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Michi’s blog » The why and the what of homological algebra

 The why and the what of homological algebra

  • July 12th, 2007
  • 7:35 pm

I seem to have become the Goto-guy in this corner of the blogosphere for homological algebra.

Our beloved Dr. Mathochist just gave me the task of taking care of any readers prematurely interested in it while telling us all just a tad too little for satisfaction about Khovanov homology.

And I received a letter from the Haskellite crowd – more specifically from alpheccar, who keeps on reading me writing about homological algebra, but doesn’t know where to begin with it, or why.

I have already a few times written about homological algebra, algebraic topology and what it is I do, on various levels of difficulty, but I guess – especially with the carnival dry-out I’ve been having – that it never hurts writing more about it, and even trying to get it so that the non-converts understand what’s so great about it.

So here goes.

Alpheccar writes that to his understanding, the idea is to build topological spaces out of algebraic gadgets, and then do topology on them. This is a part of the story, and certainly historically very important, but it is far from all of it.

Motivations

The revolution for homological algebra pretty much started with Eilenberg-MacLane – who wrote an articlebook that did the constructions necessary for the very topological versions of homological algebra – but without ever involving the actual topological spaces.

The point is that the way you do algebraic topology is that you tend to set up a functor Top → R-ChMod that assigns a chain complex of R-modules to each (nice enough) topological space, and then you add functors R-ChMod → R-Mod that extract informations from these. Typical examples are cellular chain complexes with coefficients somewhere nice for the first functor, and then homology or cohomology for the second functor – depending on what viewpoint is the most obvious.

The revolution was that we simply throw out that first functor.

In order to study (co)homology, we don’t really need to care that there was a topological space somewhere to begin with. We only, really, need a nice enough category of chain complexes (if it’s abelian, then that’s fine – we get the long exact sequences in homology and other niftiness easily then, but if it’s not, triangulated will do…) and we study certain types of functors from these to module categories.

Homological algebra as a tool for algebraic topology

Since, in the viewpoint introduced above, homological algebra is a part of the process used in algebraic topology, it turns out to be really neat to sit down and just prove a lot of neat results in homological algebra – with the background that at some later point, these might be useful once you sit down with the topology. I got hold of this particular point early – I had started my MSc thesis work in homological algebra before I took my first real topology course, and during that course, the less pointset topology and the more algebraic topology we did, the easier everything got. The fundamental results we needed to grasp to do algebraic topology in any amount of seriosity were basically just applications of all the cornerstone results in homological algebra, and thus perfectly obvious to my clique of arrogant undergrads.

This particular piece goes far. Why don’t we need to worry about whether we’re doing homology or cohomology? Answer: since Ext and Tor are dual in certain specific ways, which ends up meaning that although internal algebraic structures might be finicky, the module structure is very neat, and in k-Mod = Vectk, we end up with no worries anywhere.

Testing grounds

The viewpoint of homological algebra as a tool for algebraic topology goes pretty deep. When I ask my advisor what to put in texts where I motivate why our field is important, in the standard answer he gives me the following pops up:

Group cohomology is important, since it is a field where topological methods can be tested reasonably safely, since we have the group theoretic attack vectors in addition to the purely topological.

On the other hand, group cohomology also turns out to be important, since we get important information for the study of groups out of the homological algebra side of things.

Low order Ext

The area where this is most notable is in representation theory. This field comes in several flavours: group representations, where we study kG-Mod for some (sometimes finite) group G; Lie algebra representations, where we study g-Mod for some Lie algebra g; quiver representations, where we study kQ-Mod for some (finite) quiver algebra kQ – and so on. One question that tends to crop up here, and with a high degree of importance for the non-homological algebraists around me – is what happens if we know only parts of our group? Can we say something about the entire group based on that?

It turns out that we can. There are very neat correspondences between the lower order Ext groups over kG and the behaviour of G itself. I’m going to stick to group representations here, since that’s the area I know best – however, this is something that pops up analogously all over the place.

Extensions

Suppose you have some R-module K that you know embeds, in some specific way, into some larger R-module M. And suppose you find the quotient L=M/K in some manner. What could, then, M be? One obvious answer is G=K\oplus L, but is this enough? This ends up depending on Ext1R(L,K), with each element of this particular Ext group indexing precisely one such extension.

This is at the core of Maschke’s theorem, by the way, which says that if the characteristic of the field k doesn’t divide the group order |G|, then by a specific construction, the only extensions possible for any kG-modules are the split extension – the one where we just take the direct sum.

This all leads to a wealth of useful information and ideas in representation theory. For instance, there is a way to describe modules proposed by Dave Benson and some co-authors, where you draw diagrams with each vertex being a simple module, occupying that spot in a composition series, and the edges being taken from the relevant Ext1.

