derived smooth geometry
Traditionally, a smooth manifold is defined as follows.
So a smooth manifold is a -differentiable manifold for all .
A smooth manifold is equivalently a locally ringed space which is locally isomorphic to the ringed space .
In an exercise of his 1973 Perugia lectures F. William Lawvere reported a somewhat surprising observation:
In the case of smooth manifolds the process of piecing together the local data can be elegantly summed up as splitting of idempotents in a category of open subsets of Euclidean spaces. More precisely:
Let be the category of smooth manifolds and smooth maps, where by a “smooth manifold”, we mean a finite-dimensional, second-countable, Hausdorff, manifold without boundary. Let be the full subcategory whose objects are the open subspaces of finite-dimensional Cartesian spaces.
The subcategory exhibits as an idempotent-splitting completion of .
By a general lemma for idempotent splittings, it suffices to prove that
Every smooth manifold is a smooth retract of an open set in Euclidean space;
If is a smooth idempotent on an open set , then the subset is an embedded submanifold.
For the first statement, we use the fact that any manifold can be realized as a closed submanifold of some , and every closed submanifold has a tubular neighborhood . In this case carries a structure of vector bundle over in such a way that the inclusion is identified with the zero section, so that the bundle projection provides a retraction, with right inverse given by the zero section.
For the second statement, assume that the origin is a fixed point of , and let be its tangent space (observe the presence of a canonical isomorphism to ). Thus we have idempotent linear maps where the latter factors through the inclusion via a projection map . We have a map that takes to ; let denote the composite
Now we make some easy observations:
The map restricts to a map , by idempotence of .
The derivative is again since is idempotent. Thus has full rank ( say), and so the restriction of to some neighborhood has as a regular value, and is a manifold of dimension by the implicit function theorem. The tangent space is canonically identified with .
There are smaller neighborhoods so that restricts to maps as in the following diagram ( are inclusion maps, all taking a domain element to itself):
and such that are diffeomorphisms by the inverse function theorem (noting here that is the identity map, by idempotence of ).
Letting denote the smooth inverse to , we calculate , and
so that for every . Hence .
From all this it follows that , meaning is locally diffeomorphic to , and so is an embedded submanifold of .
Lawvere comments on this fact as follows:
“This powerful theorem justifies bypassing the complicated considerations of charts, coordinate transformations, and atlases commonly offered as a ”basic“ definition of the concept of manifold. For example the 2-sphere, a manifold but not an open set of any Euclidean space, may be fully specified with its smooth structure by considering any open set in 3-space which contains it but not its center (taken to be ) and the smooth idempotent endomap of given by . All general constructions (i.e., functors into categories which are Cauchy complete) on manifolds now follow easily (without any need to check whether they are compatible with coverings, etc.) provided they are known on the opens of Euclidean spaces: for example, the tangent bundle on the sphere is obtained by splitting the idempotent on the tangent bundle of ( being the vector space of translations of ) which is obtained by differentiating . The same for cohomology groups, etc.” (Lawvere 1989, p.267)
In this context one specifies for instance a geometry (for structured (∞,1)-toposes) and then plenty of geometric notions are defined canonically in terms of . The theory of smooth manifolds appears if one takes CartSp.
Alternatively one can specify differential cohesion and proceed as discussed at differential cohesion – structures - Cohesive manifolds (separated).
This is discussed in The geometry CartSp below.
It is important in the context of locally representable locally ringed toposes that we regard as equipped with this local -algebra. This is what remembers the site and gives a notion of local representability in the first place.
The big topos is a cohesive topos of generalized smooth spaces. Its concrete sheaves are precisely the diffeological spaces. See there for more details. We now discuss how with regarded as a -structured topos, smooth manifolds are precisely its locally representable objects.
The representables themselves should evidently be locally representable and canonically have the structure of -structured toposes.
Indeed, every object is canonically a CartSp-ringed space, meaning a topological space equipped with a local sheaf of smooth algebras. More generally: every object is canonically incarnated as the -structured (∞,1)-topos
given by the over-(∞,1)-topos of the big (∞,1)-sheaf (∞,1)-topos over and the structure sheaf given by the composite of the (∞,1)-Yoneda embedding and the inverse image of the etale geometric morphism induced by .
is an effective epimorphism in .
Let be the full subcategory on locally representable sheaves.
There is an equivalence of categories
Define a functor by sending each smooth manifold to the sheaf over that it naturally represents. By definition of manifold there is an open cover . We claim that is an effective epimorphism, so that this functor indeed lands in . (This is a standard argument of sheaf theory in Diff, we really only need to observe that it goes through over CartSp, too.)
For that we need to show that
is a coequalizer diagram in (that the Cech groupoid of the cover is equivalent to .). Notice that the fiber product here is just the intersection in . By the fact that the sheaf topos is by definition a reflective subcategory of the presheaf topos we have that colimits in are computed as the sheafification of the corresponding colimit in . The colimit in in turn is computed objectwise. Using this, we see that that we have a coequalizer diagram
Essentially by the definition of the coverage on , it follows that sheafification takes this subfunctor inclusion to an isomorphism. This shows that is indeed the tip of the coequalizer in as above, and hence that it is a locally representable sheaf.
Conversely, suppose that for there is a family of open embeddings such that we have a coequalizer diagram
in , which is the sheafification of the corresponding coequalizer in . By evaluating this on the point, we find that the underlying set of is the coequalizer of the underlying set of the in . Since every plot of factors locally through one of the it follows that is a diffeological space.
It follows that in the pullback diagrams
the object is the diffeological space whose underlying topological space is the intersection of and in the topological space underlying . In particular the inclusions are open embeddings.
We may switch from regarding smooth manifolds as objects in the big topos to regrading them as toposes themselves, by passing to the over-topos . This remembers the extra (smooth) structure on the topological space by being canonically a locally ringed topos with the structure sheaf of smooth functions on : a CartSp-structured (∞,1)-toposes
is a 0-localic (∞,1)-topos;
For every there is an equivalence
The second and third condition say in words that is locally equivalent to the ordinary cannonically CartSp-locally ringed space (for the dimension. The first condition then says that these local identifications cover .
A textbook reference is
An algebraic approach to classical smooth manifolds with an eye toward physics is in
Discussion of smooth manifolds as colimits of the Cech nerves of their good open covers is also at
The general abstract framework of higher geometry referred to above is discussed in
The proof that idempotents split in the category of smooth manifolds was adapted from this MO answer:
Which provides a solution to exercise 3.21 in
The above comment by Lawvere is taken from