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synthetic differential geometry
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from point-set topology to differentiable manifolds
geometry of physics: coordinate systems, smooth spaces, manifolds, smooth homotopy types, supergeometry
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Tangency
The magic algebraic facts
Theorems
Axiomatics
Models
differential equations, variational calculus
Chern-Weil theory, ∞-Chern-Weil theory
Cartan geometry (super, higher)
∞-Lie theory (higher geometry)
Background
Smooth structure
Higher groupoids
Lie theory
∞-Lie groupoids
∞-Lie algebroids
Formal Lie groupoids
Cohomology
Homotopy
Related topics
Examples
-Lie groupoids
-Lie groups
-Lie algebroids
-Lie algebras
A formal smooth -groupoid is an ∞-groupoid equipped with a cohesive structure that subsumes that of smooth ∞-groupoids as well as of infinitesimal -groupoids – ∞-Lie algebroids, hence equipped with “differential cohesion”.
In the cohesive (∞,1)-topos of formal smooth -groupoids the canonical fundamental ∞-groupoid in a locally ∞-connected (∞,1)-topos factors through a version relative to Smooth∞Grpd: the infinitesimal path ∞-functor . In traditional terms this is the object modeled by the tangent Lie algebroid and the de Rham space of . The quasicoherent ∞-stacks on are D-modules on .
We consider (∞,1)-sheaves over a “twisted semidirect product” site or (∞,1)-site of
Cartesian spaces with smooth functions between them as for smooth ∞-groupoids,
and a category or (∞,1)-category of infinitesimally thickened points.
First in
we consider the 1-site, then in
we consider the -site.
Let CartSp be the syntactic category of the Lawvere theory of smooth algebras: the category of Cartesian spaces and smooth functions between them.
Write
for its category of T-algebras – the smooth algebras (-rings).
Let
be the full subcategory on the infinitesimally thickened points: this smooth algebras whose underlying abelian group is a vector space of the form with a finite-dimensional real vector space and nilpotent in the algebra structure.
Let
be the full subcategory of the category of smooth loci on the objects of the form
that are products of a Cartesian space CartSp for and an infinitesimally thickened point . (See at FormalCartSp.)
Equip this category with the coverage where a family of morphisms is covering precisely if it is of the form for a covering in CartSp (a good open cover).
This appears as ([Kock 86, (5.1)]).
The sheaf topos over FormalCartSp is (equivalent to) the topos known as the Cahiers topos, a smooth topos that constitutes a well adapted model for synthetic differential geometry. See at Cahiers topos for further references.
We say the (∞,1)-topos of formal smooth -groupoids is the (∞,1)-category of (∞,1)-sheaves
on .
We now generalize the 1-category of infinitesimally thickened points to the (∞,1)-category of “derived infinitesimally thickened points”, the formal dual of “small commutative -algebras” from (Hinich, Lurie).
(…)
is a cohesive (∞,1)-topos.
Because FormalCartSp is an ∞-cohesive site. See there for details.
Write for the full subcategory of smooth loci on the formal smooth manifolds: those modeled on FormalCartSp equipped with the evident coverage.
is a dense sub-site of .
With the above observation this is directly analogous to the corresponding proof at ETop∞Grpd.
Write for the canonical embedding.
The functor given by restriction along exhibits as an infinitesimal cohesive neighbourhood of the (∞,1)-topos Smooth∞Grpd of smooth ∞-groupoids in that we have a quadruple of adjoint (∞,1)-functors
such that is a full and faithful (∞,1)-functor.
Since is an infinitesimally ∞-cohesive site this follows with a proposition discussed at cohesive (infinity,1)-topos – infinitesimal cohesion.
We discuss the realization of the general abstract structures in a cohesive (∞,1)-topos in .
Since by the above discussion is strongly -connected relative to Smooth∞Grpd all of these structures that depend only on -connectedness (and not on locality) acquire a relative version.
This subsection is at
We discuss the intrinsic infinitesimal path adjunction realized in .
For we have that
is the reduced smooth locus: the formal dual of the smooth algebra obtained by quotienting out all nilpotent elements in the smooth algebra .
By the model category presentation of of the above proof and using that every representable is cofibrant and fibrant in the local projective model structure on simplicial presheaves we have
(using that is a full and faithful functor).
For a smooth locus, we have that is the corresponding de Rham space, the object in which all infinitesimal neighbour points in are equivalent, characterized by
By the -adjunction relation
We discuss the intrinsic cohomology in a cohesive (∞,1)-topos realized in .
The canonical line object of the Lawvere theory CartSp is the real line, regarded as an object of the Cahiers topos, and hence of
We shall write also for the underlying additive group
regarded as an abelian ∞-group object in . For write for its -fold delooping.
For and write
for the cohomology group of with coefficients in the canonical line object in degree .
Write
for the cohomology localization of at -cohomology: the full sub-(∞,1)-category on the -local object with respect to the class of morphisms that induce isomorphisms in all -cohomology groups.