Invariants and coinvariants

Suppose you have a group acting on a vector space. This can be taken extremely physical – quantum mechanics is all about this kind of situation, or so I’ve heard. Then it might be interesting to figure out the invariant subspace: {a|ga=a for all g in G}. This is Ext0. Or we might want a basis for the complement: representatives for every way that things can move. This is the coinvariant vector space, defined as A/(ga-a), and this is just Tor0.

Simples, projectives and the stable module category

Simple modules are nice. They don’t have invariant subspaces. In the best of all worlds – which is when Maschke holds – simple modules are precisely the irreducible modules. However, when Maschke doesn’t hold, we can have non-trivial Ext1, and thus we can build larger modules out of simples by a kind of gluing: they aren’t just a nice direct sum of simples, but something ickier.

Thus, unless Maschke holds, there will be weird things happening in the module category.

These weird things, though, are controllable. To be specific, we can consider the smallest possible irreducible modules. These will end up being building blocks, and for nice enough worlds, these will also end up corresponding closely to the simples – in the way that we can allocate a simple to an irreducible projective in a bijective manner.

So … what is this projective I keep throwing around? Take a free module. This is a direct sum of a finite number of copies of the ring R. This will have direct summands. By picking apart all summands into further direct summands, at some point we hit bottom: we cannot pick anything apart any longer. This is, by the theorem of Krull-Schmidt, a well-defined state of being. We can permute things, but in essence, a module is just its decomposition into irreducibles.

So, anything that is a direct sum of a free module is a projective. We can lose projectivity by taking quotients – so if we add relations, we may well get lost. But as long as we just look for direct summands, we’re pretty much home free. Now, the irreducible projectives have to be summands of the ring R itself, so they end up actually being (left) ideals in the ring. And each of them corresponds intimately to a simple module.

One trick that’s very beloved among the people who worry about these things is to get rid of anything projective, and look at the stable module category. In this, we just quotient away anything projective – morphism sets are taken modulo morphisms that factor through a projective… This way, we only have the “essential”, or as it is known to the experts of the field “difficult” information left. Then Extn(M,N)=Hom(ΩnM,N), where Ωn is the nth syzygy – see below for more on this.

So, homological algebra lets us understand the stable module category, which in turn lets us understand the parts that are essential to the module category structure.

Resolutions

I just promised you I’d tell you about syzygies. First off, some personal information – because readers always love that!

If you find me on IRC, on EFNet or on Freenode, you’ll find me under the nick Syzygy-. The – is there because there is someone who’s been using Syzygy for years and years on EFNet and because I’m not deliberately trying to be a bastard if I can help it. The rest of the nick is there to a certain extent because I like the way I write it in longhand.

And to a certain extent because it is an epitome of why homological algebra is interesting in my eyes.

Suppose we are interested in a finitely presented module, which we might be for many reasons, including being interested in algebraic geometry and in solving systems of polynomial equations. We might then just figure out what relations hold within a set of generators, which gives us a generating set, and some relation set.

These, relations, though are far from guaranteed to be the whole story. It’s probable that there are non-trivial relations between the relations. What do we do? We figure out what these are. They span the first syzygy module of the module we started with, denoted by ΩM. But this is unlikely to be free, so we can keep on going.

This way, we get a sequence of modules, all of which are free – since we just choose a generating set in each step – and with maps between them adding all the extra relations. But this is nothing other than a free resolution of the starting module. And here comes the candy that hooked me for this discipline: studying modules over their resolutions is the same thing as studying what chain complexes are, deep down, which in turn is the same thing as studying homological algebra.

Want to figure out what a module map means for the family of syzygies? What you really want is a chain map in the chain complex category. But some of these maps – or even portions of maps – will not carry relevant information. So we factor those away, and we get a slightly weirder category. But here, equality doesn’t quite mean what it should, so we add in more equality relations. And suddenly, we live in a derived category – and in here, the Hom sets are Ext groups, and the tensor products are Tor groups.

Number theory, geometry, and computation!

To continue this tour de force, consider the theorems in vector calculus relating various triple and double integrals. (note – I never dealt with this. I rode on technicalities to root out calculus from my curriculum so it would fit more algebra….) These theorems, in the end, only state that \partial^2=0, which is at the core of what homological algebra is all about.

If we formalize this particular recognition a bit, and tug at the corners, we end up with de Rham cohomology, which deals with what you can do with differential forms on manifolds (layman speak: things you can integrate. The f(x)dx after the integral sign is a typical differential form) – and this is one of the many many places where cohomology rather than homology ends up being the “right” way to view things just because you started out as a geometer instead of .. well .. topologist or algebraist.

The same kind of thing happens in algebraic geometry as well. You start out happily with your varieties, you conclude that as soon as things get interesting, the nice and pretty concepts of coordinate rings don’t hold up, and you’re forced to go to coordinate sheaves. And then you try to figure out what you can do with sheaves of functions on a variety – and before you know it, you reconstructed sheaf cohomology. This, by the way, a quick look at wikipedia told me, lets you define euler characteristics for varieties in a way consistent with the classical uses of it.

