(Note: WordPress seems to be having some intermittent technical problem parsing my math markup in this post, so please bear with me until it, hopefully, goes away…)

As August is the month in which Portugal goes on vacation, and we had several family visitors toward the end of the summer, I haven’t posted in a while, but the term has now started up at IST, and seminars are underway, so there should be some interesting stuff coming up to talk about.

New Blog

First, I’ll point out that that Derek Wise has started a new blog, called simply “Simplicity“, which is (I imagine) what it aims to contain: things which seem complex explained so as to reveal their simplicity.  Unless I’m reading too much into the title.  As of this writing, he’s posted only one entry, but a lengthy one that gives a nice explanation of a program for categorified Klein geometries which he’s been thinking a bunch about.  Klein’s program for describing the geometry of homogeneous spaces (such as spherical, Euclidean, and hyperbolic spaces with constant curvature, for example) was developed at Erlangen, and goes by the name “The Erlangen Program”.  Since Derek is now doing a postdoc at Erlangen, and this is supposed to be a categorification of Klein’s approach, he’s referred to it the “2-Erlangen Program”.  There’s more discussion about it in a (somewhat) recent post by John Baez at the n-Category Cafe.  Both of them note the recent draft paper they did relating a higher gauge theory based on the Poincare 2-group to a theory known as teleparallel gravity.  I don’t know this theory so well, except that it’s some almost-equivalent way of formulating General Relativity

I’ll refer you to Derek’s own post for full details of what’s going on in this approach, but the basic motivation isn’t too hard to set out.  The Erlangen program takes the view that a homogeneous space is a space X (let’s say we mean by this a topological space) which “looks the same everywhere”.  More precisely, there’s a group action by some G, which we understand to be “symmetries” of the space, which is transitive.  Since every point is taken to every other point by some symmetry, the space is “homogeneous”.  Some symmetries leave certain points x \in X where they are – they form the stabilizer subgroup H = Stab(x).  When the space is homogeneous, it is isomorphic to the coset space, X \cong G / H.  So Klein’s idea is to say that any time you have a Lie group G and a closed subgroup H < G, this quotient will be called a “homogeneous space”.  A familiar example would be Euclidean space, \mathbb{R}^n \cong E(n) / O(n), where E is the Euclidean group and O is the orthogonal group, but there are plenty of others.

This example indicates what Cartan geometry is all about, though – this is the next natural step after Klein geometry (Edit:  Derek’s blog now has a visual explanation of Cartan geometry, a.k.a. “generalized hamsterology”, new since I originally posted this).  We can say that Cartan is to Klein as Riemann is to Euclid.  (Or that Cartan is to Riemann as Klein is to Euclid – or if you want to get maybe too-precisely metaphorical, Cartan is the pushout of Klein and Riemann over Euclid).  The point is that Riemannian geometry studies manifolds – spaces which are not homogeneous, but look like Euclidean space locally.  Cartan geometry studies spaces which aren’t homogeneous, but can be locally modelled by Klein geometries.  Now, a Riemannian geometry is essentially a manifold with a metric, describing how it locally looks like Euclidean space.  An equivalent way to talk about it is a manifold with a bundle of Euclidean spaces (the tangent spaces) with a connection (the Levi-Civita connection associated to the metric).  A Cartan geometry can likewise be described as a G-bundle with fibre X with a connection

Then the point of the “2-Erlangen program” is to develop similar geometric machinery for 2-groups (a.k.a. categorical groups).  This is, as usual, a bit more complicated since actions of 2-groups are trickier than group-actions.  In their paper, though, the point is to look at spaces which are locally modelled by some sort of 2-Klein geometry which derives from the Poincare 2-group.  By analogy with Cartan geometry, one can talk about such Poincare 2-group connections on a space – that is, some kind of “higher gauge theory”.  This is the sort of framework where John and Derek’s draft paper formulates teleparallel gravity.  It turns out that the 2-group connection ends up looking like a regular connection with torsion, and this plays a role in that theory.  Their draft will give you a lot more detail.

