DGLAs and obstruction theory

The purpose of this post, the third in a series on deformation theory and DGLAs, is to describe the obstruction theory for a formal moduli problem associated to a DGLA.

1. Tangent-obstruction theories

Standard problems in classical deformation theory usually have a “tangent-obstruction theory” parametrized by certain successive cohomology groups. For example, let’s consider the problem of deformations of a smooth variety over an algebraically closed field , over finite-dimensional local -algebras. Then:

• The “infinitesimal automorphisms” of —that is, automorphisms of the trivial deformation over —are given by where is the tangent bundle (i.e., vector fields).
• The isomorphism classes of deformations of over the dual numbers are given by .
• There is an obstruction theory with . Specifically, given a square-zero extension of finite-dimensional local -algebras

  

  and given a deformation of over , there is a functorial obstruction in to extending the deformation over the inclusion .

• In the previous item, if the obstruction vanishes, then the isomorphism classes of extensions of over are a torsor for .

One has a similar picture for other deformation problems, for example deformations of vector bundles or closed subschemes. The “derived” approach to deformation theory provides (at least in characteristic zero) a general explanation for this phenomenon.

 

Deformation theory and DGLAs II

Let be a field of characteristic zero. In the previous post, we introduced the category (i.e., -category) of formal moduli problems over . A formal moduli problem over is a moduli problem, taking values in spaces, that can be evaluated on the class of “derived” artinian -algebras with residue field : this was the category introduced in the previous post.

In other words, a formal moduli problem was a functor

which was required to send itself to a point, and satisfy a certain cohesiveness condition: respects certain pullbacks in (which corresponded geometrically to pushouts of schemes).

The main goal of the series of posts was to sketch a proof of (and define everything in) the following result:

Theorem 7 (Lurie; Pridham) There is an equivalence of categories between and the -category of DGLAs over .

 

4. Overview

Here’s a rough sketch of the idea. Given a formal moduli problem , we should think of as something like a small space, concentrated at a point but with lots of “infinitesimal” thickening. (Something like a .) Moreover, has a canonical basepoint corresponding to the “trivial deformation.” That is, we can think of as taking values in pointed spaces rather than spaces.

It follows that we can form the loop space  of , which is a new formal moduli problem. However, has more structure: it’s a group object in the category of formal moduli problems — that is, it’s some sort of derived formal Lie group. Moreover, knowledge of the original is equivalent to knowledge of together with its group structure: we can recover as (modulo connectivity issues that end up not being a problem). This relation between ordinary objects and group objects (via ) is something very specific to the derived or homotopy world, and it’s what leads to phenomena such as Koszul duality.

 

Fundamental groups of unirational varieties

Let be a variety over an algebraically closed field . is said to be rational if is birational to . In general, it is difficult to determine when a variety in higher dimensions is rational, although there are numerical invariants in dimensions one and two.

• Let be a smooth projective curve. Then is rational if and only if its genus is zero.
• Let be a smooth projective surface. Then is rational if and only if there are no global 1-forms on (i.e., ) and the second plurigenus vanishes. This is a statement about the negativity of the cotangent bundle (or, equivalently, of the positivity of the tangent bundle) which is a birational invariant and which holds for . The result is a criterion of Castelnuovo, extended by Zariski to characteristic .

In higher dimensions, it is harder to tell when a variety is rational. An easier problem is to determine when a variety is unirational: that is, when there is a dominant rational map

or, equivalently, when the function field has a finite extension which is purely transcendental. In dimensions one and two (and in characteristic zero), the above invariants imply that a unirational variety is rational. In higher dimensions, there are many more unirational varieties: for example, a theorem of Harris, Mazur, and Pandharipande states that a degree hypersurface in , is always unirational.

The purpose of this post is to describe a theorem of Serre that shows the difficulty of distinguishing rationality from unirationality. Let’s work over . The fundamental group of a smooth projective variety is a birational invariant, and so any rational variety has trivial .

Theorem 1 (Serre) A unirational (smooth, projective) variety over has trivial .

The reference is Serre’s paper “On the fundamental group of a unirational variety,” in J. London Math Soc. 1959.

