Wanted To Experiment With Some Brushes On Csp So I Just Sacrificed Regan From @zzztlk And Doodled Her

Generalized Golf

I have looked up nothing about golf to write this.

Let C be any topological space. We will call this the ‘course’. For any two points x,y ∈ C we have a collection S_xy of ‘shots from x to y’, where each ‘shot’ s ∈ S_xy is a path in C from x to y, which is to say a continuous function s: [0,1] → C with s(0) = x and s(1) = y. For a shot s ∈ S_xy we call x its ‘start’ and y its ‘end’. Let S denote the collection of all shots in C between any two points.

A ‘hole’ on C is a triple (t,h,p) where t ∈ C is a point called the ‘tee’, h ⊂ C is a subset called (confusingly) the ‘hole’, and p is an ordinal number called the ‘par’. For any cardinal number κ we define a ‘golf’ of length κ to be a function g: κ → H, where H is a set of holes on C. A golf g is called ‘finite’ if κ is finite and the par of every hole in the image of g is finite. We define the par of a finite golf as the sum of the pars of its constituent holes.

A quintuple (C,S,κ,H,g) defined like above is called a ‘game of (generalized) golf’.

Take a hole (t,h,p), a successor ordinal ω+1. Let F: ω+1 → S be a function such that F(0) is a shot from t, for every i < ω the end of F(i) equals the start of F(i+1), the end of F(ω) is an element of h, and no F(i) ends in h before this. Such an F is called a ‘play’. We call ω the ‘score’ of F.

A ‘golfer’ is a collection of probability spaces, which for any shot s ∈ S with start x and end y gives a probability space on the set of shots from x. This is to be interpreted as the ways in which a shot can deviate from the golfer’s intent.

…

Now to define the real numbers by way of games of golf on ℚ.

If your 5 favorite Billy Joel songs were birds what would they be and why

Hmmmmm, that's a really good question that I hadn't thought of before.....let's see:

The Stranger (1977)

I think it'd have to be either a mimicking bird or a brood parasite. So I'm thinking either a Common Cuckoo, a brood parasite:

Or a European Starling, an expert mimic and also an invasive species here in North America:

It's Still Rock and Roll to Me (1980)

This song screams Rock Pigeon, with its stubborness and memory of a better time:

Pressure (1982)

This one has to be a bird that has a really long migration route, so why not the one with the longest, the Arctic Tern:

Lullabye (1994)

This song makes me think of endangered and extinct birds. I think of the last song of the Kauaʻi ʻōʻō, a bird that was formally declared extinct in 2023, but was last heard in 1987:

I also think of the Zebra Finch, often the subject of experiments about communication development, that can involve isolating a baby bird from their parents so they cannot learn their proper song:

Goodnight Saigon (1982)

This makes me think of the Wake Island Rail, which went extinct as a consequence of WW2 (this song is about Vietnam, but I think it can apply to a lot of war):

I also think of Edward's Pheasant, which is believed to be extinct in the wild, in a large part due to the effects of the Vietnam War:

Thank you so much for the question!!!

It’s impossible to give directions in Boston. Nothing makes sense. There are inexplicable one way streets, there are streets that change their names as you move from one area to another. There’s a road in my area where the gps literally tells you to take TWO full right turns to “stay on” the same road which is at a right angle to the original. There’s like four different Massachusetts Avenues. Sometimes you have to be in the left lane to turn right. The gps can’t even get directions to my workplace correct; it tells me to take a left on a road where lefts are not allowed, and the only way to not have to go across the river and take 15 minutes to turn around is to remember this shit one block early and make the left THERE. Recently, they’ve restricted 1 of 2 lanes on each side of major thoroughfares to only allow bikes and buses, and the government officials seemed to genuinely believe that would somehow EASE traffic. Oh and don't try to drive on Memorial Drive on Sundays; they close it for pedestrians. Just because. And when you DO drive on Memorial, there's one exit that will make your gps lose its mind and start chanting random sequences of numbers for four minutes straight. You can't take a Uhaul on Storrow Drive because the bridges that go over it are too short, and every year some doofus college student ignores this rule and proceeds to "get Storrowed" when they shave off the top of the truck on the overpass and get stuck. I-93 turns into I-95 and makes a big circle around the city, so a lot of the time you'll be on I-95 north but driving east or west.

