Born To Go To Beautiful Libraries And Study, Forced To Hustle In My Room

This is very interesting! It makes sense that, multiplication being repeated addition, it should be able to arise from just the group structure of the integers, but it never occurred to me before reading this. Thanks for the write-up.

Okay so to get the additive group of integers we just take the free (abelian) group on one generator. Perfectly natural. But given this group, how do we get the multiplication operation that makes it into the ring of integers, without just defining it to be what we already know the answer should be? Actually, we can leverage the fact that the underlying group is free on one generator.

So if you have two abelian groups A,B, then the set of group homorphisms A -> B can be equipped with the structure of an abelian group. If the values of homorphisms f and g at a group element a are f(a) and g(a), then the value of f + g at a is f(a) + g(a). Note that for this sum function to be a homomorphism in general, you do need B to be abelian. This abelian group structure is natural in the sense that Hom(A ⊗ B,C) is isomorphic in a natural way to Hom(A,Hom(B,C)) for all abelian groups A,B,C, where ⊗ denotes the tensor product of abelian groups. In jargon, this says that these constructions make the category of abelian groups into a monoidal closed category.

In particular, the set End(A) = Hom(A,A) of endomorphisms of A is itself an abelian group. What's more, we get an entirely new operation on End(A) for free: function composition! For f,g: A -> A, define f ∘ g to map a onto f(g(a)). Because the elements of End(A) are group homorphisms, we can derive a few identities that relate its addition to composition. If f,g,h are endomorphisms, then for all a in A we have [f ∘ (g + h)](a) = f(g(a) + h(a)) = f(g(a)) + f(h(a)) = [(f ∘ g) + (f ∘ h)](a), so f ∘ (g + h) = (f ∘ g) + (f ∘ h). In other words, composition distributes over addition on the left. We can similarly show that it distributes on the right. Because composition is associative and the identity function A -> A is always a homomorphism, we find that we have equipped End(A) with the structure of a unital ring.

Here's the punchline: because ℤ is the free group on one generator, a group homomorphism out of ℤ is completely determined by where it maps the generator 1, and every choice of image of 1 gives you a homomorphism. This means that we can identify the elements of ℤ with those of End(ℤ) bijectively; a non-negative number n corresponds to the endomorphism [n]: ℤ -> ℤ that maps k onto k added to itself n times, and a negative number n gives the endomorphism [n] that maps k onto -k added together -n times. Going from endomorphisms to integers is even simpler: evaluate the endomorphism at 1. Note that because (f + g)(1) = f(1) + g(1), this bijection is actually an isomorphism of abelian groups

This means that we can transfer the multiplication (i.e. composition) on End(ℤ) to ℤ. What's this ring structure on ℤ? Well if you have the endomorphism that maps 1 onto 2, and you then compose it with the one that maps 1 onto 3, then the resulting endomorphism maps 1 onto 2 added together 3 times, which among other names is known as 6. The multiplication is exactly the standard multiplication on ℤ!

A lot of things had to line up for this to work. For instance, the pointwise sum of endomorphisms needs to be itself an endomorphism. This is why we can't play the same game again; the free commutative ring on one generator is the integer polynomial ring ℤ[X], and indeed the set of ring endomorphisms ℤ[X] -> ℤ[X] correspond naturally to elements of ℤ[X], but because the pointwise product of ring endomorphisms does not generally respect addition, the pointwise operations do not equip End(ℤ[X]) with a ring structure (and in fact, no ring structure on Hom(R,S) can make the category of commutative rings monoidal closed for the tensor product of rings (this is because the monoidal unit is initial)). We can relax the rules slightly, though.

Who says we need the multiplication (or addition, for that matter) on End(ℤ[X])? We still have the bijection ℤ[X] ↔ End(ℤ[X]), so we can just give ℤ[X] the composition operation by transfering along the correspondence anyway. If p and q are polynomials in ℤ[X], then p ∘ q is the polynomial you get by substituting q for every instance of X in p. By construction, this satisfies (p + q) ∘ r = (p ∘ r) + (q ∘ r) and (p × q) ∘ r = (p ∘ r) × (q ∘ r), but we no longer have left-distributivity. Furthermore, composition is associative and the monomial X serves as its unit element. The resulting structure is an example of a composition ring!