Let be the projective model structure on cosimplicial smooth algebras and let be the prolonged external Yoneda embedding.
This constitutes the right adjoint of a Quillen adjunction
Restricted to simplicial formal smooth manifolds along
the right derived functor of is a full and faithful (∞,1)-functor that factors through the cohomology localization and thus identifies a full reflective sub-(∞,1)-category
The intrinsic -cohomology of any object is computed by the ordinary cochain cohomology of the Moore cochain complex underlying the cosimplicial abelian group of the image under the left derived functor under the Dold-Kan correspondence:
First a remark on the sites. By the above proposition is equivalent to the hypercomplete (∞,1)-topos over formal smooth manifolds. This is presented by the left Bousfield localization of at the ∞-connected morphisms. But a fibrant object in that is also n-truncated for is also fibrant in the hyperlocalization (only for the untruncated object there is an additional condition). Therefore the right Quillen functor claimed above indeed lands in truncated objects in .
The proof of the above statements is given in (Stel), following (Toën). Some details are spelled out at function algebras on ∞-stacks.
Let be a Lie group.
Then the intrinsic group cohomology in Smooth∞Grpd and in of with coefficients in coincides with Segal‘s refined Lie group cohomology (Segal, Brylinski).
For the full proof please see here, section 3.4 for the moment.
This follows from applying the above result to the fiber sequence induced by the sequence .
This means that the intrinsic cohomology of compact Lie groups in Smooth∞Grpd and coincides for these coefficients with the Segal-Blanc-Brylinski refined Lie group cohomology (Brylinski).
Let be an L-∞ algebroid. Then its intrinsic real cohomology in
coincides with its ordinary L-∞ algebroid cohomology: the cochain cohomology of its Chevalley-Eilenberg algebra
By the above propoposition we have that
By this lemma this is
Observe that is cofibrant in the Reedy model structure relative to the opposite of the projective model structure on cosimplicial algebras:
the map from the latching object in degree in is dually in the projection
hence is a surjection, hence a fibration in and therefore indeed a cofibration in .
Therefore using the Quillen bifunctor property of the coend over the tensoring in reverse to this lemma, the above is equivalent to
with the fat simplex replaced again by the ordinary simplex. But in brackets this is now by definition the image under the monoidal Dold-Kan correspondence of the Chevalley-Eilenberg algebra
By the Dold-Kan correspondence we have hence
It follows that a degree- -cocycle on is presented by a morphism
where on the right we have the -algebroid whose -algebra is concentrated in degree . Notice that if is the delooping of an - algebra this is equivalently a morphism of -algebras
under construction
We consider the de Rham theorem in , with respect to the infinitesimal de Rham cohomology
This means that for all the infinitesimal de Rham cohomology coincides with the ordinary real cohomology of the geometric realization of
Since all representables are formally smooth, we have a colimit
In the presentation over the site we have that
Therefore a morphism is equivalently a morphism such that for all that coincide on we have that all the composites
are equals. If is the point, then is necessarily constant. If is not the point, there is one nontrivial tangent vector in . The composite produces the corresponding tangent vector in . But all these tangent vectors must be equal. Hence must be constant.
This kind of argument is indicated in (Simpson-Teleman, prop. 3.4).
Let SmoothMfd and write for the tangent Lie algebroid regarded as a simplicial object (see L-infinity algebroid for the details).
Then there is a morphism which is an equivalence in -cohomology.
(…)
We discuss formally étale morphisms and étale objects with respect to the cohesive infinitesimal neighbourhood Smooth∞Grpd .
Let be a degreewise smooth paracompact simplicial manifold, canonically regarded as an object of Smooth∞Grpd.
Then in is presented by the same simplicial manifold.
First consider an ordinary smooth paracompact manifold . It admits a good open cover and the corresponding Cech nerve is a cofibrant resolution of . Therefore the -functor is computed on by evaluating the corresponding simplicial functor (of which it is the derived functor) on .
Since the simplicial functor
is a left adjoint (indeed a left Quillen functor) it preserves the coproducts and coend that the Cech nerve is built from:
Here we used that, by assumption on a good open cover, all the are Cartesian spaces, and that coincides on representables with the inclusion .
Let now be a general simplicial manifold. Assume that in each degree there is a good open cover such that these fit into a simplicial system giving a bisimplicial Cech nerve such that there is a morphism of bisimplicial presheaves
with regarded as simplicially constant in one direction. Each is a cofibrant resolution.
We claim that the coend
is a cofibrant resolution of , where is the fat simplex. From this the proposition follows by again applying the left Quillen functor degreewise and pulling it through all the colimits.
This remaining claim follows from the same argument as used above in prop. .
A morphism in , is a formally étale morphism with respect to the infinitesimal cohesion precisely if for all infinitesimally thickened points the diagram
is an -pullback under .
