There is a brief entry at bar construction together with a blog link
There is some discussion of the bar-cobar adjointness as it related to twisting cochains, at that entry.
Here we will concentrate on the bar-cobar adjointness itself and start exploring the links with other parts of differential algebra.
One of the earliest examples of a pair of adjoint functors studied in algebraic topology was that giving the relationship between the functors for reduced suspension and based loop space. If we take a pointed connected space , then its reduced suspension is obtained by taking the cylinder and identifying the subspace to a point. (Think of crushing the two ends of the cylinder and the line through the base point to a point.) This can also be thought of as forming the smash product of the circle with .
Adjoint to is the loop space functor: is the space of pointed maps from to . This has a monoid structure (up to homotopy) given by concatenation of loops. (Back in , we have a comonoid structure with respect to the pointed coproduct as described at interval object. This in some sense is ‘subdivision as an inverse for composition’.)
(perhaps: Picture to go here?)
Using ordinary (co)homology to study spaces such as CW-complexes, we naturally use the complexes of (cellular) chains on spaces. The structure of chains on the suspension is easy to work out using the obvious cellular structure, but that on the loop space is much harder as is given the compact open topology and only has the homotopy type of a CW-complex, so no nice cellular structure is given us ‘on a plate’. The idea is thus to start with a chain complex model, , for a CW-complex, , (usually the complex of cellular chains on ), and we try to construct from a ‘model’ for the chain complex of the loop space of . Adams’ cobar construction was such a method (see below). This was adjoint to a bar construction defined by Eilenberg and MacLane.
Both directions use an abstract algebraic model of concatenation of paths and so their construction is linked to that of free monoids, and through those to monads, operads and related abstract machinery to handle concatenation and its higher categorical analogues in categorical contexts.
The chain complex has a rich coalgebraic structure coming from induced by a cellular diagonal approximation on so the cobar construction will start with a dg-coalgebra as ‘input’ and as output we will hope for both a coalgebra structure (reflecting the chain coalgebra idea) and an algebra structure (coming from modelling the concatenation of loops). We therefore might hope for, and in fact do get, a differential graded Hopf algebra.
Going the other way, we start with a differential graded algebra and use ‘coconcatenation’ or ‘subdivision’ to get a coalgebra structure. In fact, once again, this is a Hopf algebra.
These topologically motivated constructions can be applied in much greater generality as we will see both here and elsewhere:
(due originally Eilenberg-MacLane) Remember this goes from ‘algebras’ to Hopf algebras in general.
Urs: This is a bit abrupt. Could we just say this in words, once, like “The bar construction, originally due to MacLane, is a map that sends xyz to abc such that rst.”? I would also greatly appreciate some more motivational background and other helpful side remarks. Such as: why does the bar/cobar-construction go between algebras and coalgebras, instead of being an endomorphism of either?
Tim: I have tried to motivate this a bit above. This needs more filling in (and also I think that (slowly) I will go through changing pre-gvs etc to gvs and then talk about positively or negatively graded gvs and so on.)
Let be a commutative, augmented differential -graded algebra, , .
The bar construction is given by
Note that the image of a 1-connected cdga is a connected commutative Hopf algebra.
It uses the suspension operator on the graded vector spaces. This mirrors the reduced suspension at the cell complex level.
It uses a tensor algebra construction. This from one point of view handles the formal concatenation aspect,
but has also a rich structure of a coalgebraic structure with reduced diagonal, given by
(see differential graded coalgebra). This can be interpreted as looking at how a formal concatenation can be ‘subdivided’ into its various parts.
(due to J. F. Adams)
We define a functor:
Let be a cocommutative differential -graded coaugmented coalgeba:
The Cobar construction is the cocommutative pre-dgha defined by
is the cocommutative Hopf algebra generated by , as before(in differential graded coalgebra) is the cokernel of the coaugmentation, )
The image of a 1-connected cdgc is a connected cocommutative dgha.
If is of finite type, is isomorphic to as a differential -graded Hopf algebra.
If is not (graded) commutative, the differential of does not respect the shuffle product on ; thus becomes merely a differential -graded coalgebra. Similarly if is not (graded) cocommutative is merely a differential -graded algebra.
In particular, let
be the category of augmented differential graded algebras, ().
, the category of connected differential graded coalgebras,
then the Bar and Cobar constructions yield functors
is right adjoint to .
For any objects in , and of , the natural adjunction morphisms
are weak equivalences / quasi-isomorphisms.
These latter morphisms are defined by
is the zero mapping on and the natural isomorphism on .
is the unique lifting of
The source used for the above was
D. Tanré, Homotopie rationnelle: Modèles de Chen, Quillen, Sullivan, Lecture Notes in Maths No. 1025, Springer, 1983.
This was augmented with material from
H. J. Baues, Geometry of loop spaces and the cobar construction, Mem. Amer. Math. Soc. 25 (230) (1980) ix+171.