It’s not limited to driving either. The Arlington train station is not in Arlington, it’s in the middle of downtown. Harvard Square is not a square, it’s more like a pentagon. There are four different green line train routes, and they’re labeled B for Boston College, C for Cleveland Circle, D for… Riverside, and E for… Heath Street. The Silver line is listed on the train map but is entirely run on buses which have to be connected and disconnected from power lines every time you go through the route. The Blue line goes to (and I’m not joking) Wonderland. The two red lines are labeled for their southern points: Braintree line goes to Braintree, and the Ashmont line goes to… Mattapan. To be fair, the train itself stops in Ashmont and you continue to Mattapan on a trolley, but that doesn't make it better. South Station and North Station are 1 mile apart and the easiest way to get from one to the other is just to walk it because otherwise you have to travel through 4 or 5 train stops on two different lines. But make sure you memorize the route because there's a good chance your gps will lose signal in the Financial district because it can't get through the buildings. In Boston Commons there are two train stops within line of sight of each other, on the same street, and one of them screams. To get to the trains at Porter Square, you have to ride down escalators 105 feet below street level, or you could just take the 3 flights of stairs totaling 199 steps (presumably because the engineers had something against nice even numbers). The North End is south of East Boston. Castle Island is part of the mainland.

No matter where you're going or how you're getting there, it takes 45 minutes (no wrong turns) or an hour and a half (one wrong turn). It doesn't matter if you're going one stop on a train; it will take 45 minutes. If it's summer, there's a better than 50% chance you'll be in the train car that lost it's AC; if it's winter, you're guaranteed to be in the car where the heat has it up to 80 degrees and the inside of your winter coat will be a sauna. Check the Red Sox schedule before you go south of the river, or you'll be trapped in the waves of baseball fans flooding the streets and days will go by before you're found again. And just... don't go outside on September 1.

If you're thinking that this sounds eldritch as shit, you're right. The entire city is an arcane lock keeping the ghoulies and ghosties from haunting the rest of the nation. We charge it with every "fuck" we utter while we travel our labyrinthine paths and drink our Dunks. You're welcome.

Just adding the former Twitter thread of analysis here for posterity.

threadreaderapp.com

Thread by @normative on Thread Reader App

@normative: I just got a graphic novel adaptation of Nineteen Eighty-Four and was pleasantly surprised to see they included the Appendix “Pr

Actually the implied timeline for the fall of Ingsoc is even shorter than that, on a second look. The Appendix says the Newspeak spoken in 1984 is captured in the 9th & 10th editions of the Newspeak dictionary, and that the 11th edition was the “final” one.  We don’t know exactly when the Party rose to power or started Newspeak, but if they’re already on the 10th edition by 1984 or shortly thereafter, and the Party doesn’t take power until after 1949, that implies new editions being released at absolute most every 3-4 years.  If the 10th edition is released roughly contemporaneously with the events of the book & Ingsoc lasts long enough to produce an 11th edition, but not a 12th, that suggests the collapse of the regime within about a decade of those events.  Another clue: Translation of English Lit classics to Newspeak were planned, and it “was not expected that they would be completed before the first or second decade of the 21st century,” after which the originals would be destroyed.  The use of the subjunctive—and the fact that the future authors of the Appendix assume readers remain familiar with works of Shakespeare, Milton, Jefferson etc.—also implies the fall of Ingsoc before “the first or second decade of the twenty-first century”.  So that’s at least two very strong clues Orwell plants that Ingsoc did not outlast the 20th Century, despite its seeming efficiency at ferreting out and breaking dissenters.

Absolutely amazing money quote too:

The large number of people responding that they did indeed miss this when they read Nineteen Eighty-Four suggests Orwell pulled off a rather brilliant literary prank: He wrote an optimistic epilogue, but hid it in the guise of an Appendix which many (perhaps most) readers either skipped or took at face value as just an essay about Newspeak, rather than a continuation of the story. And this is perfect. It would have been a literary crime to spoil the gut punch of “He loved Big Brother” by tacking on an overt happy ending epilogue. The happy ending is there, but Orwell makes you work for it.

Besties I am reading about the appendix of 1984 by George Orwell and I just re-read the actual appendix last night and I stg I'm gonna scream and cry and throw up because it's fucking past perfect tense and it's the "woulds" and it's the fact that the appendix is written in modern English. 1984 secret ending where Big Brother is long gone it makes me cry.

Today on the blog I start a new project: where do numbers come from?

By which I mean, mathematicians deal with lots of weird kinds of numbers. Real numbers, complex numbers, p-adic numbers, quaternions, surreal numbers, and more. And if you try to describe the more abstract types of "numbers" you sound completely incomprehensible.

