nLab C^∞-ring


C \mathrm{C}^\infty-rings should not be confused with C \mathrm{C}_\infty -algebras.


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)



A smooth algebra or C C^\infty-ring is an algebra AA over the reals \mathbb{R} for which not only the product operation :×\cdot : \mathbb{R} \times \mathbb{R} \to \mathbb{R} lifts to the algebra product A×AAA \times A \to A, but for which every smooth map f: n mf : \mathbb{R}^n \to \mathbb{R}^m (morphism in Diff) lifts to a map A(f):A nA mA(f) : A^n \to A^m in a compatible way.

In short this means that AA is

The smoothness of such C C^\infty-rings is witnessed by the fact that this Lawvere theory is even a Fermat theory.

The opposite category of the category of C C^\infty-rings is the category of smooth loci. This and its subcategories play a major role as sites for categories of sheaves that serve as models for synthetic differential geometry.

Motivating example

For XX a smooth manifold, the assignment

nC (X, n)=Hom Diff(X, n) \mathbb{R}^n \mapsto C^\infty(X,\mathbb{R}^n) = Hom_{Diff}(X,\mathbb{R}^n)

of the set of smooth n\mathbb{R}^n-valued functions on XX is clearly covariant and hence yields a copresheaf on CartSp \subset Diff: a functor

C (X,):CartSpSet. C^\infty(X,-) : CartSp \to Set \,.

Since the hom-functor sends limits to limits in its second argument this is clearly product preserving.

C (X, n× m)C (X, n)×C (X, m) C^\infty(X, \mathbb{R}^n \times \mathbb{R}^m) \simeq C^\infty(X,\mathbb{R}^n) \times C^\infty(X, \mathbb{R}^m)

If as usual we write C (X):=C (X,)C^\infty(X) := C^\infty(X,\mathbb{R}) for the set of just \mathbb{R}-valued smooth functions, then the usual pointwise product of functions

:C (X)×C (X)C (X) \cdot : C^\infty(X) \times C^\infty(X) \to C^\infty(X)

can be regarded as the image of our copresheaf under the muliplication map ×\mathbb{R} \times \mathbb{R} \stackrel{-\cdot -}{\to} \mathbb{R} on the algebra of real numbers:

:C (X)×C (X):=C (X,)×C (X,)C (X,×)C (X,)C (X,)=:C (X). \cdot : C^\infty(X) \times C^\infty(X) := C^\infty(X,\mathbb{R}) \times C^\infty(X,\mathbb{R}) \simeq C^\infty(X,\mathbb{R}\times \mathbb{R}) \stackrel{C^\infty(X,-\cdot-)}{\to} C^\infty(X,\mathbb{R}) =: C^\infty(X) \,.



CartSp is the full subcategory of Diff

on manifolds of the form n\mathbb{R}^n.


A C C^\infty-algebra is a finite product-preserving copresheaf on CartSp, i.e. a finite product preserving functor

A:CartSpSet. A : CartSp \to Set \,.

The category of such functors and natural transformations between them we denote by C AlgC^\infty Alg.

Remark on terminology

The standard name in the literature for generalized smooth algebras is C C^\infty-rings. Even though standard, this has the disadvantage that it collides badly with the use of \infty- for higher categorical structures.

Tensor product

Definition (smooth tensor product)

The coproduct in C AlgC^\infty Alg we call the smooth tensor product

:C Alg×C AlgC Alg. \otimes_\infty : C^\infty Alg \times C^\infty Alg \to C^\infty Alg \,.

More generally, for i:CAi : C \to A and j:CBj : C \to B two morphisms in C AlgC^\infty Alg, we call the pushout

C i A B j A CB \array{ C &\stackrel{i}{\to}& A \\ \downarrow && \downarrow \\ B &\stackrel{j}{\to}& A \otimes_C B }

the smooth tensor product over CC of AA and BB.

Finitely generated C C^\infty-rings


For XX a smooth manifold, the smooth algebra C (X)C^\infty(X) is the functor

C (X):=Hom Diff(X,) C^\infty(X) := Hom_{Diff}(X,-)

(finitely generated and finitely presented C C^\infty-rings)

For RR a C C^\infty-ring, and IU(R)I \in U(R) an ideal in the underlying ordinary ring, there is a canonical C C^\infty-ring structure R/IR/I on the ordinary quotient ring U(R)/IU(R)/I.

A C C^\infty-ring RR is called finitely generated if it is of the form C ( n)/IC^\infty(\mathbb{R}^n)/I for nn \in \mathbb{N} and II an ideal in U(C ( n))U(C^\infty(\mathbb{R}^n)).

