smooth infinity-groupoid

**structures in a cohesive (∞,1)-topos**

**infinitesimal cohesion?**

A *smooth $\infty$-groupoid* is an ∞-groupoid equipped with cohesion in the form of smooth structure. Examples include smooth manifolds, Lie groups and Lie groupoids.

The (∞,1)-topos $Smooth \infty Grpd$ of all smooth $\infty$-groupoids is a cohesive (∞,1)-topos. It realizes a higher geometry version of differential geometry.

Many properties of smooth $\infty$-groupoids are inherited from the underlying Euclidean-topological ∞-groupoids. See ETop∞Grpd for more.

There is a refinement of smooth $\infty$-groupoids to synthetic differential ∞-groupoids. See SynthDiff∞Grpd for more on that.

For $X$ a smooth manifold, say an open cover $\{U_i \to X\}$ is a **differentiably good open cover** if each non-empty finite intersection of the $U_i$ is diffeomorphic to a Cartesian space.

Every paracompact smooth manifold admits a differentiably good open cover.

This is a folk theorem. A detailed proof is at good open cover.

Let SmoothMfd be the large site of paracompact smooth manifolds with smooth functions between them and equipped with the coverage of differentiably good open covers.

This does indeed define a coverage. The Grothendieck topology that is generated from it is the standard open cover topology.

For $\{U_i \to X\}$ any open cover of a paracompact manifold also $\coprod_i U_i$ is paracompact. Hence we may find a differentiably good open cover $\{K_j \to \coprod_i U_i\}$. This is then a refinement of the original open cover of $X$.

Let CartSp${}_{smooth}$ be the site of Cartesian spaces with smooth functions between them and equipped with the coverage of differentiably good open covers.

The (∞,1)-topos of **smooth $\infty$-groupoids** is the (∞,1)-category of (∞,1)-sheaves on CartSp${}_{smooth}$:

$Smooth \infty Grpd
:=
Sh_{(\infty,1)}(CartSp_{smooth})
\,.$

$Smooth \infty Grpd$ is a cohesive (∞,1)-topos.

The site CartSp${}_{smooth}$ is (as discussed there) an ∞-cohesive site (see there).

Let SmoothMfd be the large site of paracompact smooth manifolds with smooth functions between them and equipped with the coverage whose covering families are *differentiably good open covers* : open covers $\{U_i \to U\}$ where each non-empty open intersection is diffeomorphic to a Cartesian space.

This does indeed define a coverage and the Grothendieck topology generated by it is the standard open cover topology.

This is discussed in detail at good open cover.

The (∞,1)-topos $Smooth \infty Grpd$ is equivalent to the hypercompletion $\hat Sh_{(\infty,1)}(SmoothMfd)$ of the (∞,1)-category of (∞,1)-sheaves on the large site SmoothMfd

$Smooth \infty Grpd \simeq \hat Sh_{(\infty,1)}(SmoothMfd)
\,.$

By the above we have that CartSp${}_{smooth}$ is a dense sub-site of SmoothMfd. With this the claim follows as in the analogous discussion at ETop∞Grpd.

The canonical embedding of smooth manifolds as 0-truncated objects in $Smooth\infty Grpd$ is a full and faithful (∞,1)-functor

$SmoothMfd \hookrightarrow Smooth \infty Grpd
,.$

We discuss the relation of $Smooth\infty Grpd$ to other cohesive (∞,1)-toposes.

The cohesive (∞,1)-topos ETop∞Grpd of Euclidean-topological ∞-groupoids has as site of definition CartSp${}_{top}$. There is a canonical forgetful functor

$i : CartSp_{smooth} \to CartSp_{top}$

The functor $i$ extends to an essential (∞,1)-geometric morphism

$(i_! \dashv i^* \dashv i_*)
:
Smooth\infty Grpd
\stackrel{\overset{i_!}{\to}}{\stackrel{\overset{i^*}{\leftarrow}}{\underset{i_*}{\to}}}
ETop\infty Grpd$

such that the (∞,1)-Yoneda embedding is factored through the induced inclusion SmoothMfd $\stackrel{i}{\hookrightarrow}$ Mfd as

$\array{
SmoothMfd &\hookrightarrow& Smooth\infty Grpd
\\
\downarrow^{\mathrlap{i}} && \downarrow^{\mathrlap{i_!}}
\\
Mfd &\hookrightarrow& ETop\infty Grpd
}
\,$

Using the observation that $i$ preserves coverings and pullbacks along morphism in covering families, the proof follows precisely the steps of the proof of this proposition.

(Both of these are special cases of a general statement about morphisms of (∞,1)-sites, which should eventually be stated in full generality somewhere).

