nLab Cahiers topos



Synthetic differential geometry

synthetic differential geometry


from point-set topology to differentiable manifolds

geometry of physics: coordinate systems, smooth spaces, manifolds, smooth homotopy types, supergeometry



smooth space


The magic algebraic facts




infinitesimal cohesion

tangent cohesion

differential cohesion

graded differential cohesion

singular cohesion

id id fermionic bosonic bosonic Rh rheonomic reduced infinitesimal infinitesimal & étale cohesive ʃ discrete discrete continuous * \array{ && id &\dashv& id \\ && \vee && \vee \\ &\stackrel{fermionic}{}& \rightrightarrows &\dashv& \rightsquigarrow & \stackrel{bosonic}{} \\ && \bot && \bot \\ &\stackrel{bosonic}{} & \rightsquigarrow &\dashv& \mathrm{R}\!\!\mathrm{h} & \stackrel{rheonomic}{} \\ && \vee && \vee \\ &\stackrel{reduced}{} & \Re &\dashv& \Im & \stackrel{infinitesimal}{} \\ && \bot && \bot \\ &\stackrel{infinitesimal}{}& \Im &\dashv& \& & \stackrel{\text{étale}}{} \\ && \vee && \vee \\ &\stackrel{cohesive}{}& \esh &\dashv& \flat & \stackrel{discrete}{} \\ && \bot && \bot \\ &\stackrel{discrete}{}& \flat &\dashv& \sharp & \stackrel{continuous}{} \\ && \vee && \vee \\ && \emptyset &\dashv& \ast }


Lie theory, ∞-Lie theory

differential equations, variational calculus

Chern-Weil theory, ∞-Chern-Weil theory

Cartan geometry (super, higher)

Cohesive toposes



The Cahier topos is a cohesive topos that constitutes a well-adapted model for synthetic differential geometry (a “smooth topos”).

It is the sheaf topos on the site FormalCartSp of infinitesimally thickened Cartesian spaces.



Let FormalCartSp be the full subcategory of the category of smooth loci on those of the form

n×W, \mathbb{R}^n \times \ell W \,,

consisting of a product of a Cartesian space with an infinitesimally thickened point, i.e. a formal dual of a Weil algebra .

Dually, the opposite category is the full subcategory FormalCartSp opSmoothAlgFormalCartSp^{op} \hookrightarrow SmoothAlg of smooth algebras on those of the form

C ( k×W)=C ( k)W. C^\infty( \mathbb{R}^k \times \ell W) = C^\infty(\mathbb{R}^k) \otimes W \,.

This appears for instance in Kock Reyes (1).


Define a structure of a site on FormalCartSp by declaring a covering family to be a family of the form

{U i×Wp i×IdU×W} \{ U_i \times \ell W \stackrel{p_i \times Id}{\to} U \times \ell W \}

where {U ip iU}\{U_i \stackrel{p_i}{\to} U\} is an open cover of the Cartesian space UU by Cartesian spaces U iU_i.

This appears as Kock (5.1).


The Cahiers topos 𝒞𝒯\mathcal{CT} is the category of sheaves on this site:

𝒞𝒯:=Sh(FormalCartSp). \mathcal{CT} := Sh(FormalCartSp) \,.

This site of definition appears in Kock, Reyes. The original definition is due to Dubuc 79


Synthetic differential geometry


The Cahiers topos is a well-adapted model for synthetic differential geometry.

This is due to Dubuc 79.

Connectedness, locality and cohesion


The Cahiers topos is a cohesive topos. See synthetic differential infinity-groupoid for details.

Convenient vector spaces


The category of convenient vector spaces with smooth functions between them embeds as a full subcategory into the Cahiers topos.

The embedding is given by sending a convenient vector space VV to the sheaf given by

V: k×WC ( k,V)W. V : \mathbb{R}^k \times \ell W \mapsto C^\infty(\mathbb{R}^k, V) \otimes W \,.

This result was announced in Kock. See the corrected proof in (KockReyes).


Together with prop. this means that the differential geometry on convenient vector spaces may be treated synthetically in the Cahiers topos.

Synthetic tangent bundles of smooth spaces

Synthetic tangent spaces

We discuss here induced synthetic tangent spaces of smooth spaces in the sense of diffeological spaces and more general sheaves on the site of smooth manifolds after their canonical embedding into the Cahiers topos.


Write SmoothLocSmoothLoc for the category of smooth loci. Write

CartSpSmoothLoc CartSp \hookrightarrow SmoothLoc

for the full subcategory on the Cartesian spaces n\mathbb{R}^n (nn \in \mathbb{N}). Write

InfThPointSmoothLoc InfThPoint \hookrightarrow SmoothLoc

for the full subcategory on the infinitesimally thickened points, and write

CartSp synthdiffSmoothLoc CartSp_{synthdiff} \hookrightarrow SmoothLoc

for the full subcategory on those smooth loci which are the cartesian product of a Cartesian space n\mathbb{R}^n (nn \in \mathbb{N}) and an infinitesimally thickened point.