But these numbers all come from somewhere. So I'm going to take you through a fictional history of numbers. Not the real history of the actual people who developed these concepts, but the way they could have developed them, cleaned up and organized. So in the end you can see how you, too, could have developed all these seemingly strange and abstract concepts.

This week in part 1, we cover the most sensible numbers. We start with the basic ability to count, and invent negative numbers, fractions, square roots, and more.

But that will still leave some important questions open—like, what is π? So we'll have to come back for that in part 2.

fields of mathematics

number theory: The Queen of Mathematics, in that it takes a lot from other fields and provides little in return, and people are weirdly sentimental about it.

combinatorics: Somehow simultaneously the kind of people who get really excited about Martin Gardner puzzles and very serious no-nonsense types who don’t care about understanding why something is true as long as they can prove that it’s true.

algebraic geometry: Here’s an interesting metaphor, and here’s several thousand pages of work fleshing it out.

differential geometry: There’s a lot of really cool stuff built on top of a lot of boring technical details, but they frequently fill entire textbooks or courses full of just the boring stuff, and they seem to think students will find this interesting in itself rather than as a necessary prerequisite to something better. So there’s definitely something wrong with them.

category theory: They don’t really seem to understand that the point of generalizing a result is so that you can apply it to other situations.

differential equations: physicists

real analysis: What if we took the most boring parts of a proof and just spent all our time studying those?

point-set topology: See real analysis, but less relevant to the real world.

complex analysis: Sorcery. I thought it seemed like sorcery because I didn’t know much about it, but then I learned more, and now the stuff I learned just seems like sorcery that I know how to do.

algebraic topology: Some of them are part of a conspiracy with category theorists to take over mathematics. I’m pretty sure that most algebraic topologists aren’t involved in that, but I don’t really know what else they’re up to.

functional analysis: Like real analysis but with category theorists’ generalization fetish.

group theory: Probably masochists? It’s hard to imagine how else someone could be motivated to read a thousand-page paper, let alone write one.

operator algebras: Seems cool but I can’t understand a word of it, so I can’t be sure they’re not just bullshitting the whole thing.

commutative/homological algebra: Diagram chases are of the devil, and these people are his worshipers.

Explaining math jokes

So yeah I said I was gonna do it and now is the time I think (if I wait any longer I'm gonna have too much jokes to explain) IMPORTANT: I had a lot of trouble writing this because I don't really know what background to assume the reader has. So at each new explanation, assumed background changes. Difficulty of the concepts explained is in no particular order. So if there is something you don't understand, that's fine, just go read something else. Dually, if there is something you already know well, don't throw away the whole post. Though it is very possible you already know everything I'm gonna rant about lol Anyways let's get to it

geometric group theory talk but on the speaker’s slide instead of the Cayley graph of the free group on two generators there’s just loss

(link) Geometric group theory is a subfield of math that studies groups using geometry. A particular geometric thing that is often interesting to study for a given group is its Cayley graph, which roughly speaking is a graph that reflects in a way the structure of the group. My neurodivergent brain thought the Calyey graph of the free group on two generators lowkey looks like the abstracted loss meme in the original post:

sooo, turns out the #latex tag is not for typography enthusiasts

(link) LaTeX is a typesetting engine and the industry standard for math. It's the thing almost everyone uses to typeset beautiful math equations and stuff on computers. If you're seriously interested in math I'd recommend you learn it. A good place to start would be Overleaf which is a free online LaTeX editor and has some tutorials on how to get started, though eventually you may want to switch to doing LaTeX on your computer directly

so a homological algebraist goes to see their therapist and says “doc, i’ve got complexes”

(link) Homological algebra is a branch of algebra that was born from algebraic topology. It has become widely used in many parts of math because of the computational power it brings. The gist of it is that people define a thing called a chain complex, that is a sequence of abelian groups (or modules, or maybe something even more general, look up abelian categories), with homomorphisms from one abelian group to the next called differentials, such that doing one differential then the next always gives you zero:

If you're more comfortable with linear algebra, you can replace "abelian group" with "vector space" and "homomorphism" by "linear map". The fact that doing one differential then the next gives you zero means that the image of one differential is contained in the kernel of the next. Homological algebra is about finding ways to calculate exactly how far away we are from the image of a differential being exactly the kernel of the next. This is made precise when one defines homology groups, which are the quotient Ker(d_n)/Im(d_{n+1}). What happens in practice when we apply homological algebra is that we try to define an interesting chain complex related to what we are doing, so that the homology groups tell us something interesting about what's going on with whatever math thing interests us, and then we apply methods from homological algebra to calculate them. Any serious example of homological algebra being used is going to require a bit of math background, but two examples I can give are singular homology, from algebraic topology, and de Rham cohomology, from differential geometry (don't worry about the co-, it just means the indices go up instead of down). So yeah a homological algebraist would have complexes