Talk on Manifold Calculus

On a different note, one of the first talks I went to so far this semester was one by Pedro Brito about “Manifold Calculus and Operads” (though he ran out of time in the seminar before getting to talk about the connection to operads).  This was about motivating and introducing the Goodwillie Calculus for functors between categories of spaces.  (There are various references on this, but see for instance these notes by Hal Sadofsky). In some sense this is a generalization of calculus from functions to functors, and one of the main results Goodwillie introduced with this subject, is a functorial analog of Taylor’s theorem.  I’d seen some of this before, but this talk was a nice and accessible intro to the topic.

So the starting point for this “Manifold Calculus” is that we’d like to study functors from spaces to spaces (in fact this all applies to spectra, which are more general, but Pedro Brito’s talk was focused on spaces).  The sort of thing we’re talking about is a functor which, given a space M, gives a moduli space of some sort of geometric structures we can put on M, or of mappings from M.  The main motivating example he gave was the functor

Imm(-,N) : [Spaces] \rightarrow [Spaces]

for some fixed manifold N. Given a manifold M, this gives the mapping space of all immersions of M into N.

(Recalling some terminology: immersions are maps of manifolds where the differential is nondegenerate – the induced map of tangent spaces is everywhere injective, meaning essentially that there are no points, cusps, or kinks in the image, but there might be self-intersections. Embeddings are, in addition, local homeomorphisms.)

Studying this functor Imm(-,N) means, among other things, looking at the various spaces Imm(M,N) of immersions of each M into N. We might first ask: can M be immersed in N at all – in other words, is \pi_0(Imm(M,N)) nonempty?

So, for example, the Whitney Embedding Theorem says that if dim(N) is at least 2 dim(M), then there is an embedding of M into N (which is therefore also an immersion).

In more detail, we might want to know what \pi_0(Imm(M,N)) is, which tells how many connected components of immersions there are: in other words, distinct classes of immersions which can’t be deformed into one another by a family of immersions. Or, indeed, we might ask about all the homotopy groups of Imm(M,N), not just the zeroth: what’s the homotopy type of Imm(M,N)? (Once we have a handle on this, we would then want to vary M).

It turns out this question is manageable, party due to a theorem of Smale and Hirsch, which is a generalization of Gromov’s h-principle – the original principle applies to solutions of certain kinds of PDE’s, saying that any solution can be deformed to a holomorphic one, so if you want to study the space of solutions up to homotopy, you may as well just study the holomorphic solutions.

The Smale-Hirsch theorem likewise gives a homotopy equivalence of two spaces, one of which is Imm(M,N). The other is the space of “formal immersions”, called Imm^f(M,N). It consists of all (f,F), where f : M \rightarrow N is smooth, and F : TM \rightarrow TN is a map of tangent spaces which restricts to f, and is injective. These are “formally” like immersions, and indeed Imm(M,N) has an inclusion into Imm^f(M,N), which happens to be a homotopy equivalence: it induces isomorphisms of all the homotopy groups. These come from homotopies taking each “formal immersion” to some actual immersion. So we’ve approximated Imm(-,N), up to homotopy, by Imm^f(-,N). (This “homotopy” of functors makes sense because we’re talking about an enriched functor – the source and target categories are enriched in spaces, where the concepts of homotopy theory are all available).

We still haven’t got to manifold calculus, but it will be all about approximating one functor by another – or rather, by a chain of functors which are supposed to be like the Taylor series for a function. The way to get this series has to do with sheafification, so first it’s handy to re-describe what the Smale-Hirsch theorem says in terms of sheaves. This means we want to talk about some category of spaces with a Grothendieck topology.

