WEEK 1: first lecture of #VisualAlgebra.

"The mathematician's patterns, like the painter's or the poet's must be beautiful; the ideas like the colours or the words, must fit together in a harmonious way."
― G.H. Hardy, A Mathematician's Apology

1/4
We have not yet defined a group. Rather, we are exploring the intuition of them via symmetries. This will *motivate the axioms*, rather than the other way around.

What properties does this group have what might not hold more generally?

2/4
For further insight, consider the symmetries of a triangle. This motivates the idea of relations, and why this "group calculator" tool is useful.

I got so many really good questions and comments in class today. How often does that happen on *Day 1* of abstract algebra??

3/4
Finally, we ended with a Cayley diagram using a different set of generators. (Henceforth, bi-directed arrows will be un-directed).

From a distance, these don't look like the same group. But they are! Surprised? And that's a wrap!

Questions & comments welcome!

4/4

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More from @mattmacauley

1 Jan
Happy New Year #MathTwitter! Let's start 2022 w/ Part 1 of a fun series: "Groups you Never Knew Existed...and others you can't POSSIBLY live without!"

Today we'll see the "diquaternions", a term you've never heard of b/c I made it up last month. Let's dig in! 🧵👇

1/17 Image
We'll start with the familiar quaternion group Q_8. Shown here are several Cayley diagrams, a Cayley table, cycle diagram, subgroup lattice, its partition by conjugacy classes, and an action diagram of Aut(Q_8). Each of these highlights different structural features.

2/17 ImageImageImageImage
Next, have you ever wondered what would happen if you replace i=e^{2\pi i/4} in Q_8 with a larger root of unity?

These are the dicyclic groups. Here is Dic_6, for n=6. Note that n=4 gives Q_8.

The last two pictures highlight the orbit structure (cyclic subgroups).

3/17 ImageImageImageImage
Read 19 tweets
4 Dec 21
What does it really mean for a group to be "nilpotent"?

This year, I've asked many people to describe it in simple, memorable terms, and have yet to get a good answer.

Usually: something something about an ascending series. But what exactly, and WHY? Let's dig in! 🧵👇

1/17
First, I'm wasn't at all picking on anyone, but rather, at how this concept (and so many others) are taught in nearly every algebra class and book.

By the end of this thread, you'll learn what nilpotent really means in a memorable visual way you'll never forget!

2/17
First, a quick refresher. In the subgroup lattice of G:

--subgroups H≤G appear as down-sets, like stalagmites
--quotients G/N appear as up-sets, like stalactites.

Here are two groups of order 20. The dihedral group D_5 is a subgroup of one and a quotient of the other.

3/17
Read 18 tweets
13 Nov 21
Last week I did the Sylow theorems in class, and I want to share how I do them with my visual approach to groups.

To start, here are the 5 groups of order 12. Note how there are "towers of p-groups", for p=2 and p=3.

This is what the 1st Sylow theorem guarantees.

1/17
The key lemma needed for the Sylow theorems is:

"If a p-group G acts on S, then |Fix|≡|S| mod p."

Here's a picture proof of that, adapted from @nathancarter5's fantastic "Visual Group Theory" book.

2/17
As a corollary of this, by letting a p-group act on its subgroups by conjugation:

"p-groups cannot have fully unnormal subgroups"

Said differently, the normalizer of any subgroup strictly gets bigger. Here's what I mean by that concept.

3/17
Read 19 tweets

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