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I'm trying to read Quillen's paper "Rational homotopy theory" and am a little confused about the construction. As I understand, he associates a dg-Lie algebra over $\mathbb{Q}$ to every 1-reduced simplicial set via a somewhat long series of Quillen equivalences. But the construction that I had heard before makes spaces (rationally) Quillen equivalent to commutative dgas over $\mathbb{Q}$ via the polynomial de Rham functor. Is there a simple reason why dg-Lie algebras and commutative DGAs should be Quillen equivalent? I believe this should be Koszul duality, but I don't really understand that right now. If someone has a (preferably lowbrow) explanation for this phenomenon (even in this specific case), I'd be interested.

In addition, I would be very interested in a "high concept" explanation of why Quillen's construction works. It seems that the crux of the proof is the Quillen equivalence between simplicial groups (localized at $\mathbb{Q}$, I guess) and complete simplicial Hopf algebras. I've been struggling with why this should proof should work, since I was not familiar with the work of Curtis on lower central series filtrations referred to there.

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At the far end of Quillen's long series of equivalences, beyond the DG Lie algebras, you will find DG commutative coalgebras, yes? That's the Koszul duality step. (And from these DGCs to DGAs it's basically "vector space dual of a coalgebra is an algebra"; but without finiteness conditions not every algebra is the dual of a coalgebra, so DGAs are better than DGCs for general simply connected spaces.) –  Tom Goodwillie Oct 24 '11 at 0:59
    
Ah, I see; thanks. I guess I hadn't paid sufficient attention to the DGC part of the equivalences. –  Akhil Mathew Oct 24 '11 at 1:35
    
"but without finiteness conditions not every algebra is the dual of a coalgebra, so DGAs are better than DGCs for general simply connected spaces." To the contrary, I think that DGCs do a much better job of capturing "spaces" as defined by, say, simplicial sets, than do DGAs. Here is one reason: DGCs comprise a (infty-)presentable category, whereas the opposite category of DGAs is not presentable. More generally, my understanding (based entirely on J. Francis's class last spring --- I haven't read the papers) is that Spaces=DGAs is only true with some "smallness" conditions. –  Theo Johnson-Freyd Oct 24 '11 at 3:14
    
I took a course where the lecturer said that the Quillen equivalence was between the rational homotopy category of simplicial sets and the opposite category of DGAs, but I didn't understand as much as I should have, and I don't know a good source for the model category stuff here. Your point about presentability is intriguing, though. –  Akhil Mathew Oct 24 '11 at 3:36
    
One thing that might be helpful is the difference between Sullivan models and Quillen models for rational homotopy types. The Sullivan model is a DGA that is pretty easy to construct given the rational cohomology of the space. The Quillen model is a bit more difficult to construct, but I think it gets closer to the homotopy of the space. The construction you heard before about the polynomial de Rham functor is that of Sullivan. –  Sean Tilson Oct 24 '11 at 4:25
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up vote 19 down vote accepted

I'm not sure if this will still be helpful, but here is my understanding of the Quillen model. I'm a little more comfortable with the Sullivan approach, which replaces a space $X$ with a commutative DGA over $\mathbb{Q}$. So my understanding of the Quillen model might be a bit off (if so, someone please correct me!). Also, everything correct that I write below, I learned from John Francis. (Probably in the same lecture that Theo mentioned in his comment above.) Oh, but any mistakes are probably not his fault---more likely an error in my understanding.

Before we begin: Quillen v Sullivan.

As others have mentioned, Quillen gets you a DG Lie algebra, where as the Sullivan model will get you a commutative DG algebra. As you write, the passage from one to the other is (almost) Koszul duality. Really, a Lie algebra will get you a co-commutative coalgebra by Koszul duality, and a commutative algebra will get you a coLie algebra. You can bridge the world of coalgebras and algebras when you have some finiteness conditions--for instance, if the rational homotopy groups are finite-dimensional in each degree. Then I think you can simply take linear duals to get from coalgebras to algebras.

A way to find Lie algebras.

So where do (DG) Lie algebras come from? First let me point out that there is a natural place that one finds Lie algebras, before knowing about the Quillen model: Lie algebras arise as the tangent space (at the identity) of a Lie group $G$.