So lets let \mathcal{E} be the category whose objects are d-dimensional manifolds and whose morphisms are embeddings (which, of course, are necessarily codimension 0). Now, the point here is that if f : M \rightarrow M' is an embedding in \mathcal{E}, and M' has an immersion into N, this induces an immersion of M into N. This amounst to saying Imm(-,N) is a contravariant functor:

Imm(-,N) : \mathcal{E}^{op} \rightarrow [Spaces]

That makes Imm(-,N) a presheaf. What the Smale-Hirsch theorem tells us is that this presheaf is a homotopy sheaf – but to understand that, we need a few things first.

First, what’s a homotopy sheaf? Well, the condition for a sheaf says that if we have an open cover of M, then

So to say how Imm(-,N) : \mathcal{E}^{op} \rightarrow [Spaces] is a homotopy sheaf, we have to give \mathcal{E} a topology, which means defining a “cover”, which we do in the obvious way – a cover is a collection of morphisms f_i : U_i \rightarrow M such that the union of all the images \cup f_i(U_i) is just M. The topology where this is the definition of a cover can be called J_1, because it has the property that given any open cover and choice of 1 point in M, that point will be in some U_i of the cover.

This is part of a family of topologies, where J_k only allows those covers with the property that given any choice of k points in M, some open set of the cover contains them all. These conditions, clearly, get increasingly restrictive, so we have a sequence of inclusions (a “filtration”):

J_1 \leftarrow J_2 \leftarrow J_3 \leftarrow \dots

Now, with respect to any given one of these topologies J_k, we have the usual situation relating sheaves and presheaves.  Sheaves are defined relative to a given topology (i.e. a notion of cover).  A presheaf on \mathcal{E} is just a contravariant functor from \mathcal{E} (in this case valued in spaces); a sheaf is one which satisfies a descent condition (I’ve discussed this before, for instance here, when I was running the Stacks Seminar at UWO).  The point of a descent condition, for a given topology is that if we can take the values of a functor F “locally” – on the various objects of a cover for M – and “glue” them to find the value for M itself.  In particular, given a cover for M \in \mathcal{E}, and a cover, there’s a diagram consisting of the inclusions of all the double-overlaps of sets in the cover into the original sets.  Then the descent condition for sheaves of spaces is that

The general fact is that there’s a reflective inclusion of sheaves into presheaves (see some discussion about reflective inclusions, also in an earlier post).  Any sheaf is a contravariant functor – this is the inclusion of Sh( \mathcal{E} ) into $latex PSh( \mathcal{E} )$.  The reflection has a left adjoint, sheafification, which takes any presheaf in PSh( \mathcal{E} ) to a sheaf which is the “best approximation” to it.  It’s the fact this is an adjoint which makes the inclusion “reflective”, and provides the sense in which the sheafification is an approximation to the original functor.

The way sheafification works can be worked out from the fact that it’s an adjoint to the inclusion, but it also has a fairly concrete description.  Given any one of the topologies J_k,  we have a whole collection of special diagrams, such as:

U_i \leftarrow U_{ij} \rightarrow U_j

(using the usual notation where U_{ij} = U_i \cap U_j is the intersection of two sets in a cover, and the maps here are the inclusions of that intersection).  This and the various other diagrams involving these inclusions are special, given the topology J_k.  The descent condition for a sheaf F says that if we take the image of this diagram:

F(U_i) \rightarrow F(U_{ij}) \leftarrow F(U_j)

then we can “glue together” the objects F(U_i) and F(U_j) on the overlap to get one on the union.  That is, F is a sheaf if F(U_i \cup U_j) is a colimit of the diagram above (intuitively, by “gluing on the overlap”).  In a presheaf, it would come equipped with some maps into the F(U_i) and F(U_j): in a sheaf, this object and the maps satisfy some universal property.  Sheafification takes a presheaf F to a sheaf F^{(k)} which does this, essentially by taking all these colimits.  More accurately, since these sheaves are valued in spaces, what we really want are homotopy sheaves, where we can replace “colimit” with “homotopy colimit” in the above – which satisfies a universal property only up to homotopy, and which has a slightly weaker notion of “gluing”.   This (homotopy) sheaf is called F^{(k)} because it depends on the topology J_k which we were using to get the class of special diagrams.