If you’re not careful and you noclip out of reality in the wrong areas, you’ll end up in Hilbert’s Hotel

(link) Hilbert's Hotel is an imaginary hotel with infinitely many rooms, that is one room labeled zero, one room labeled one, one room labeled two, and so on. One room for every natural number. It reminds me of the backrooms, hence the joke. Hilbert's hotel is commonly used as a metaphor to think about how infinity behaves and how bijections work. For instance, if the hotel was full, but one new guest showed up, you could still get them a room: simply tell the person occupying room number n to move to room number n+1. Then room 0 will be empty and the new guest can stay in it. However, quite interestingly, it is possible for too many guests to show up and the hotel to be unable to give a room to all of them.

If you speak French, excellent math youtuber El jj (I heavily recommend you subscribe to his channel!) has a very good video on Hilbert's Hotel.

If you don't speak French but still speak English, here's a Veritasium video on Hilbert's hotel, and a Ted-Ed video on it.

“cats are liquid” factoid actually formalized by mathematicians as saying a cat is only truly defined up to homeomorphism

(link) Topological spaces are mathematical objects that abstract away the concept of nearness. What do I mean? Well, a topological space is a set X, together with a collection of subsets called "a topology", that in way specifies which points are "near" each other. This allows us to generalize a lot of concepts from real analysis, for instance limits: if you have a sequence of points (x_n), and they get "nearer" and "nearer" to some point x, well that point can be called the limit of the sequence. But topology also turns out to be massively useful to geometry: if I only gave you the set of points of a sphere, you wouldn't know they make up a sphere because you wouldn't know how to assemble them. But if I give you the set of points of a sphere and the correct topology on it, then you can actually know it is a sphere and do stuff with it. But as always, in math, we only consider things up to some notion of being "the same". This notion of "the same" for topological spaces is called "homeomorphism", and two things being homeomorphic corresponds to the more intuitive geometric intuition of "I can continuously deform one thing into the other without cutting or gluing stuff". For instance, a cube is homeomorphic to a sphere:

Or more famously, a mug is homeomorphic to a donut:

So cats only truly being defined up to homeomorphism kinda works to say they're liquid. Not math, but physicist Marc-Antoine Fardin did actual physics on cats being liquid and was award the 2017 IgNobel prize in physics for it.

i would describe my body type as only defined up to homotopy equivalence

(link) Homotopy equivalence is a weaker notion of two topological spaces "being the same". I won't go into details but I have seen it being describe as kind of like a homeomorphism, but you are also allowed to inflate/deflate objects. For instance, a filled cube is a "3d object" in a way (when you are inside the cube, you can move in 3 directions). This means it will never be homeomorphic to a 2d square because a property of homeomorphisms is that they preserve dimensions. But, the cube is homotopy equivalent to the square, because you can "deflate" the cube and squish it to make it a square. In fact, it is even homotopy equivalent to a point (you can just deflate it completely). Homotopy equivalence is weaker (more permissive) than homeomorphism, that is if two things are homeomorphic, then they must be homotopy equivalent, but not the other way around. You may ask yourself why we would care about a notion weaker than homeomorphisms that can't even tell apart points and cubes, and that's a fair question. I will provide one answer but there are definitely many more I haven't even learned yet. In algebraic topology, we are concerned with studying spaces by attaching algebraic thingies to them. Why do we do that? Because telling apart spaces is hard. Think about it: how do you prove a donut is not homeomorphic to a sphere? You'd have to consider all possible deformations of a donut and show none of them is a sphere. This is mathematically hopeless. Algebraic topology solves this by attaching algebraic invariants to spaces. What do I mean? Well we have a way of saying that a donut has "one hole" and a sphere has "zero holes", and we have a theorem saying that if two things are homeorphic they must have the same number of holes (the number of holes is an invariant). Therefore we know that a donut cannot be homeomorphic to a sphere. Usually we have more sophisticated invariants (homotopy groups, homology groups, the cohomology ring, and other stuff) that are not just numbers but algebraic structures, but the same principle remains. It turns out a lot of these invariants are actually invariants for homotopy equivalence, that is, they will not be able to tell apart homotopy equivalent spaces. This is useful to know: for instance, a band and a Möbius band are both homotopic to a circle, so you know that if you want to tell them apart, you're going to need more than the classical algebraic invariants (if you know a bit of algebraic topology and you're curious about that, this can be done by thinking of them as vector bundles, but also through more elementary methods, see this stackexchange post). Also, if you want to calculate some invariants for a complicated space, a good place to start can be to try to find a less complicated space that is homotopy equivalent to the original space (and this is often doable since homotopy equivalence is a kind of weak notion).