It is finitely presented if also II is finitely generated, as an ideal, I=(i 1,,i k)I = (i_1, \cdots, i_k) with i jU(R)i_j \in U(R).

This is equivalent to RR being a pushout of the form

C ( k) C (*) C ( n) R. \array{ C^\infty(\mathbb{R}^k) &\to& C^{\infty}(*) \simeq \mathbb{R} \\ \downarrow && \downarrow \\ C^\infty(\mathbb{R}^n) &\to& R } \,.

(germ-determined ideal)

For p np \in \mathbb{R}^n let

π p:C ( n)C p ( n) \pi_p : C^\infty(\mathbb{R}^n) \to C^\infty_p(\mathbb{R}^n)

be the natural projection onto the smooth algebra of germs of functions at pp.

A C C^\infty-ring CC is called fair or finitely generated and germ-determined if it is finitely generated CC ( n)/IC \simeq C^\infty(\mathbb{R}^n)/I and the ideal II has the property that fC ( n)f \in C^\infty(\mathbb{R}^n) is an element of II if (and hence precisely if) for all p np \in \mathbb{R}^n the germ π p(f)C p () n\pi_p(f) \in C^\infty_p(\mathbb{R})^n is in the germ π p(I)\pi_p(I) of the ideal.

Internal C C^\infty-rings

For any smooth topos (𝒯,R)(\mathcal{T}, R), there is an internal notion of generalized smooth algebra:

Definition (internal generalized smooth algebra)

For (𝒯,R)(\mathcal{T}, R) a topos equipped with an internal ring object RR (possibly but not necessarily a smooth topos), let CartSp(𝒯,R)CartSp(\mathcal{T},R) be the full subcategory of 𝒯\mathcal{T} on objects of the form R nR^n for nn \in \mathbb{N}. Then a (𝒯,R)(\mathcal{T},R)-algebra is a product-preserving functor A:CartSp(𝒯,R)SetA : CartSp(\mathcal{T}, R) \to Set.

All constructions on smooth algebras generalize to (𝒯,R)(\mathcal{T},R)-algebras. In particular for X𝒯X \in \mathcal{T} any object we have the function (𝒯,R)(\mathcal{T},R)-algebra

C(X):R n𝒯(X,R n). C(X) : R^n \mapsto \mathcal{T}(X,R^n) \,.

The following remark asserts that when 𝒯\mathcal{T} is itself a sufficiently nice category of sheaves on formal duals of (Set,)(Set,\mathbb{R})-algebras, then the internal notion of smooth function algebras on formal duals of external smooth algebras reproduces these external smooth algebras.


Let AA be a finitely generated C C^\infty-ring, A\ell A its incarnation as an object in 𝕃=(C Ring fin) op\mathbb{L} = (C^\infty Ring^{fin})^{op} and YAY\ell A its incarnation in Sh(𝕃)PSh(𝕃)Sh(\mathbb{L}) \subset PSh(\mathbb{L}), with YY the Yoneda embedding and using the assumption that the Grothendieck topology used to form Sh(𝕃)Sh(\mathbb{L}) is subcanonical.

Also suppose that the line object RR is represented by C ()\ell C^\infty(\mathbb{R})

Then we have for all AC Ring finA \in C^\infty Ring^{fin} that

C(YA):R nA(*) n C(Y\ell A) : R^n \mapsto A({*})^n

This is a straightforward manipulation:

Sh 𝕃(Y(A),R n) =Sh 𝕃(Y(A),Y(C ( n))) =PSh 𝕃(Y(A),Y(C ( n))) 𝕃(A,C ( n)) C Ring fin(C ( n),A) A(*) n \begin{aligned} Sh_{\mathbb{L}}(Y(\ell A), R^n) & = Sh_{\mathbb{L}}(Y(\ell A), Y(\ell C^\infty(\mathbb{R}^n))) \\ & = PSh_{\mathbb{L}}(Y(\ell A), Y(\ell C^\infty(\mathbb{R}^n))) \\ & \simeq \mathbb{L}(\ell A, \ell C^\infty(\mathbb{R}^n)) \\ & \simeq C^\infty Ring^{fin}(C^\infty(\mathbb{R}^n), A) \\ & \simeq A({*})^n \end{aligned}


  1. the first step expresses the nature of the line object in the models under consideration

  2. the second step expresses that the embedding Sh(𝕃)PSh(𝕃)Sh(\mathbb{L}) \to PSh(\mathbb{L}) is a full and faithful functor

  3. the third step expresses that the Yoneda embedding is a full and faithful functor

  4. the fourth step is the definition of 𝕃\mathbb{L} as the opposite category of C Ring finC^\infty Ring^{fin}

  5. the fifth step expresses that C (R n)C^\infty(R^n) is the free generalized smooth algebra on nn generators (MSIA, chaper I, prop 1.1)

Local C C^\infty-algebra

The category CartSp carries a natural Grothendieck topology.