Now, if you're an algebraist, you might claim another origin of Lie algebras: If you have any kind of Hopf algebra, you can look at the primitives of the Hopf algebra. These always form a Lie algebra.

(Recall that a Hopf algebra has a coproduct $\Delta: H \to H \otimes H$, and a primitive of $H$ is defined to be an element $x$ such that $\Delta(x) = 1 \otimes x + x \otimes 1.$)

One link between the algebraist's fountain of Lie algebras, and the geometer's, is that many Hopf algebras arise as functions on finite groups. If you are well-versed in algebra, one natural place to find Lie algebras, then, would be to take a finite group, take functions on that group, then take primitives.

A cooler link arises when a geometer looks at distributions near the identity of $G$ (which are dual to 'functions on $G$') rather than functions themselves. This isn't so obviously the right thing to look at in the finite groups example, but if you believe that functions on a Lie group $G$ are like de Rham forms on $G$, then you'd believe that something like 'the duals to functions on $G$' (which are closer to vector fields) would somehow safeguard the Lie algebra structure. The point being, you should expect to find Lie structures to arise from things that look like 'duals to functions on a group'. So one should take 'distributions' to be the Hopf algebra in question, and look at its primitives to find the Lie algebra of 'vector fields.'

A (fantastical) summary of the Quillen model.

Let us assume for a moment that your space $X$ happens to equal $BG$ for some Lie group, and you want to make a Lie algebra out of it. Then, by the above, what you could do is take $\Omega X = \Omega B G = G$, then look at the primitives of the Hopf algebra known as `distributions on $\Omega X$'.

Now, instead of considering just Lie groups, let's believe in a fantasy world (later made reality) in which all the heuristics I outlined for a Lie group $G$ will also work for a based loop space $\Omega Y$. A loop space is `like a group' because it has a space of multiplications, all invertible (up to homotopy). Moreover, any space $X$ is the $B$ (classifying space) of a loop space--namely, $X \cong B \Omega X$. So this will give us a way to associate a Lie algebra to any space, if you believe in the fantasy.

Blindly following the analogy, `functions on $\Omega X$' is like cochains on $\Omega X$, and the dual to this (i.e., distributions) is now chains on $\Omega X$. That is, $C_\bullet \Omega X$ should have the structure of what looks like a Hopf algebra. And its primitives should be the Lie algebra you're looking for.

What Quillen Does.

So if that's the story, what else is there? Of course, there is the fantasy, which I have to explain. Loop spaces are most definitely not Lie groups. Their products have $A_\infty$ structure, and correspondigly, we should be talking about things like homotopy Hopf algebras, not Hopf algebras on the nose. What Quillen does is not to take care of all the coherence issues, but to change the models of the objects he's working with.

For instance, one can get an actual simplicial group out of a space $X$ by Kan's construction $G$. This is a model for the loop space $\Omega X$, and I think this is what Quillen looks at instead of looking only at $\Omega X$, which is too flimsy. From this, taking group algebras over $\mathbb{Q}$ and completing (these are the simplicial chains, i.e., distributions), he obtains completed simplicial Hopf algebras. Again, instead of trying to make my fantasy precise in a world where one has to deal with higher algebraic structures (homotopy up to homotopy, et cetera) he uses this nice simplicial model. To complete the story, he takes level-wise primitives, obtaining DG Lie algebras.

Edit: This is from Tom's comment below. To recover a $k$-connected group or a $k$-connected Lie algebra from the associated $k$-connected complete Hopf algebra, you need $k \geq 0$. And $k$-connected groups correspond to $k+1$-connected spaces. This is why you need simply connected spaces in the equivalence.

I'm not sure I gave any 'high concept' as to 'why Quillen's construction works', but this is at least a road map I can remember.

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This is helpful, and maybe explains why DG-Lie algebras or coalgebras are more natural than commutative dgas. Thanks! –  Akhil Mathew Oct 27 '11 at 19:29
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$(k+1)$-connected spaces or simplicial sets correspond to $k$-connected groups even if $k$ is just $-1$. But to recover a $k$-connected group or a $k$-connected Lie algebra from the associated $k$-connected complete Hopf algebra you need $k\ge 0$. –  Tom Goodwillie Oct 27 '11 at 21:54
    
Thanks for the correction! I've updated the answer accordingly. –  Hiro Lee Tanaka Oct 27 '11 at 22:11
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