I am no geometer, and I’m not the person to tell you about the intricacies of these things. If you understand them, though, I’d love to figure them out at some point.

The discussion of Khovanov homology is a slightly (though not very) similar thing to this. Again, I have no real idea, and am treading on thin ice here.

So, alpheccar. Is this what you asked for? Please tell me what more you want covered, and I’ll write up some more! This was fun writing!

14 People had this to say...

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Don’t worry, I’ll back you up a bit once I can say what an Abelian category is, which will require enriched categories, which is one of my goals in going through monoidal categories.

And one of these days I might even start in on topology…

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  • Michi
  • July 12th, 2007
  • 19:46

John: Are you seriously trying to write a blog that you can typeset, staple and hand in as a textbook in your field? Sometimes I kinda get that feeling.

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This is amazing stuff! What books/articles did you read to learn this stuff (or is much of it your own work)? Somehow Weibel doesn’t make it seem this interesting, but maybe it’s all there if I can work through it.

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Well, it’s sort of become that. Really I just wanted to talk about my stuff. But I don’t know what everybody else knows, and I don’t want it to just be accessible to a narrow cluster of experts, so I have to back up a bit.

And after you back up a bit more and a bit more, you just say “screw it” and start at the beginning.

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  • Michi
  • July 12th, 2007
  • 20:00

John: I can, at times, sympathize with that. However, I did the other way around: I said “screw it” and wrote so that some of my posts are, really, only ever accessible to me and me alone.

[...] asked me about the origins of my intuitions for homological algebra in my recent post. The answer got a bit lengthy, so I’ll put it in a [...]

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Thank you so much Mikael ! I was not expecting such a quick and such a long answer to my email. I am not understanding everything and I’ll have to read this post again. But, it helps me to connect the dots. It is much better than anything I have read so far.

I have the Weibel book but I never found the energy to decrypt it. I needed some explanation of why homological algebra is useful, and how before reading that book. Your post is a great help.

I’ll post more comments when I have digested this first post :-)

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So, if I have understood, in the context of the category of modules, homological algebra is used as a kind of Xray machine. Instead of studying modules and the morphisms between modules we replace them by injective/projective/free resolutions and study the morphisms in this new category of chain complexes. And, thanks to that we get a lot of information about the original problem.

The last point which is remaining fuzzy for me is why we have so many different homological and cohomological theories. Are they all used with the category of modules or are they needed to study different algebraic gadgets ?

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  • Michi
  • July 16th, 2007
  • 22:13

So, the different theories deal with various contexts. For one, there is the case that you can read different amounts out of your Exts depending on what kind of algebra you started with. And things developed historically slightly differently – so that it need not have been immediately obvious that group- and Lie cohomology are the same thing, for instance.

On the topological side, we have similar things going on: singular and simplicial and cellular (co)homologies coincide whenever they all exist, but they cover different classes of spaces.

And then there is Tate – where you have a double ended resolution. This is a bit like approximating a function with power series in a variable and its inverse, instead of just power series as such: you go in both directions, and get a slightly different presentation that way.

All theories (well … kinda) tend to be used to study algebraic gadgets by studying properties of a corresponding module category. This characterization breaks down, among other places, for Khovanov homology and for certain kinds of poset homology – unless I missed something about these. But for most the algebraic kind of theories, we do intend all along to study the module categories anyway.

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This characterization breaks down … for Khovanov homology

Well, not really (in my view). Basically, he replaces a tangle with a cobordism in the cobordism Temperley-Lieb bicategory. Then he replaces the cobordism with a complex of graded modules to get a bicomplex whose graded Euler characteristic is the Jones polynomial.

And it’s really not so surprising. Remember that (for a post-Jones knot theorist like me) a knot, a link, or a tangle is an algebraic gadget: a morphism in the free braided monoidal category with duals on one unframed, self-dual object.

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  • Michi
  • July 16th, 2007
  • 23:00

So that leaves with poset homology, among the things I can think of, and that isn’t really that much of a good example either. We take a poset, and we equate this with a cellular complex that has this poset as a face poset. And then, boom, we’re back in topology and can take the usual route.

The main hook is that the poset isn’t even remotely close to a ring, the modules of which we want to study, which might make this just barely a decent example.

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Ahh, but a poset is a category, which is a generalization of a monoid, and a ring is a monoid object in the category of abelian groups :D

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  • Michi
  • July 17th, 2007
  • 8:02

John: Of course I can figure out module categories over a poset, and of course I can view poset cohomology over the category cohomology construction as well! You’re missing my point right now, viz. that this particular kind of poset cohomology does not seem to deal with a module structure over poset directly.

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Thanks to all. It is helpful.

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