## Guide to Starting Javaplex (With Matlab)

Step 1)

Visit https://appliedtopology.github.io/javaplex/ and download the Persistent Homology and Topological Data Analysis Library

2)

3)

In Matlab, change Matlab’s “Current Folder” to the directory matlab examples that you just extracted from the zip file.

(See https://www.mathworks.com/help/matlab/ref/cd.html to change current folder)

Type this in Matlab: cd /…/matlab_examples

Where … depends on where you put the folder

4) In the tutorial (from the link given in step 2), proceed to follow the instructions starting from “In Matlab, change Matlab’s “Current Folder” to the directory matlab examples that you just extracted from the zip file. In the Matlab command window, run the load javaplex.m file.”.

5) Test: Run example 3.2 (House example) by typing in the code (following the tutorial)

## How to use Kenzo (Algebraic Topology CAS) on Clozure CL

After some trials, finally got Kenzo to run on Mac:

Type:

followed by

(in-package “CAT”)

followed by any Kenzo commands.

Screenshot:

## List of Fundamental Group, Homology Group (integral), and Covering Spaces

Just to compile a list of Fundamental groups, Homology Groups, and Covering Spaces for common spaces like the Circle, n-sphere ($S^n$), torus ($T$), real projective plane ($\mathbb{R}P^2$), and the Klein bottle ($K$).

# Fundamental Group

Circle: $\pi_1(S^1)=\mathbb{Z}$

n-Sphere: $\pi_1(S^n)=0$, for $n>1$

n-Torus: $\pi_1(T^n)=\mathbb{Z}^n$ (Here n-Torus refers to the n-dimensional torus, not the Torus with n holes)

$\pi_1(T^2)=\mathbb{Z}^2$ (usual torus with one hole in 2 dimensions)

Real projective plane: $\pi_1(\mathbb{R}P^2)=\mathbb{Z}_2$

Klein bottle $K$: $\pi_1(K)=(\mathbb{Z}\amalg\mathbb{Z})/\langle aba^{-1}b\rangle$

# Homology Group (Integral)

$H_0(S^1)=H_1(S^1)=\mathbb{Z}$. Higher homology groups are zero.

$H_k(S^n)=\begin{cases}\mathbb{Z}&k=0,n\\ 0&\text{otherwise} \end{cases}$

$H_k(T)=\begin{cases}\mathbb{Z}\ \ \ &k=0,2\\ \mathbb{Z}\times\mathbb{Z}\ \ \ &k=1\\ 0\ \ \ &\text{otherwise} \end{cases}$

$H_k(\mathbb{R}P^2)=\begin{cases}\mathbb{Z}\ \ \ &k=0\\ \mathbb{Z}_2\ \ \ &k=1\\ 0\ \ \ &\text{otherwise} \end{cases}$

Klein bottle, $K$: $H_k(K)=\begin{cases}\mathbb{Z}&k=0\\ \mathbb{Z}\oplus(\mathbb{Z}/2\mathbb{Z})&k=1\\ 0&\text{otherwise} \end{cases}$

# Covering Spaces

A universal cover of a connected topological space $X$ is a simply connected space $Y$ with a map $f:Y\to X$ that is a covering map. Since there are many covering spaces, we will list the universal cover instead.

$\mathbb{R}$ is the universal cover of the unit circle $S^1$

$S^n$ is its own universal cover for $n>1$. (General result: If $X$ is simply connected, i.e. has a trivial fundamental group, then it is its own universal cover.)

$\mathbb{R}^2$ is the universal cover of $T$.

$S^2$ is universal cover of real projective plane $RP^2$.

$\mathbb{R}^2$ is universal cover of Klein bottle $K$.

## Contractible space as Codomain implies any two maps Homotopic

Recall that a space Y is contractible if the identity map $\text{id}_Y$ is homotopic to a constant map. Let Y be contractible space and let X be any space. Then, for any maps $f,g: X\to Y$, $f\simeq g$.

