One of my projects in life is to (i) become “fluent in mathematics" in the sense that my intuition should incorporate the objects and relationships of 20th-century mathematical discoveries, and (ii) share that feeling with people who are interested in doing the same in a shorter timeframe.

Inspired by the theory of Plato’s Republic that “philosopher kings” should learn Geometry—pure logic or the way any universe must necessarily work—and my belief that the shapes

and feelings thereof operate on a pre-linguistic, pre-rational “gut feeling” level, this may be a worthwhile pursuit. The commercial application would come in the sense that, once you’re in a situation where you have to make big decisions, the only tools you have, in some sense, are who you have become. (Who knows if that would work—but hey, it might! At least one historical wise guy believed the decision-makers should prepare their minds with the shapes of ultimate logic in the universe—and the topologists have told us by now of many more shapes and relations.)

To that end I owe the interested a few more blogposts on:

- automorphisms / homomorphisms
- the logic of shape, the shape of logic
- breadth of functions
- "to equivalence-class"

which I think relate mathematical discoveries to unfamiliar ways of thinking.

Today I’ll talk about **the breadth of functions.**

If you remember Descartes’ concept of a function, it is merely a one-to-at-least-one association. “Associate” is about as blah and general and nothing a verb as I could come up with. How could it say anything worthwhile?

The breadth of functions-as-verbs, I think, comes from which codomains you choose to associate to which domains.

The biggest contrast I can come up with is between

- a function that associates a non-scalar domain to a ≥0 scalar domain, and
- a domain to itself.

If I impose further conditions on the second kind of function, it becomes an automorphism. The conditions being surjectivity ≥↓ and injectivity ≤↑: coveringness ≥ ↓

If I impose those two conditions then I’m talking about an isomorphism (bijection) from a space to itself, which I could also call “turning the abstract space over and around and inside out in my hands” — playing with the space. If I biject the space to another version of itself, I’m looking at the same thing in a different way.

Back to the first case, where I associate a ≥0 scalar (i.e., a “regular number” like 12.8) to an object of a complicated space, like

- the space of possible neuron weightings;
- the space of 2-person dynamical systems (like the “love equations”);
- a space containing weird objects that twist in a way that’s easier to describe than to draw;
- a space of possible things that could happen;
- the space of paths through London that spend 90% of their time along the Thames;
- the space of possible protein configurations;

then I could call that “assigning a size to the object”. Again I should add some more constraints to the mapping in order to really call it a “size assignment”. For example continuity, if reasonable—I would like similar things to have a similar size. Or the standard definition of a metric: `dist(a,b)=dist(b,a)`

; `dist(x,x)=0`

; no other zeroes besides `dist(self,self)`

, and triangle law.

Since the word “size" itself could have many meanings as well, such as:

- volume
- angle measure
- likelihood
- length/height
- correlation
- mass
- how long an algorithm takes to run
- how different from the typical an observation is
- how skewed a statistical distribution is
- (the inverse of) how far I go until my sampling method encounters the farthest-away next observation
- surface area
- density
- number of tines (or “points” if you’re measuring a buck’s antlers)
- how big of a suitcase you need to fit the thing in (L-∞ norm)

which would order objects differently (e.g., lungs have more surface area in less volume; fractals have more points but needn’t be large to have many points; a delicate sculpture could have small mass, small surface area, large height, and be hard to fit into a box; and osmium would look small but be very heavy—heavier than gold).

Let’s stay with the weighted-neurons example, because it’s evocative and because posets and graphs model a variety of things.

An isomorphism from graphs to graphs might be just to interchange certain wires for dots. So roads become cities and cities become roads. Weird, right? But mathematically these can be dual. I might also take an observation from depth-first versus breadth-first search from computer science (algorithm execution as trees) and apply it to a network-as-brain, if the tree-ness is sufficiently similar between the two and if trees are really a good metaphor after all for either algorithms or brains.

More broadly, one hopes that theorems about automorphism groups on trees (like automorphism groups on T-shirts) could evoke interesting or useful thoughts about all the tree-like things and web-like things: be they social networks, roads, or brains.

So that’s one example of a pre-linguistic “shape” that’s evoked by 20th-century mathematics. Today I feel like I could do two: so how about **To Equivalence-Class**.

Probably due to the invention of set theory, mathematics offers a way of bunching all alike things together. This is something people have done since at least Aristotle; it’s basically like Aristotle’s categories.

