Model category theory
Producing new model structures
Presentation of -categories
for stable/spectrum objects
for stable -categories
for -sheaves / -stacks
A model category structure on a category of dg-algebras tends to present an (∞,1)-category of ∞-algebras.
For dg-algebras bounded in negative or positive degrees, the monoidal Dold-Kan correspondence asserts that their model category structures are Quillen equivalent to the corresponding model structure on (co)simplicial algebras. This case plays a central role in rational homotopy theory.
The case of model structures on unbounded dg-algebras may be thought of as induced from this by passage to the derived geometry modeled on formal duals of the bounded dg-algebras. This is described at dg-geometry.
The category of dg-algebras is that of monoids in a category of chain complexes. Accordingly general results on a model structure on monoids in a monoidal model category apply.
Below we spellout special cases, such as restricting to commutative monoids when working over a field of characteristic 0, or restricting to non-negatively graded cochain dg-algebras.
Non-negatively graded cochain dg-algebras
Write for the category of cochain dg-algebras in non-negative degree over a field of characteristic 0. Write for the subcategory of (graded-)commutative dg-algebras.
The projective model category structure on and on is given by setting:
See the references below.
The nature of the cofibrations is discussed below.
Cofibrations: Sullivan algebras
In this section we describe the cofibrations in the model structure on of non-negatively graded dg-algebras. Notice that it is these that are in the image of the dual monoidal Dold-Kan correspondence.
Before we characterize the cofibrations, first some notation.
For a -graded vector space write for the Grassmann algebra over it. Equipped with the trivial differential this is a semifree dga .
With our ground field we write for the corresponding dg-algebra, the tensor unit for the standard monoidal structure on . This is the Grassmann algebra on the 0-vector space .
A relatived Sullivan algebra is a morphism of dg-algebras that is an inclusion
for some dg-algebra and for some graded vector space, such that
there is a well ordered set?
indexing a basis of ;
such that with for all basis elements we have that
This is called a minimal relative Sullivan algebra if in addition the condition
holds. For a Sullivan algebra relative to the tensor unit we call the semifree dga simply a Sullivan algebra. And a minimal Sullivan algebra if is a minimal relative Sullivan algebra.
(cofibrations are relative Sullivan algebras)
The cofibration in are precisely the retracts of relative Sullivan algebras .
Accordingly, the cofibrant objects in are precisely the Sullivan algebras
(sphere and disk algebras)
Write for the graded vector space which is the ground field in degree and 0 in all other degrees. For , consider the semifree dgas
and for the semifree dga
for the obvious morphism that takes the generator in degree to the generator in degree (and for it is the unique morphism from the initial object ).
are sets of generating cofibrations and acyclic cofibrations, respectively, exhibiting as a cofibrantly generated model category.
Commutative vs. non-commutative dg-algebras
The forgetful functor clearly preserves fibrations and cofibrations. It has a left adjoint, the free abelianization functor , which sends a dg-algebra to its quotient .
So this says that the weak equivalence classes of the commutative dg-algebras in the model category of all dg-algebras already exhaust the most general non-commutative but homotopy-commutative dg-algebras.
This is in II.1.5 of
- Igor Kriz and Peter May, Operads, algebras, modules and motives , Astérisque No 233 (1995)
We discuss now the case of unbounded dg-algebras. For these there is no longer the monoidal Dold-Kan correspondence available. Instead, these can be understood as arising naturally as function -algebras in the derived dg-geometry over formal duals of bounded dg-algebras, see function algebras on ∞-stacks.
Gradings and conventions
In derived geometry two categorical gradings interact: a cohesive -groupoid has a space of k-morphisms for all non-negative , and each such has itself a simplicial T-algebra of functions with a component in each non-positive degree. But the directions of the face maps are opposite. We recall the grading situation from function algebras on ∞-stacks.