One way to think about F^{(k)} is that we take the restriction to manifolds which are made by pasting together at most k open balls.  Then, knowing only this part of the functor F, we extend it back to all manifolds by a Kan extension (this is the technical sense in which it’s a “best approximation”).

Now the point of all this is that we’re building a tower of functors that are “approximately” like F, agreeing on ever-more-complicated manifolds, which in our motivating example is F = Imm(-,N).  Whichever functor we use, we get a tower of functors connected by natural transformations:

F^{(1)} \leftarrow F^{(2)} \leftarrow F^{(3)} \leftarrow \dots

This happens because we had that chain of inclusions of the topologies J_k.  Now the idea is that if we start with a reasonably nice functor (like F = Imm(-,N) for example), then F is just the limit of this diagram.  That is, it’s the universal thing F which has a map into each F^{(k)} commuting with all these connecting maps in the tower.  The tower of approximations – along with its limit (as a diagram in the category of functors) – is what Goodwillie called the “Taylor tower” for F.  Then we say the functor F is analytic if it’s just (up to homotopy!) the limit of this tower.

By analogy, think of an inclusion of a vector space V with inner product into another such space W which has higher dimension.  Then there’s an orthogonal projection onto the smaller space, which is an adjoint (as a map of inner product spaces) to the inclusion – so these are like our reflective inclusions.  So the smaller space can “reflect” the bigger one, while not being able to capture anything in the orthogonal complement.  Now suppose we have a tower of inclusions V \leftarrow V' \leftarrow V'' \dots, where each space is of higher dimension, such that each of the V is included into W in a way that agrees with their maps to each other.  Then given a vector w \in W, we can take a sequence of approximations (v,v',v'',\dots) in the V spaces.  If w was “nice” to begin with, this series of approximations will eventually at least converge to it – but it may be that our tower of V spaces doesn’t let us approximate every w in this way.

That’s precisely what one does in calculus with Taylor series: we have a big vector space W of smooth functions, and a tower of spaces we use to approximate.  These are polynomial functions of different degrees: first linear, then quadratic, and so forth.  The approximations to a function f are orthogonal projections onto these smaller spaces.  The sequence of approximations, or rather its limit (as a sequence in the inner product space W), is just what we mean by a “Taylor series for f“.  If f is analytic in the first place, then this sequence will converge to it.

The same sort of phenomenon is happening with the Goodwillie calculus for functors: our tower of sheafifications of some functor F are just “projections” onto smaller categories (of sheaves) inside the category of all contravariant functors.  (Actually, “reflections”, via the reflective inclusions of the sheaf categories for each of the topologies J_k).  The Taylor Tower for this functor is just like the Taylor series approximating a function.  Indeed, this analogy is fairly close, since the topologies J_k will give approximations of F which are in some sense based on k points (so-called k-excisive functors, which in our terminology here are sheaves in these topologies).  Likewise, a degree-k polynomial approximation approximates a smooth function, in general in a way that can be made to agree at k points.

Finally, I’ll point out that I mentioned that the Goodwillie calculus is actually more general than this, and applies not only to spaces but to spectra. The point is that the functor Imm(-,N) defines a kind of generalized cohomology theory – the cohomology groups for M are the \pi_i(Imm(M,N)). So the point is, functors satisfying the axioms of a generalized cohomology theory are represented by spectra, whereas N here is a special case that happens to be a space.

Lots of geometric problems can be thought of as classified by this sort of functor – if N = BG, the classifying space of a group, and we drop the requirement that the map be an immersion, then we’re looking at the functor that gives the moduli space of G-connections on each M.  The point is that the Goodwillie calculus gives a sense in which we can understand such functors by simpler approximations to them.

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