Proof: Let Y be a contractible space and let X be any space. $\text{id}_Y\simeq c$, where $c$ is a constant map. There exists a map $F: Y\times [0,1]\to Y$ such that $F(y,0)=\text{id}_Y(y)=y$, for $y\in Y$. $F(y,1)=c(y)=b$ for some point $b\in Y$.

Let $f,g: X\to Y$ be any two maps. Consider $G:X\times [0,1]\to Y$ where $G(x,t)=\begin{cases} F(f(x),2t),&\ \ \ \text{for}\ 0\leq t\leq 1/2\\ F(g(x),-2t+2),&\ \ \ \text{for}\ \frac 12

When $t=\frac 12$, $F(f(x),1)=b$, $F(g(x),1)=b$. Therefore G is cts.

$G(x,0)=F(f(x),0)=f(x)$,

$G(x,1)=F(g(x),0)=g(x)$.

Therefore $f\simeq g$.

## Video on Simplices and Simplicial Complexes

Professor Wildberger is extremely kind to upload his videos which would be very useful to any Math student studying Topology. Simplices / Simplicial Complexes are usually the first chapter in a Algebraic Topology book.

Check out also Professor Wildberger’s book on Rational Trigonometry, something that is quite novel and a new approach to the subject of Trigonometry. For instance, it can be used for rational parametrisation of a circle.

## AlgTop1: One-dimensional objects

This is a continuation of the series of Algebraic Topology videos. Previous post was AlgTop 0.

Professor Wildberger is an interesting speaker. He holds some unorthodox views, for instance he doesn’t believe in “real numbers” or “infinite sets”. Nevertheless, his videos are excellent and educational. Highly recommended to watch!

The basic topological objects, the line and the circle are viewed in a new light. This is the full first lecture of this beginner’s course in Algebraic Topology, given by N J Wildberger at UNSW. Here we begin to introduce basic one dimensional objects, namely the line and the circle. However each can appear in rather a remarkable variety of different ways.

Author: NJ Wildberger

This revolutionary book establishes new foundations for trigonometry and Euclidean geometry. It shows how to replace transcendental trig functions with high school arithmetic and algebra to dramatically simplify the subject, increase accuracy in practical problems, and allow metrical geometry to be systematically developed over a general field. This new theory brings together geometry, algebra and number theory and sets out new directions for algebraic geometry, combinatorics, special functions and computer graphics. The treatment is careful and precise, with over one hundred theorems and 170 diagrams, and is meant for a mathematically mature audience. Gifted high school students will find most of the material accessible, although a few chapters require calculus. Applications include surveying and engineering problems, Platonic solids, spherical and cylindrical coordinate systems, and selected physics problems, such as projectile motion and Snell’s law. Examples over finite fields are also included.

## Algebraic Topology Video by Professor N J Wildberger

This is the Introductory lecture to a beginner’s course in Algebraic Topology given by N J Wildberger of the School of Mathematics and Statistics at UNSW in 2010.

This first lecture introduces some of the topics of the course and three problems.

If you are curious about how to make the interesting flap of paper (Problem 1), the solution can be found here. 🙂

Author: N J Wildberger

This revolutionary book establishes new foundations for trigonometry and Euclidean geometry. It shows how to replace transcendental trig functions with high school arithmetic and algebra to dramatically simplify the subject, increase accuracy in practical problems, and allow metrical geometry to be systematically developed over a general field. This new theory brings together geometry, algebra and number theory and sets out new directions for algebraic geometry, combinatorics, special functions and computer graphics. The treatment is careful and precise, with over one hundred theorems and 170 diagrams, and is meant for a mathematically mature audience. Gifted high school students will find most of the material accessible, although a few chapters require calculus. Applications include surveying and engineering problems, Platonic solids, spherical and cylindrical coordinate systems, and selected physics problems, such as projectile motion and Snell’s law. Examples over finite fields are also included.