It's My Turn (1980) Snake Lemma

[adric137]

What is a lemma?

[killingbuddha1]

You just CAN'T do cohomology of groups just after explaining Snake Lemma :D

[bakethehake]

No sound.

[faritop]

Ok I admit I'm not a fan of Harry Potter movies. Buut it was a clever joke i will give you that much.

[79dharv]

I asked an ex-math prof friend of mine whether he'd ever had any students as annoying as the one depicted here. He replied, "Yes, the really good ones."

[bel1689]

the proof of this lemma is fun. definitly no magic in there. because there are so few given pre-conditions there is nearly no freedom how to act in every single step of this proof. just take one element, see that the preimage is not empty, map one element of the preimage via the given homomorphism and so on. the mystery of diagram based lemmas may be, that the diagrams are just a simplification to present such problems graphically. if u dont like this you may also formulate it in pure formulars.

[nonindividual]

Wow, not all movies were as badly researched as Good Will Hunting!

[belyi3]

i guess i'm not too good at it, but homological algebra always seemed like magic to me. the snake lemma, the 4-, 5- and 9-lemmas, derived triangles, the equivalence between various definitions of Tor and Ext -- of course if you're told these things are true you can follow your nose to prove them, but -- WHY really are they true? and how did people come up with them? i'm mystified.

[weylwargot]

@belyi3 They are true cuz you prove them. DIagram chasing is a technique based on which one writes a proof, not the proof itself. Once one gets used to, one obviates the proof. There is no magic, and if there is, then there is magic also in complex variables, diferential geometry and so on. Anyway, thats an opinion and I am not mathematician.

[faritop]

@belyi3

More like Module Theory... besides there is nothing misterous about Snake's Lemma and 4-,5- and 9-lemmas. And if you really know what you are talking about you wouldn't be mistyfied. THERE IS NOTHING SPECIAL HERE just some technical lemmas than in practice are not even used.Lastly they are really true because people showed proof already.

[wdgarraway]

@faritop Just to be pedantic - it's "the snake lemma", not Snake's lemma. (unless of course it gets used in your average potions class with professor Snake).

[faritop]

@wdgarraway

Just so you know : My native lenguage is spanish so for me it's the "Lema de la serpiente". I apologize for my english grammar, it's a common mistake for me to add " 's " where they aren't supposed to be. And why in god's( god??) green earth are you talking about potion classes, my major is in mathematics not in potion brewing.

[wdgarraway]

@faritop No need to apologize. Clearly your English is much better than my Spanish.

The potions joke was a reference to Harry Potter. One of the characters is Professor Snape who is a member of slitherin house which is represented by a snake.

OK it was a very bad joke :)

[abc75]

@belyi3 Valid point: how do these things come about? The answer is: from algebraic topology. Starting with Euler characteristic and Betti numbers of surfaces, one generalises to homology and cohomology groups. Then some very smart people realised that the underlying operations in homological & cohomological computations can be abstractised so that only the algebraic remnants remain. So this is what you get.

Like you, I'd love to see a more historically motivated account of cohomology.

[woodchiu]

@belyi3 eh, actually in the proof to snake lemma, we did not use any homological algebra-- just simply diagram chasing, using the property of the exact sequence and homomorphisms. In my opinion, people come up with those you talked about because they are useful, just as the spectral sequences, which is a pain when you learn it, with lots of formal stuffs but is a treat when you need to compute certain groups.

[scholzie]

Are you people seriously debating the teacher's lesson plan and whether or not the student was being disruptive? IT IS A MOVIE.

[Gebstadter]

fuckface

[yoda31419]

This was part of my algebraic topology homework, to watch this video (after proving the Lemma ourselves of course :D)

[gregersj]

I was surprised I could'nt find this on netflix

[Fermatprime]

I'm impressed -- her gestures are pretty much what I'd expect from someone actually teaching this proof. Did they use a math double for this scene?

[dan131m]

Have you ever observed students in a math class? If this guy was the only one who was confused, the other students would immediately jump in and try to help him out. They're not, though -- they just kind of stare ahead, which is a pretty good indicator that they've just totally glazed over -- as you would expect if you tried to give someone their first exposure to homological algebra by running through a relatively technical lemma at breakneck pace and leaving no time for questions.

[trigalg693]

@dan131m Actually I don't know about that. In my experience, sometimes you just keep quiet because they're hopeless :shrug: Either that, or people get tired of trying to help out, and give up and let the teacher take care of it.

[dan131m]

That's some good professoring there... just shout people down when they have questions and proceed to talk over them.

[thx291]

I didn't see her shout him down. She was teaching the class and he was being disruptive.

[dan131m]

I don't see "disruptive" by any means. He has a legitimate concern about the argument -- he can see that g^{-1}(x) is a set, not necessarily a particular point, and so it's natural to wonder about well-definedness when she starts hitting it with other maps. Given the reaction of the other students, who don't seem to be reacting as though he's holding the class back, it seems they probably don't understand what's going on either.

[thx291]

He asked a question, she then answered it and moved on. It's a math class not a caucus.

[quiprimusaboris]

He is definitely being disruptive. She hasn't made any claims of uniqueness or well-definedness yet and he insists on talking over her and demanding that she talk about about these issue before she had planned to.

It isn't completely clear if he's just trying to show off or if he's genuinely confused about this point, but either way, for the sake of his fellow students, he should wait to ask until she's done presenting the proof, in the (unlikely) event that she doesn't address these points.

[DreamFairy0]

He was being inquisitive because he was interested in understanding, not memorizing.

[dalailana88]

he reminds me of Andy Samberg.

[hypermorphism]

LoL, thanks, I've been looking for this movie. Not a lot of real math in movies anymore.