in ‘Murica land of the free every module is born with a basis

(link) In 1st-year linear algebra, we study vector spaces over fields. But in more advanced linear algebra, we study modules over rings, which are basically vector spaces, but over rings instead of fields. It turns out dropping the condition that every non-zero scalar must be invertible makes the algebra much more complicated (and interesting!). When a module has a basis, we say it is free, hence the joke. If this basis is finite, we say that the module has finite rank, and the length of the basis is the rank of the module (exactly like dimension for vector spaces!), hence the tag "not every module ranks the same though".

testicular torsion? this wouldn’t happen over a field

(link) Continuing on modules, modules can sometimes have what is called torsion. Let's take Z-modules, or as you may know them, abelian groups! Indeed, a "vector space over Z" is actually the same as an abelian group: any module has an underlying abelian group (just forget you know how to scale elements) and conversely, if you take an abelian group, you know that any element a is supposed to be 1a, so 2a must be (1+1)a = 1a + 1a = a + a. More generally, for any positive integer n, n.a = a + ... + a, n times, and if n is negative, n.a = (-a) + ... + (-a), n times. So knowing how addition works actually tells us how Z must scale elements. With that out of the way, take the Z-module formed by the integers mod n, Z/nZ. It is an abelian group, so a Z-module, but something weird happens here that doesn't happen in a vector space: n.1 = 0. You can scale something, by a non-zero scalar (in fact a non-zero-divisor scalar), and still end up with 0. This is known as torsion, and vector spaces (modules over fields) don't have that. So yeah, testicular torsion? that wouldn't happen over a field. Also, watch out: the notion of torsion for a module over a ring is not necessarily the same as the notion of torsion for the underlying abelian group. Z/4Z doesn't have torsion, when seen as a Z/4Z-module.

Mathematical band names

(link) For these posts, I'll be quickly explaining each band name, and i'll be including good additions from other peeps! (with proper credit of course, you can't expect a wannabe-academic to not cite their sources) (also plagiarism is bad) (if no one is credit that means I thought of the band name)

Algebrasmith, The Smathing Pumpkins, System of an Equations, My Mathematical Romance, I Don't Know How But They Found X, Will Wood and the Tape Measures (by @dorothytheexplorothy), DECO*3^3 (by @associativeglassdesert), The Teach Boys, Dire Straight Lines, n Directions, XYZ Top, Mathallica: I have nothing to explain here

Rage against the Module: if you've read the parts of this post about modules, you get it (partly inspired by the commutative algebra class I'm taking right now, I love it, but I've been stuck on a problem for some time)

The pRofinite Stones: a profinite space is a topological space obtained by some process involving finite, discrete spaces. They are usually called Stone spaces, hence the joke

Mariah Cayley: Arthur Cayley was a mathematician. It's the same Cayley from the Cayley graphs! (also Cayley-Hamilton, if you've heard of the theorem)

Billie Eigen: eigenvalues and eigenvectors are linear algebra concepts. For a given operator on a space, its eigenvalues are scalars that tell us a lot about the operator. This is not my field but I have heard in quantum physics physical quantities like mass, speed, etc are replaced by operators, and eigenvalues correspond to states the physical system can be in

Smash Product: in algebraic topology/homotopy theory, the smash product is an operation on pointed topological spaces that is interesting for categorical purposes (it gives a symmetric monoidal category structure to the category of compactly generated pointed topological spaces, if you know what that means) somebody once told me, the world is categories

FOIL out boy (by @mathsbian): FOIL is a way of remembering how to expand products that some people learn. It means First, Outer, Inner, Last. So if you expand (a+b)(c+d) using FOIL, you get ac + ad + bc + bd.

