{S}^1$ as the unit circle in the complex plane. Next, consider the surjective function $\varphi:\mathbb{R}\to\mathbb{S}^1$ as $\varphi(x) = e^{2\pi ix}$. Note that $\varphi$ is a Group Homomorphism, and that $Ker(\varphi)=\mathbb{Z}$. Therefore, under the first group isomorphism theorem, we have that $\mathbb{R}/\mathbb{Z}\cong\mathbb{S}^1$. Imprecisely, this can be visualized in the following way Alt text Alt text Alt text

Tangent Bundles

Consider the curve $\gamma:I\to M$, with $\gamma’(x)$ representing the derivative of $\gamma$ in an appropriate chart at the point $x$. The tangent fiber at a point x is the set of all curves $\gamma$ that have the same derivative at $x$, denoted by $T_x M$. The tangent bundle $TM = \sqcup_{x\in M}T_x M$, or the disjoint union of the tangent fibers as seen in Alt text

A section of the tangent bundle $\xi\in\Gamma(M)$ provides a vector field on $M$. Alt text Alt text

Nontrivial Vector Bundle

Let $M=[0,1]\subset\mathbb{R}$ be an embedded submanifold. For the proceeding discussion, let $\pi:V\to M$ be the projection mapping from the vector bundle $V$ to the base manifold. Now consider the equivalence relation $\mathcal{R}_1=\{((0,y),(1,y)):y\in\mathbb{R}\}$, as visualized by Alt text The vector bundle $V_1=M\times \mathbb{R}/~$ is isomorphic to $TM$ because of the parallelizable nature of $M$, and is a cylinder as seen in
Alt text If instead we consider the equivalence class $\mathcal{R}_2=\{((0,y),(1,-y)):y\in\mathbb{R}\}$ Alt text the vector bundle $V_2=M\times\mathbb{R}/\sim$ is the infinite Möbius band, as seen in Alt text Note that $\pi(V_i)\cong\mathbb{S}^1$ for $i=\{1,2\}$.