The essential global section (∞,1)-geometric morphism of $Smooth \infty Grpd$ factors through that of ETop∞Grpd

$(\Pi_{Smooth} \dashv Disc_{Smooth} \dashv \Gamma_{Smooth})
:
Smooth \infty Grpd
\stackrel{\overset{i_!}{\to}}{\stackrel{\overset{i^*}{\leftarrow}}{\underset{i_*}{\to}}}
ETop\infty Grpd
\stackrel{\overset{\Pi_{ETop}}{\to}}{\stackrel{\overset{Disc_{ETop}}{\leftarrow}}{\underset{\Gamma_{ETop}}{\to}}}
\infty Grpd$

This follows from the essential uniqueness of the global section (∞,1)-geometric morphism and of adjoint (∞,1)-functors.

The functor $i_!$ here is the forgetful functor that forgets smooth structure and only remembers Euclidean topology-structure.

Observe that CartSp${}_{smooth}$ is (the syntactic category of) a Lawvere theory: the algebraic theory of smooth algebras ($C^\infty$-rings). Write $SmoothAlg := Alg(C)$ for the category of its algebras. Let $InfPoint \hookrightarrow SmoothAlg^{op}$ be the full subcategory on the infinitesimally thickened points.

Let CartSp${}_{synthdiff} \hookrightarrow SmoothAlg^{op}$ be the full subcategory on the objects of the form $U \times D$ with $D \in CartSp_{smooth} \hookrightarrow SmoothAlg^{op}$ and $D \in InfPoint \hookrightarrow SmoothAlg^{op}$. Write

$i : CartSp_{smooth} \hookrightarrow CartSp_{synthdiff}$

for the canonical inclusion.

The inclusion exhibits an infinitesimal cohesive neighbourhood of $Smooth \infty Grpd$

$(i_! \dashv i^* \dashv i_* \dashv i^!)
:
Smooth \infty Grpd
\hookrightarrow
SynthDiff\infty Grpd
\,,$

where SynthDiff∞Grpd is the cohesive (∞,1)-topos of synthetic differential ∞-groupoids: the (∞,1)-category of (∞,1)-sheaves over $CartSp_{synthdiff}$.

This follows as a special case of this proposition after observing that $CartSp_{synthdiff}$ is an infinitesimal neighbourhood site of $CartSp_{smooth}$ in the sense defined there.

In SynthDiff∞Grpd we have ∞-Lie algebras and ∞-Lie algebroids as actual infinitesimal objects. See there for more details.

The (1,1)-topos on the 0-truncated smooth $\infty$-groupoids is

$Sh(CartSp)
\simeq
Smooth \infty Grpd_{\leq 0} \hookrightarrow Smooth\infty Grpd
\,,$

the sheaf topos on SmthMfd/CartSp discussed at *smooth space*.

The concrete objects in there

$Smooth\infty Grpd_{\leq 0}^{conc}
\hookrightarrow
Smooth \infty Grpd$

are precisely the diffeological spaces.

We discuss the general abstract structures in a cohesive (∞,1)-topos realized in $Smooth \infty Grpd$.

This section is at

Smooth $\infty$-groupoids and related cohesive structures play a central role in the discussion at

For standard references on differential geometry and Lie groupoids see there.

The $(\infty,1)$-topos $Smooth \infty Grpd$ is discussed in section 3.3 of

A discussion of smooth $\infty$-groupoids as $(\infty,1)$-sheaves on $CartSp$ and the presentaton of the $\infty$-Chern-Weil homomorphism on these is in

- Domenico Fiorenza, Urs Schreiber, Jim Stasheff,
*Cech cocycles for differential characteristic classes – An $\infty$-Lie theoretic construction*(web).

For references on Chern-Weil theory in Smooth∞Grpd and connection on a smooth principal ∞-bundle, see there.

The results on differentiable Lie group cohomology used above are in

- P. Blanc,
*Cohomologie différentiable et changement de groupes*Astérisque, vol. 124-125 (1985), pp. 113-130.

and

- Jean-Luc Brylinski,
*Differentiable Cohomology of Gauge Groups*(arXiv)

which parallels

- Graeme Segal,
*Cohomology of topological groups*, Symposia Mathematica, Vol IV (1970) (1986?) p. 377

A review is in section 4 of

- Chris Schommer-Pries,
*A finite-dimensional String 2-group*(arXiv:0911.2483)

Classification of topological principal 2-bundles is discussed in

- John Baez, Danny Stevenson,
*The classifying space of a topological 2-group*Algebraic Topology Abel Symposia, 2009, Volume 4, 1-31 (arXiv:0801.3843)

and the generalization to classification of smooth principal 2-bundles is in

- Thomas Nikolaus, Konrad Waldorf,
*Four Equivalent Versions of Non-Abelian Gerbes*(arXiv:1103.4815)

Further discussion of the shape modality on smooth $\infty$-groupoids is in

- David Carchedi,
*On The Homotopy Type of Higher Orbifolds and Haefliger Classifying Spaces*(arXiv:1504.02394)

Revised on April 14, 2015 15:10:35
by Urs Schreiber
(195.113.30.252)