Sheaf in a Birdcage (by @dorothytheexplorothy): a (pre)sheaf is a way of assigning algebraic data to a topological space (or a generalized notion of space). A presheaf is a sheaf if the data respects some locality condition. (pre)sheaves were introduced by Jean Leray but really used by Grothendieck to completely transform algebraic geometry, and are now widely used in modern geometry (they show up to abstract the notion of "a geometric thing"). I can't explain much more as I am still learning about sheaf theory!

The Curry-Howard correspondents (by @dorothytheexplorothy): the Curry-Howard correspondence, in logic/theoretical computer science, essentially says that algorithms (computer programs) correspond to mathematical (constructive) proofs. I'm no computer scientist or logician so I'll avoid saying dumb stuff by not trying to explain more, but I know it can be made more precise using lambda calculus.

Le(ast com)mon Demoninator (by @dorothytheexplorothy): I don't think I have to explain anything here (let's be honest, if you're reading this, you probably already know what a least common denominator is), but I will say that the band name being spoofed here is Lemon Demon (Neil Cicierega's musical project) and I love his music go listen to it. Also I love the word demoninator thank you for that dottie

Taylor Serieswift (by @associativeglassdesert): a Taylor series is an infinite sum that approximates a nice-enough (analytic) function around a point. This is useful because the Taylor series only depends on the derivatives of the function at one point but can approximate its behavior on more that one point, and also because the Taylor series is a power series, so a more tractable kind of function. In particular if we truncate it, that is stop at some term, we get a polynomial that approximates our function well around a point, and polynomials are very nice to work with (this is where kinda cursed stuff you may have seen in physics like sin(x) = x or tan(x) = x comes from!)

mxmmatrix (by @associativeglassdesert): you may have heard a matrix is a table of numbers. Actually, it's much more than that. Matrices are secretly functions! In fact, very special kind of functions (linear maps) between very special kind of objects (finite-dimensional vector spaces). And if you've seen how to multiply matrices before but have not been told why we do it that way, be not afraid, there is actually an answer. The answer is that when we take some x, do one linear map f to get f(x), then another linear map g to get g(f(x)), we actually end up with a new linear map, gf. And if you take a matrix representing f and multiply it (left) by a matrix representing g, you get a matrix representing gf. This is why the matrix product is done like that: it's actually composition of functions! If this interests you, consider reading more about abstract linear algebra.

Ring Starr (by @associativeglassdesert): a ring is an algebraic structure. Take the integers. What can we do with them? We can add them together, addition is associative (when adding a bunch of stuff we don't need parentheses), commutative (a+b = b+a), we have zero that doesn't do anything when adding (a+0 = a), and we have opposites: for any integer a, we have another integer -a such that (a + (-a) = 0). But we also have multiplication: multiplication is associative (no need for parentheses again), commutative, we have 1 and multiplying by 1 doesn't do anything, and multiplication distributes over addition. Now, re-read what I just said but replace "integer" by "real number". Or "complex number". When seeing such similar behavior by different things (there are in fact many more examples that those I just gave), mathematicians are compelled to abstract away and imagine rings. A ring is a set of stuff, with some way to add the stuff and some way to multiply the stuff that satisfies the properties I talked about above. Sometimes we also drop some properties, for instance we allow multiplication to not be commutative (ab =/= ba). By allowing this, square matrices of a given dimension form a ring! Quaternions, if you know what they are, also form a ring. A lot of things are rings. Rings are cool. Learn about rings.

WLOGic (by @associativeglassdesert): WLOG is mathematician speak for "without loss of generality".

Alice and Bob Cooper: in many math problems, people are called Alice and Bob. Because A and B. Yes there is a Wikipedia page for this

The four Toposes: a topos is a kind of category meant to resemble a topological space. Grothendieck toposes are used in algebraic geometry and elementary topoi are used in logic. I can't explain more since I don't really know anything about topoises besides that they are kinda scary and that people really like to argue about what the plural of "topos" should be

Green-Tao Day: the Green-Tao theorem says that if you have a positive integer n, then you can find prime numbers p1, p2, ..., pn, such that they are evenly spaced (or equivalently, in an arithmetic progression). It's pretty neat. I have no idea how the proof goes, though. It must be pretty complicated, since it was proven in 2004.

Aut(Kast)/Inn(Kast): I'm really proud of that one. So if you have a group G, you can look at bijective group homomorphisms from G to G, or as they are more well-known, automorphisms of G. Together with composition, they form a group, called Aut(G). Now we already know of some automorphisms of G: if g is any element of G, then x ↦ gxg^{-1} is an automorphism of G (proof is left as an exercise to the tumblr). These automorphisms are called inner automorphisms of G, and they form a normal subgroup of Aut(G). The quotient group Aut(G)/Inn(G) is called the outer automorphisms of G and denoted Out(G), which is reason behind the band name.