- The set of all librarians;
- The set of all hats;
- The set of all sciences;
- Quine’s (extensional) definition of the number three as “the class of all sets with cardinality three”. (Don’t try the “intensional” definition or “What is it
*intrinsically*that makes three, three? What does three*really mean*?” unless you’re trying to drive yourself insane to get out of the capital punishment.) - The set of all cars;
- The set of all cats;
- The set of all computers;
- The set of all even numbers;
- The set of all planes oriented any way in 𝔸³
- The set of all equal-area blobs in any plane 𝔸² that’s parallel to the one you’re talking about (but could be shifted anywhere within 𝔸³)
- The set of all successful people;
- The set of all companies that pay enough tax;
- The set of all borrowers who will make at least three late payments during the life of their mortgage;
- The set of all borrowers with between 1% and 5% chance of defaulting on their mortgage;
- The set of all Extraverted Sensing Feeling Perceivers;
- The set of all janitors within 5 years of retirement age, who have worked in the custodial services at some point during at least 15 of the last 25 years;
- The set of all orchids;
- The set of all ungulates;

The boundaries of some of these (Aristotelian, not Lawverean) categories may be fuzzy or vague—

- if you cut off a cat’s leg is it still a cat?

What if you shave it? What if you replace the heart with a fish heart? - Is economics a science? Is cognitive science a science? Is mathematics a science? Is Is the particular idea you’re trying to get a grant for scientific?

and in fact membership in any of these equivalence classes could be part of a rhetorical contest. If you already have positive associations with “science”, then if I frame what I do as scientific then you will perceive it as e.g. precise, valuable, truthful, honourable, accurate, important, serious, valid, worthwhile, and so on. Scientists put Man on the Moon. **Scientists cured polio.** Scientists discovered Germ Theory. (But did “computer scientists” or “statisticians” or “Bayesian quantum communication” or “full professors” or “mathematical élite” or “string theorists” do those things? Yet they are classed together under the STEM label. Related: engineers, artisans, scientists, and intelligentsia in Leonardo da Vinci’s time.)

But even though it is an old thought-form, mathematicians have done such interesting things with the equivalence-class concept that it’s maybe worth connecting the mathematical type with the everyday type and see where it leads you.

What mathematics adds to the equivalence-class concept is the idea of “quotienting” to make a new equivalence-class. For example if you take the set of integers you can quotient it in two to get either the odd numbers or the even numbers.

- If you take a manifold and quotient it you get an orbifold—an example of which would be Dmitri Tymoczko’s mathematical model of Bach/Mozart/Western theory of harmonious musical chords.
- If you take the real plane ℝ² and quotient it by ℤ²

(ℤ being the integers) you get the torus 𝕋² - Likewise if you take ℝ and quotient it by the integers ℤ you get a circle.

- If you take connected orientable topological surfaces
`S`

with genus`g`

and`p`

punctures, and quotient by the group of orientation-preserving diffeomorphisms of it, you get Riemann’s moduli space of deformations of complex structures`S`

. (I don’t understand that one but you can read about it in*Introduction to Teichmüller theory, old and new*by Athanase Papadopoulos. It’s meant to just suggest that there are many interesting things in moduli space, surgery theory, and other late-20th-century mathematics that use quotients.) - If you quotient the disk D² by its boundary ∂D² you get the globe S².
- Klein bottles are quotients of the unit rectangle I²=[0,1]².

So equivalence-classing is something we engage in plenty in life and business. Whether it is

- grouping individuals together for stereotypes (maybe based on the way they dress or talk or spell),
- or arguing about what constitutes “science” and therefore should get the funding,
- or about which borrowers should be classed together to create a MBS with a certain default probabilities and covariance (correlation) with other things like the S&P.

Even any time one refers to a group of distinct people under one word—like “Southerners” or “NGO’s” or “small business owners”—that’s effectively creating an (Aristotelian) category and presuming certain properties hold—or hold approximately—for each member of the set.

Of course there are valid and invalid ways of doing this—but before I started using the verb “to equivalence-class” to myself, I didn’t have as good of a rhetoric for interrogating the people who want to generalise. Linking together the process of abstraction-from-experience—going from many particular observations of being cheated to a model of “untrustworthy person”—with the mathematical operations of

- slicing off outliers,
- quotienting along properties,
- foliating,
- considering subsets that are tamer than the vast freeness of generally-the-way-anything-can-be

—formed a new vocabulary that’s helpfully guided my thinking on that subject.

*Ordine geometrico demonstrata!*