 

Genetics of homotopy theory and the Adams conjecture

The purpose of this post is to describe Sullivan’s proof of the Adams conjecture via algebraic geometry; the conjecture and its motivation were described in the previous post (from where the notation is taken). The classical reference is Sullivan’s paper “Genetics of homotopy theory and the Adams conjecture,” and the MIT notes on “Geometric topology.”

1. First step: completion at a prime

Sullivan’s proof of the Adams conjecture  is based on interpreting the Adams operations via a surprising Galois symmetry in the (profinitely completed) homotopy types of classifying spaces. Let’s work in the complex case for simplicity. Our goal is to show that the composite

is nullhomotopic. (The map was defined in the previous post.)

Since the homotopy groups of are finite, it will follow (by the Milnor exact sequence) that we can let and conclude that the map

is nullhomotopic (i.e., there are no phantom maps into a spectrum with finite homotopy groups).

Using again the finiteness of the homotopy groups of , we can get a splitting

into the respective profinite completions. There is a well-behaved theory of profinite completions for connective spectra, or for sufficiently nice (e.g. simply connected with finitely generated homology) spaces, which will be the subject of a different post.

 

The Adams conjecture I

The Sullivan conjecture on maps from classifying spaces originated in Sullivan’s work on localizations and completions in topology, which together with étale homotopy theory led him to a proof of the classical Adams conjecture. The purpose of this post is to briefly explain the conjecture; in the next post I’ll discuss Sullivan’s proof.

1. Vector bundles and spherical fibrations

Let be a finite CW complex. Given a real -dimensional vector bundle , one can form the associated spherical fibration  with fiber by endowing with a euclidean metric and taking the vectors of length one in each fiber. The Adams conjecture is a criterion on when the sphere bundles associated to vector bundles are fiber homotopy trivial. It will be stated in terms of the following definition:

Definition 1 Let be the quotient of the Grothendieck group of vector bundles on by the relation that if have fiber homotopy equivalent sphere bundles (or rather, the group generated by it).

Observe that if and have fiber homotopy equivalent sphere bundles, then so do ; for example, this is because the sphere bundle of is the fiberwise join of that of and . It is sometimes more convenient to work with pointed spherical fibrations instead: that is, to take the fiberwise one-point compactification of a vector bundle rather than the sphere bundle. In this case, the fiberwise join is replaced with the fiberwise smash product; we have

where denotes a fiberwise smash product.

One reason is that this is of interest is that the group of vector bundles on a space is often very computable, thanks to Bott periodicity which identifies the -groups of a point. The set of spherical fibrations is much harder to describe: to describe the spherical fibrations over essentially amounts to computing a bunch of homotopy groups of spheres.

 

The category U IV

This post is part of a series (started here) of posts on the structure of the category of unstable modules over the mod Steenrod algebra , which plays an important role in the proof of the Sullivan conjecture (and its variants).

In the previous post, we introduced some additional structure on the category .

• First, using the (cocommutative) Hopf algebra structure on , we got a symmetric monoidal structure on , which was an algebraic version of the Künneth theorem.
• Second, we described a “Frobenius” functor

  

  which was symmetric monoidal, and which came with a Frobenius map .

• We constructed an exact sequence natural in ,

  

  where was the suspension and the left adjoint. In particular, we showed that all the higher derived functors of (after ) vanish.

The first goal of this post is to use this extra structure to prove the following:

 

Theorem 39 The category is locally noetherian: the subobjects of the free unstable module satisfy the ascending chain condition (equivalently, are finitely generated as -modules).

 

In order to prove this theorem, we’ll use induction on and the technology developed in the previous post as a way to make Nakayama-type arguments. Namely, the exact sequence (4) becomes

as we saw in the previous post. Observe that is clearly noetherian (it’s also not hard to check this for ). Inductively, we may assume that (and therefore ) is noetherian.

Fix a subobject ; we’d like to show that is finitely generated.

 

The category U III

This is part of a series of posts intended to understand some of the basic structure of the category of unstable modules over the (mod ) Steenrod algebra, to prepare for the proof of the Sullivan conjecture. Here’s what we’ve seen so far:

• is a Grothendieck abelian category, with a set of compact, projective generators (the free unstable module on a generator in degree ). (See this post.)
• has a tautological class in degree , and has a basis given by for an admissible sequence of excess . (This post explained the terminology and the proof.)
• was the subspace .