Functions on a bare -groupoid , modeled as a simplicial set, form a cosimplicial algebra , which under the monoidal Dold-Kan correspondence identifies with a cochain dg-algebra (meaning: with positively graded differential) in non-negative degree
On the other hand, a representable has itself a simplicial T-algebra of functions, which under the monoidal Dold-Kan correspondence also identifies with a cochain dg-algebra, but then necessarily in non-positive degree to match with the above convention. So we write
Taking this together, for a general ∞-stack, its function algebra is generally an unbounded cochain dg-algebra with mixed contributions as above, the simplicial degrees contributing in the positive direction, and the homological resolution degrees in the negative direction:
For a field of characteristic 0 let
be the category of undounded commutative dg-algebras. With fibrations the degreewise surjections and weak equivalences the quasi-isomorphisms this is a
The existence of the model structure follows from the general discussion at model structure on dg-algebras over an operad.
Properness and combinatoriality is discussed in (ToënVezzosi):
in lemma 220.127.116.11 they state that constitutes the first two items in a triple which they call an HA context .
this implies their assumption 18.104.22.168 which asserts properness and combinatoriality
Discussion of cofibrations in is in (Keller).
Derived tensor product
Let be the projective model structure on commutative unbounded dg-algebras from above
For cofibrant , the functor
For , their derived coproduct in coincides in the homotopy category with the derived tensor product in : the morphism
is an isomorphism in .
This follows by the above with (ToënVezzosi, assumption 22.214.171.124, and page 8).
The model structure on unbounded dg-algebras is almost -enriched. See the section simplicial enrichment at model structure on dg-algebras over an operad for details.
Let be a field of characteristic 0. Let be the functor that assigns polynomial differential forms on simplices.
For define the simplicial set
This extends to a functor
The functor satisfies the dual of the pushout-product axiom: for any cofibration in and any fibration, the canonical morphism
is a Kan fibration, which is acyclic if or is.
This implies in particular that for cofibrant, is a Kan complex.
The proof works along the lines of (BousfieldGugenheim, prop. 5.3). See also the discussion at model structure on dg-algebras over an operad.
We give the proof for a special case. The general case is analogous.
We show that for cofibrant, and for any (automatically fibrant), is a Kan complex.
By a standard fact in rational homotopy theory (due to BousfieldGugenheim, discussed at differential forms on simplices) we have that is a left Quillen functor, hence in particular sends acyclic cofibrations to acyclic cofibrations, hence acyclic monomorphisms of simplicial sets to acyclic fibrations of dg-algebras.
Specifically for each horn inclusion we have that the restriction map is an acyclic fibration in , hence in .
A -horn in is a morphism . A filler for this horn is a lift in
If is cofibrant, then such a lift does always exist.
For cofibrant, is the correct derived hom-space
By the assumption that is cofibrant and according to the facts discussed at derived hom-space, we need to show that
is a resolution, or simplicial frame for . (Notice that every object is fibrant in ).
Since polynomial differential forms are acyclic on simplices (discussed here) it follows that
is degreewise a weak equivalence. It remains to show that is fibrant in the Reedy model structure .
One finds that the matching object is given by
Therefore is Reedy fibrant if in each degree the morphism
is a fibration. But this follows from the fact that is a left Quillen functor (as discussed at differential forms on simplices).
Derived copowering over
We discuss a concrete model for the -copowering of over ∞Grpd in terms of an operation of over sSet.
First notice a basic fact about ordinary commutative algebras.
In the coproduct is given by the tensor product over :
We check the universal property of the coproduct: for and two morphisms, we need to show that there is a unique morphism such that the diagram
commutes. For the left triangle to commute we need that sends elements of the form to . For the right triangle to commute we need that sends elements of the form to . Since every element of is a product of two elements of this form
this already uniquely determines to be given on elements by the map
That this is indeed an -algebra homomorphism follows from the fact that and are
We have that the copowering of with the map of sets from two points to the single point
is the product morphism on . And that the tensoring with the map from the empty set to the point
is the unit morphism on . Generally, for any map of sets we have that the tensoring
is the morphism between tensor powers of of the cardinalities of and , respectively, whose component over a copy of on the right corresponding to is the iterated product on as many tensor powers of as there are elements in the preimage of under .
The analogous statements hold true with replaced by : for and we obtain a simplicial cdg-algebra
by the ordinary degreewise copowering over Set, using that has coproducts (equal to the tensor product over ).