The composition rings, like the commutative unital rings, and the abelian groups, form an equational class of algebraic structures, so they too have free objects. For sanity's sake, let's restrict ourselves to composition rings whose multiplication is commutative and unital, and whose composition is unital as well. Let C be the free composition ring with these restrictions on one generator. The elements of this ring will look like polynomials with integers coefficients, but with expressions in terms of X and a new indeterminate g (thought of as an 'unexpandable' polynomial), with various possible arrangements of multiplication, summation, and composition. It's a weird complicated object!

But again, the set of composition ring endomorphisms C -> C (that is, ring endomorphisms which respect composition) will have a bijective correspondence with elements of C, and we can transfer the composition operation to C. This gets us a fourth operation on C, which is associative with unit element g, and which distributes on the right over addition, multiplication, and composition.

This continues: every time you have a new equational class of algebraic structures with two extra operations (one binary operation for the new composition and one constant, i.e. a nullary operation, for the new unit element), and a new distributivity identity for every previous operation, as well as a unit identity and an associativity identity. We thus have an increasing countably infinite tower of algebraic structures.

Actually, taking the union of all of these equational classes still gives you an equational class, with countably infinitely many operations. This too has a free object on one generator, which has an endomorphism algebra, which is an object of a larger equational class of algebras, and so on. In this way, starting from any equational class, we construct a transfinite tower of algebraic structures indexed by the ordinal numbers with a truly senseless amount of associative unital operations, each of which distributes on the right over every previous operation.

i hate when i browse math-related tags on this site and half the posts are people ranting about how much they hate her. why are you so mean to my wife

math people scare me. math people will be like "math works in mysterious ways TO YOU. i get it though." and they do and it's fucking terrifying.

A lot of people absolutely fail to distinguish between 'I hate this because I was taught this badly' and 'I hate this because this thing is inherently bad', and I hate it.

you know I absolutely agree with the fact that the education system is fucked and traumatises students. I absolutely agree with the fact classes like english and maths and history can be piss boring and teach you nothing of worth and could be a trigger depending on how hellish of a teacher you were given.

Hell, the only reason I love maths is because I primarily engaged with it outside of class and found it fun on my own terms. I'm still bitter towards my history teachers for having clear favourites and not engaging half the class.

but like. if you're railing against the *subjects* themselves, refusing to learn any of the basics as an adult, and bringing up how much you hate them at every opportunity? At some point that becomes a problem you have to deal with yourself.

and if you don't, you're going to be the reason this happens again for the next generation.

Parents (when their child is young): "I know everything. You must obey my every command. The tiniest amount of doubt toward me is considered so disrespectful that I'm justified in hitting you for it.

Parents (when their child is grown): "Please forgive me. I'm only human. There's a lot I didn't know. I tried my best."

You took this photo? I've loved it for a long time!

when people are like “he’s not even attractive you could find a guy that looks like him at any gas station” i’m like….. well you see there’s beauty everywhere actually

ok so this is probably just me being a self-obsessed heartless dumbass but is it me or is it that when you're bad at doing something people just tell you to try harder but if you're REALLY BAD at doing something they say "oh it's [mental illness]" and try to actually give you help. what if i do have a mild version of a thing why shouldnt i get help. at least some. just a crumbe of adderall and empathy plz

Discovering something new in mathematics and then naming it after Euler just to fuck with people.

oh youre a math phd? Name 5 equations

1. The functional equation for the completed L-function associated with a modular form.

2. The Analytic Class Number Formula

3. 196883 + 1 = 196884

4. Chinese Remainder Theorem (gives an “equality” of rings)

5. Prime Number Theorem (gives an asymptotic equality of two functions)

(Can you tell what kind of mathematician I am from this? I feel like it’s pretty obvious lmao)

Also Griebach, known for the Griebach Normal Form!

Discover that a name you’ve been hearing for years belongs to a woman! (shout out Phyllis Nicolson of Crank-Nicolson method fame)