We regard CartSp as a site by equipping it with the good open cover coverage. We regard InfThPointInfThPoint as equipped with the trivial coverage and CartSp synthdiffCartSp_{synthdiff} as equipped with the induced product coverage.

The sheaf topos Sh(CartSp)Sh(CartSp) is that of smooth spaces. The sheaf topos Sh(CartSp synthdiff)Sh(CartSp_{synthdiff}) is the Cahier topos.


We write

D([ϵ]/(ϵ 2))InfThPtCartSp synthdiff D \coloneqq \ell(\mathbb{R}[\epsilon]/(\epsilon^2)) \in InfThPt \hookrightarrow CartSp_{synthdiff}

for the infinitesimal interval, the smooth locus dual to the smooth algebraof dual numbers”.


For XSh(CartSp synthdiff)X \in Sh(CartSp_{synthdiff}) any object in the Cahier topos, its synthetic tangent bundle in the sense of synthetic differential geometry is the internal hom space X DX^D, equipped with the projection map

X(*D):X DX. X(\ast \to D) \colon X^D \to X \,.

The canonical inclusion functor i:CartSpCartSp synthdiffi \colon CartSp \to CartSp_{synthdiff} induces an adjoint pair

Sh(CartSp)i *i !Sh(CartSp synthdiff) Sh(CartSp) \stackrel{\overset{i_!}{\to}}{\underset{i^\ast}{\leftarrow}} Sh(CartSp_{synthdiff})

where i *i^\ast is given by precomposing a presheaf on CartSp synthdiffCartSp_{synthdiff} with ii. The left adjoint i !i_! has the interpretation of the inclusion of smooth spaces as reduced objects in the Cahiers topos.

This is discussed in more detail at synthetic differential infinity-groupoid.


For XSh(CartSp)X \in Sh(CartSp) a smooth space, and for (W)InfThPoint\ell(W) \in InfThPoint an infinitesimally thickened point, the morphisms

(W)i !X \ell(W) \to i_! X

in Sh(CartSp synthdiff)Sh(CartSp_{synthdiff}) are in natural bijection to equivalence classes of pairs of morphisms

(W) nX \ell(W) \to \mathbb{R}^n \to X

consisting of a morphism in CartSp synthCartSp_{synth} on the left and a morphism in Sh(CartSp)Sh(CartSp) on the right (which live in different categories and hence are not composable, but usefully written in juxtaposition anyway). The equivalence relation relates two such pairs if there is a smooth function ϕ: n n\phi \colon \mathbb{R}^n \to \mathbb{R}^{n'} such that in the diagram

n (W) ϕ X n \array{ && \mathbb{R}^n \\ & \nearrow & & \searrow \\ \ell(W) && \downarrow^{\mathrlap{\phi}} && X \\ & \searrow && \nearrow \\ && \mathbb{R}^{n'} }

the left triangle commutes in CartSp synthdiffCartSp_{synthdiff} and the right one in Sh(CartSp)Sh(CartSp).


By general properties of left adjoints of functors of presheaves, i !Xi_! X is the left Kan extension of the presheaf XX along ii. By the Yoneda lemma and the coend formula for these (as discussed there), we have that the set of maps (W)i !X\ell(W) \to i_! X is naturally identified with

(i !X)((W))=(Lan iX)((W))= nCartSpHom CartSp synthdiff((W), n)×X( n). (i_! X)(\ell(W)) = (Lan_i X)(\ell(W)) = \int^{\mathbb{R}^n \in CartSp} Hom_{CartSp_{synthdiff}}(\ell(W), \mathbb{R}^n) \times X(\mathbb{R}^n) \,.

Unwinding the definition of this coend as a coequalizer yields the above description of equivalence classes.

Variants and generalizations


The Cahiers topos was introduced in

and got its name from this journal publication. The definition appears in theorem 4.10 there, which asserts that it is a well-adapted model for synthetic differential geometry.

A review discussion is in of

and with a corrected definition of the site of definition in

where it is first observed (but apparently without proof) that ThCartSp is still a site of definition for the Cahiers topos (cf. formal smooth set).

With its original site of all formal smooth manifolds, the Cahiers topos also appears briefly mentioned in:

With an eye towards Frölicher spaces the site is also considered in section 5 of

  • Hirokazu Nishimura, Beyond the Regnant Philosophy of Manifolds (arXiv:0912.0827)

The (∞,1)-topos analog of the Cahiers topos (synthetic differential ∞-groupoids) is discussed in section 3.4 of

Last revised on October 22, 2023 at 04:16:31. See the history of this page for a list of all contributions to it.