Our goal in this post is to describe some of the additional structure on the category , which will eventually enable us to prove (and make sense of!) results such as . We’ll start with the symmetric monoidal tensor product and the suspension functor, and then connect this to the Frobenius maps (which will be defined below).

 

1. The symmetric monoidal structure

Our first order of business is to describe the symmetric monoidal structure on , which will be given by the -linear tensor product. In fact, recall that the Steenrod algebra is a cocommutative Hopf algebra, under the diagonal map

The Hopf algebra structure is defined according to the following rule: we have that maps to if and only if for every two cohomology classes in the cohomology of a topological space, one has

The cocommutative Hopf algebra structure on gives a tensor product on the category of (graded) -modules, which is symmetric monoidal. It’s easy to check that if are -modules satisfying the unstability condition, then so does . This is precisely the symmetric monoidal structure on .

 

The category U II

The purpose of this post (like the previous one) is to go through some of the basic properties of the category of unstable modules over the (mod ) Steenrod algebra. An analysis of will ultimately lead to the proof of the Sullivan conjecture. Most of this material, again, is from Schwartz’s Unstable modules over the Steenrod algebra and Sullivan’s fixed point set conjecture; another useful source is Lurie’s notes. 

1. The modules

In the previous post, we showed that the category had enough projectives. More specifically, we constructed — using the adjoint functor theorem — an object , for each , which we called the free unstable module on a class of degree .The object had the universal property

To start with, we’d like to have a more explicit description of the module .

To do this, we need a little terminology. A sequence of positive integers

is called admissible if

for each . It is a basic fact, which can be proved by manipulating the Adem relations, that the squares

form a spanning set for as ranges over the admissible sequences. In fact, by looking at the representation on various cohomology rings, one can prove:

Proposition 29 The for admissible form a basis for the Steenrod algebra .

 

The category U

The purpose of this post is to introduce the basic category that enters into the unstable Adams spectral sequence and the proof of the Sullivan conjecture: the category of unstable modules over the Steenrod algebra. Throughout, I’ll focus on the (simpler) case of .

1. Recap of the Steenrod algebra

Let be a space. Then the cohomology has a great deal of algebraic structure:

• It is a graded -vector space concentrated in nonnegative degrees.
• It has an algebra structure (respecting the grading).
• It comes with an action of Steenrod operations

  

The Steenrod squares, which are constructed from the failure of strict commutativity in the cochain algebra , are themselves subject to a number of axioms:

• acts as the identity.
• is compatible with the suspension isomorphism between .
• One has the Adem relations: for ,

  

In other words, there is a (noncommutative) algebra of operations, which is the Steenrod algebra , such that the cohomology of any space is a module over . The Steenrod algebra can be defined as

 

A toy analog of the Sullivan conjecture

In this post, I’d like to describe a toy analog of the Sullivan conjecture. Recall that the Sullivan conjecture considers (pointed) maps from into a finite complex, and states that the space of such is contractible if is finite. The stable version replaces with the Eilenberg-MacLane spectrum:

 

Theorem 13 Let be the Eilenberg-MacLane spectrum. Then the mapping spectrum

is contractible. In particular, for any finite spectrum , the graded group of maps .

 

In the previous post, I sketched a proof (from Ravenel’s “Localization” paper) of this result based on a little chromatic technology. The spectrum is dissonant: that is, the Morava -theories don’t see it. However, any finite spectrum is harmonic: that is, local with respect to the wedge of Morava -theories. It follows formally that the spectrum of maps is contractible (and thus the same with replaced by any finite spectrum). The non-formal input was the fact that is in fact harmonic, which requires a little work.

In this post, I’d like to sketch an earlier proof of the above theorem. This proof is based on the Adams spectral sequence. In fact, the proof runs parallel to Miller’s proof of the Sullivan conjecture. There is a classical Adams spectral sequence for computing , with page given by

with the (mod ) Steenrod algebra.

It turns out, however, for purely algebraic reasons, that the term is trivial. Miller’s proof of the Sullivan conjecture relies on more complicated algebra to show that the unstable version of all this has the same vanishing property at . Most of this material is from Margolis’s Spectra and the Steenrod algebra.