Since preserves -limits, this is the case in particular if the diagram is an -pullback already in . In this form, restricted to 0-truncated objects, hence to the Cahiers topos, this characterization of formally étale morphisms appears axiomatized around p. 82 of (Kock 81, p. 82).
In particular, a smooth function in SmoothMfd Smooth∞Grpd between smooth manifolds is a formally étale morphism with respect to the infinitesimal cohesion precisely if it is a local diffeomorphism in the traditional sense.
We spell out the case for smooth manifolds. Here we need to to show that
is a pullback in precisely if is a local diffeomorphism. This is a pullback precisely if for all the diagram of sets of plots
is a pullback. Using, by the discussion at ∞-cohesive site, that preserves colimits and restricts to on representables, and using that , this is equivalently the diagram
where the vertical morphisms are given by restriction along the inclusion .
For one direction of the claim it is sufficient to consider this situation for the point and the first order infinitesimal interval. Then is the underlying set of points of the manifold and is the set of tangent vectors, the set of points of the tangent bundle . The pullback
is therefore the set of pairs consisting of a point and a tangent vector . This set is in fiberwise bijection with precisely if for each there is a bijection , hence precisely if is a local diffeomorphism. Therefore being a local diffeo is necessary for being formally étale with respect to the given notion of infinitesimal cohesion.
To see that this is also sufficient notice that this is evident for the case that is in fact a monomorphism, and that since smooth functions are characterized locally, we can reduce the general case to that case.
A Lie groupoid is an étale groupoid in the traditional sense, precisely if regarded as an object in Smooth∞Grpd FormalSmooth∞Grpd it is an cohesive étale ∞-groupoid.
Let be the inclusion of the smooth manifold of objects. This is an effective epimorphism. It remains to show that this is formally étale with respect to the given cohesive neighbourhood.
By the discussion at (∞,1)-pullback we may compute the characteristic -pullback by an ordinary pullback of a fibration of simplicial presheaves that presents .
By the factorization lemma such is given by
By inspection one see that this morphism is
in degree 0 the target-map ;
in degree 1 the projection .
By prop. both of these need to be étale maps in the ordinary sense. By definition, this is the case precisely if is an étale groupoid.
As a direct consequence of prop. we have the following
A smooth function between smooth manifolds, is a submersion or immersion, respectively, precisely if, when canonically regarded as a morphism in , it is a formally smooth morphism or formally unramified morphism, respectively.
As in the proof of prop. we find that the pullback is over the infinitesimal interval isomorphic to
and the canonical morphism from into this pullback is
We indicate how to formalize Lie differentiation in the context of formal smooth -groupoids.
Let
be the canonical inclusion. By (Lurie, theorem 0.0.13, remark 0.0.15, also Pridham 07) we have a full inclusion
on those objects whose space of global sections is contractible and which are infinitesimally cohesive (for a somewhat different notion of “infinitesimal cohesion”, beware the terminology). Consider then the -functor
which sends a pointed connected formal smooth -groupoid to the -presheaf of pointed morphisms
for .
By assumption that is connected (and we need to assume that it is geometric, which will gives infinitesimal cohesion by the Artin-Lurie representability theorem) this factors as
The resulting -functor
is Lie differentiation.
For differentiation of smooth groupoids with atlas to L-infinity algebroids this happens under
(…)
The site FormalCartSp is discussed in section 5 of
For more on this see at Cahiers topos.
The notion of formally étale maps as obtained here from the general abstract definition in differential cohesion coincided on 0-truncated objects with that defined on p. 82 of
The infinitesimal path ∞-groupoid adjunction and the de Rham theorem for -stacks is discussed at the level of homotopy categories in section 3 of
The -topos :
Urs Schreiber, section 4.4 of: differential cohomology in a cohesive topos (arXiv:1310.7930)
Hisham Sati, Urs Schreiber, Section 3.1.2 of: Proper Orbifold Cohomology (arXiv:2008.01101)
Urs Schreiber, Introduction to Higher Supergeometry, lecture at Higher Structures in M-Theory 2018, Durham Symposium
published in parts as: Higher Structures in M-Theory (with Branislav Jurčo, Christian Saemann, Martin Wolf), Fortschritte der Physik (2019) (arXiv:1903.02807, doi:10.1002/prop.201910001)
The cohomology localization of and the infinitesimal singular simplicial complex as a presentation for infinitesimal paths objects in is discussed in
The discussion of the cohomology localization of follows that in another context in
The construction of the infinitesimal path object has been amplified and discussed by Anders Kock under the name combinatorial differential forms, for instance in
The discussion that the normalized cosimplicial algebra of functions on the infinitesimal singular simplicial complex is the de Rham complex is further discussed in
The results on differentiable Lie group cohomology used above is in
recalled in
which parallels
The -site of derived infinitesimal points is discussed in
following
Last revised on July 14, 2021 at 10:53:09. See the history of this page for a list of all contributions to it.