This is equivalently a commutative monoid in simplicial unbounded chain complexes
By the logic of the monoidal Dold-Kan correspondence the symmetric lax monoidal Moore complex functor (via the Eilenberg-Zilber map) sends this to a commutative monoid in non-positively graded cochain complexes in unbounded cochain complexes
Since the total complex functor is itself symmetric lax monoidal (…), this finally yields
Define the functor
This appears essentially (…) as (GinotTradlerZeinalian, def 3.1.1).
This follows from (GinotTradlerZeinalian, theorem 4.2.7), which asserts that the derived functor of this tensoring is the unique (∞,1)-functor, up to equivalence, satisfying the axioms of -copowering.
preserves weak equivalences in both arguments.
This is essentially due to (Pirashvili). The full statement is (GinotTradlerZeinalian, prop. 4.2.1).
Derived powering over
Let be presented by a degreewise finite simplicial set (which we denote by the same symbol).
Then the homotopy limit in over the -shaped diagram constant on is given by .
We show dually that for degreewise finite the assignment models the -copowering in .
By the discussion at (∞,1)-copowering it is sufficient to to establish an equivalence
natural in . Consider a cofibrant model of , which we denote by the same symbol. The we compute with 1-categorical end/coend calculus
where all steps are isomorphisms and the dot denotes the ordinary 1-categorical copowering of the 1-category over Set. In the last step we are using that the tensor product commutes with finite limits of dg-algebras. (This is where the finiteness assumption is needed).
Now we use that preserves products up to quasi-isomorphism (as discussed here)
This being a weak equivalence between fibrant objects and since is assumed cofibrant, we have by the above discussion of the derived hom-functor (and using the factorization lemma) a weak equivalence
Since all this is natural in , this proves the claim.
For , a path object
for is given by
This follows along the above lines. The statement appears for instance as (Behrend, lemma 1.19).
Relation to -algebra spectra
For every ring spectrum there is the notion of algebra spectra over . Let be the Eilenberg-MacLane spectrum for the integers. Then unbounded dg-algebras (over ) are one model for -algebra spectra.
See algebra spectrum for details.
Relation to -algebras
Commutative dg-algebras over a field of characteristic 0 constitute a presentation of E-infinity algebras over ([Lurie, prop. A.126.96.36.199]).
The cofibrantly generated model structure on commutative dg-algebras is surveyed usefully for instance on p. 6 of
This makes use of the general discussion in section 3 of
that obtains the model structure from the model structure on chain complexes.
A standard textbook reference is section V.3 of
- Sergei Gelfand, Yuri Manin, Methods of homological algebra, transl. from the 1988 Russian (Nauka Publ.) original. Springer 1996. xviii+372 pp. 2nd corrected ed. 2002.
An original reference seems to be
- A. Bousfield, V. Gugenheim, On PL deRham theory and rational homotopy type Memoirs of the AMS 179 (1976)
For general non-commutative (or rather: not necessarily graded-commutative) dg-algebras a model structure is given in
This is also the structure used in
- J.L. Castiglioni G. Cortiñas, Cosimplicial versus DG-rings: a version of the Dold-Kan correspondence (arXiv)
where aspects of its relation to the model structure on cosimplicial rings is discussed. (See monoidal Dold-Kan correspondence for more on this).
Disucssion of the model structure on unbounded dg-algebras over a field of characteristic 0 is in
A general discussion of algebras over an operad in unbounded chain complexes is in
A survey of some useful facts with an eye towards dg-geometry is in
Discussion of cofibrations in unbounded dg-algebras are in
The derived copowering of unbounded commutative dg-algebras over is discussed (somewhat implicitly) in
- Grégory Ginot, Thomas Tradler, Mahmoud Zeinalian, Derived higher Hochschild homology, topological chiral homology and factorization algebras, (arxiv/1011.6483)
The commutative product on the dg-algebra of the higher order Hochschild complex is discussed in
- Grégory Ginot, Thomas Tradler, Mahmoud Zeinalian, A Chen model for mapping spaces and the surface product (pdf)
The relation to E-infinity algebras is discussed in section 7.1 of
For more see model structure on dg-algebras over an operad.