Depeche modulo: modulo is a math word that means "up to [some notion of being the same]". For instance the integers modulo 7 are the integers but we declare that two integers a and b are the same if 7 divides a-b. From there we get modular arithmetic which you may have heard of. This kind of operation is called a quotient and is insanely useful in all branches of mathematics.

Phew! We're done with the band names. For now.

"oh you like math? what's 1975 times 7869?" well that's a great question Jimmy but to answer it first I need to construct the natural numbers. [...]

(link) So this is a post about a type of response math people get when they say they do math which is that people automatically assume this give us insane mental math power. It does not. The rest of the post is about constructing the natural numbers in the ZFC axiomatic system. I'm kinda lazy and don't want to get into all that but here's a good video by certified good math channel Another Roof about it: what IS a number? The same channel has several other videos on that same topic, go watch em if you're interested

1957 times 7869 (IF IT EVEN EXISTS) is the universal object with morphisms into 1957 and 7869

This is a joke by @dorothytheexplorothy in the notes of the previous post. The joke here comes from interpreting "times" are referring to the categorical notion of product. I'm actually not gonna explain anything here because 1) this post is taking forever to write and 2) I will probably rant about category theory in the future. Here are two videos by Oliver Lugg you can watch:

27 Unhelpful Facts About Category Theory (funni video)

A Sensible Introduction to Category Theory (serius video)

and here are two books you can use to learn more if you're interested:

Seven Sketches in Compositionality (very applied, very nice, I think easy to read)

Basic Category Theory (less applied, is a typical math book)

she overfull on my \hbox till i (5.40884pt too wide)

(link) This she on my till i joke is based on a LaTeX warning you get when it can't figure out how to typeset your document well and that leads to a margin being exceeded.

Time for the math battle reblog chain

(half of the posts are by @dorothytheexplorothy)

fuck you *forgets your group is a group and only remembers it's a set now*

So any group has an underlying set, and any group homomorphism is actually a map between these underlying sets. This means that the operation of "forgetting a group is a group and only remembering it's a set" is a functor. This is less useless than you might think, because of adjunctions.

two can play at that game *constructs a free group over this set, even bigger and better than the one I had*

So basically in lots of cases the functor that forgets some structure is (right) adjoint to some other functor. You do not need to know exactly what this means to read the rest, don't worry. What it means is basically that from the operation of forgetting some structure, we can get another operation, which adds structure, in a "natural" way. In the case of forgetting a group is a group and only remembering it's a set, the adjoint functor is the "free group" functor, that takes a set and constructs the free group on it. This idea of free objects works not just for groups but for a whole lotta stuff. See this part of the Wikipedia page on forgetful functors for some information.

oh don't get me started *abelianizes your free group, now it's just a big direct sum of Z's*

A non-abelian group can be turned into one through abelianization, which is quotienting out by the commutator subgroup. This makes sense: commutativity is asking ab = ba for all a, b, which is asking aba^{-1} b^{-1} = 1 for all a, b, which is precisely what we get when quotienting by the subgroup generated by words of the form aba^{-1} b^{-1}. Abelianization is also a functor, so it fits the theme. The abelianization of a free group is a free abelian group, and a free abelian group is a direct sum of a bunch of copies of Z.

big mistake, friend *moves over to the endomorphism group over that group and treating composition as multiplication, thus replacing it with a unital ring*

The endomorphism group of an abelian group is actually a ring (like in linear algebra, endomorphisms form a unital ring with composition as multiplication). I don't think this construction is functorial, though? (correct me if I'm wrong on that. correct me if I'm wrong on anything, really. if i'm wrong about stuff send me an ask and i'll fix it)

you fool, you fell right into my trap! *takes the field of fractions of your ring* have fun working in the category of fields! now you only have monomorphisms and your eyes to shed tears

So I thought I had the advantage here because fields are, categorically speaking, very bad. This is (I think) mainly because a homormorphism of fields is always injective (so is a monomorphism, that is left-cancellable). In fact, products of fields don't exist, direct sum of fields don't exist, a lot of categorical constructions we usually like don't exist in this category. We basically only have inclusions. I will elaborate on why I was wrong in my post here in a bit

fuckkk idk enough about schemes or whatever to get out of this! you've bested me X(

Schemes are the main objects of study of algebraic geometry. I won't being to try and explain what they are because it is very abstract and I don't even really understand the definition (yet). I just know they're algebraic geometer's analogue of a "geometric object", like how smooth manifolds are to a differential geometer.

wait actually I just realized the ring of endomorphisms of a free abelian group has no business being an integral domain, or even commutative. so I think taking the field of fractions makes no sense, and I actually lost the battle.

So the field of fractions construction only makes sense for integral domains. The name of this construction is really explicit: passing from an integral domain to its field of fractions is the same thing as passing from Z to Q, or from k[X] to k(X) if you know what that is. However I made a mistake, since the ring we were talking about is almost never commutative (much like matrices).

WON ON A TECHNICALITY LET'S GOOOOO

well played, dottie

yeah, uh, we oidified your boyfriend. yeah we took his core concept and horizontally categorified it. yeah he's (or they're?) a many-object version of himself now. sorry about your one-object boyfriendoid

(link) Oidification (also known as horizontal categorification but "oidification" sounds funnier) is a way of categorifying a concept, by turning it in a "many-object" version of itself. For instance, a one-object category is precisely a monoid, so the concept of category is the oidification of the concept of a monoid. A category where every morphism is invertible is called a groupoid, and a one-object groupoid is precisely a group. The name "oidification" probably comes from the fact that after being oidified, the name of the concept gets added the suffix -oid. So a category is a monoidoid. In fact, you can even have monoidal monoidoids. Category theory really is well-suited to shitposting huh

My advisor [...] stared into my soul and noticed I liked categories. It's over for me, i am going to end up a homotopy theorist, or worse, a youtuber

(link) Category theory has the reputation of being abstract nonsense. I don't disagree. I guess I have a slightly-above-average tolerance to category theory and algebra. This has led to a not-insignificant amount of people in my life telling me I'm gonna end up in one of the abstract-nonsense-related fields like homotopy theory, infinity-category theory, etc. The "or worse, a youtuber" part was stolen from the following quote

Research shows that when someone becomes personally invested in an idea, they can become very close-minded. Or worse, a youtuber.

-hbomberguy, Vaccines and Autism: A Measured Response (4:12)

(this video is incredible, if you haven't seen it yet, go watch it)

PHEW.

I'm done. For now. This took multiple hours to write. I hope you enjoyed this post! If you enjoyed it, please let me know! If you have any questions or want to tell me "youre doing good lad" or want to yell at me, my asks are open! Thank you for reading this far! If there is a post I talked about here you found funny, you can click on the (link) to look at the original post. Give me those sweet sweet statistics. I crave them. I NEED that dopamine hit of knowing someone interacted with my blog. ok bye

physics is scary what the hell is the Riemann zeta function doing in the equation for the critical temperature of a Bose-Einstein condensate

So I recently stumbled on the Wikipedia article for the Grothendieck-Riemann-Roch theorem, which is an algebraic geometry thing that I'll hopefully learn some day once I actually have the prerequisite knowledge =w= But at the top of the article was this letter, which I thought was a wild thing to have at the top of a Wikipedia article about a niche abstract math thing - here's a translation:

Witches' Kitchen 1971 Riemann-Rochian Theorem: the latest craze*: the diagram

is commutatif**! To give this statement about f: X->Y some approximative meaning, I had to abuse the listeners' patience for nearly two hours. In black and white (in Springer's Lecture Notes) it seems like it will take up to about 400, 500 pages. A gripping example of how our thirst for knowledge and discovery indulges itself more and more in a(n il?)logical delirium far removed from life, while life itself is going to hell in thousandfold ways - and is threatened with absolute annihilation. High time to change our course! (6.12.1971) Alexander Grothendiek

* "der letzte Schrei" is a reasonably common German idiom meaning "the latest craze", but here it could alternatively be translated non-idiomatically as something like "the last cry". I think its more fun to imagine he means the idiom. ** I'm assuming this is a weird old-timey spelling probably taken from french but googling it I can find no examples of anybody using this spelling in German besides this letter

Note that this is 20 years before all of this happens:

I have a new post up on my blog, continuing the Fictional History of Numbers series. In part 1 we started with the natural numbers and built up the algebraics, which let us solve equations. In part 2 we started asking geometric questions, and constructed the real numbers.

But the real numbers are weird and hard to define. In part 3 we see one way they're extremely strange, and then talk about why we want them anyway. In the end, we shouldn't worry about the definition of the reals; we should worry about what they allow us to do. And it turns out they're exactly what we need to make calculus function as it should.