A smooth algebra

A:CartSpSet A : CartSp \to Set

is a local algebra if AA sends covering families to epimorphism families: for each covering {U iU}\{U_i \to U\} the morphism

iA(U i)A(U) \coprod_i A(U_i) \to A(U)

is an epimorphism.


Local smooth algebras are precisely the “local Archimedian” algebras (…).

This is (Bunge-Dubuc, prop. 2.1).


Functions on smooth manifolds


For XX a smooth manifold, the smooth algebra C (X)C^\infty(X) is the functor

C (X):=Hom Diff(X,) C^\infty(X) := Hom_{Diff}(X,-)

Weil algebras: functions on small infinitesimal spaces

A Weil algebra in this context is a finite-dimensional commutative \mathbb{R}-algebra WW with a maximal ideal II such that W/IW/I \simeq \mathbb{R} and I n=0I^n = 0 for some nn \in \mathbb{N}.


There is a unique C C^\infty-ring structure on a Weil algebra WW. It makes WW a finitely presented C C^\infty-ring.


The smooth loci corresponding to Weil algebras are infinitesimal spaces. Weil algebras play a crucial role in the definition of smooth toposes.

Power series

Functions on germs of manifolds

For p np \in \mathbb{R}^n, the algebra of germs of smooth \mathbb{R}-valued functions at pp carries an evident C C^\infty-ring structure C ( n) pC^\infty(\mathbb{R}^n)_p.

With I pC ( n)I_p \subset C^\infty(\mathbb{R}^n) the ideal of functions that vanish on a neighbourhood of pp we have

C p ( n)C ( n)/I p, C^\infty_p(\mathbb{R}^n) \simeq C^\infty(\mathbb{R}^n)/I_p \,,

yielding a finitely generated but not (for n>0n \gt 0) finitely presented C C^\infty-ring.


Limits and colimits


All limits and all directed colimits in C RingC^\infty Ring are computed objectwise in [CartSp,Set][CartSp,Set] as limits in Set.


As discussed at limits and colimits by example, all limits and colimits in [CartSp,Set][CartSp,Set] are computed objectwise, so the remaining question is if they preserve the property of functors CartSpSetCartSp \to Set to preserved products. The claim follows from the observation that limits and directed colimits do commute with products.

See also MSIA, p. 22.

The underlying ordinary algebra

There is a forgetful functor

U:C AlgAlg U : C^\infty Alg \to Alg

from generalized smooth algebras to ordinary algebras which is given by evaluation on \mathbb{R}

U:AA() U : A \mapsto A(\mathbb{R})

and equipping the set A()A(\mathbb{R}) with the algebra structure induced on it:

the product and sum on A()A(\mathbb{R}) is the image of the corresponding operations on the algebra \mathbb{R}

A:A()×A()A(×)A()A(). \cdot_A : A(\mathbb{R}) \times A(\mathbb{R}) \stackrel{\simeq}{\to} A(\mathbb{R}\times \mathbb{R}) \stackrel{A(\cdot)}{\to} A(\mathbb{R}) \,.
+ A:A()×A()A(+)A()A(). +_A : A(\mathbb{R}) \times A(\mathbb{R}) \stackrel{\simeq}{\to} A(\mathbb{R} + \mathbb{R}) \stackrel{A(\cdot)}{\to} A(\mathbb{R}) \,.

Moreover there is canonically a morphism of rings

A() \mathbb{R} \to A(\mathbb{R})

given by

(*= 0c)(*=A( 0)A(c)A()). (* = \mathbb{R}^0 \stackrel{c}{\to} \mathbb{R}) \mapsto (* = A(\mathbb{R}^0) \stackrel{A(c)}{\to} A(\mathbb{R})) \,.

This makes A()A(\mathbb{R}) an \mathbb{R}-algebra.


The forgetful functor UU fits into an adjunction

(FU):C Alg UFAlg . (F \dashv U) : C^\infty Alg_{\mathbb{R}} \stackrel{\overset{F}{\leftarrow}}{\underset{U}{\to}} Alg_{\mathbb{R}} \,.

This statement may be understood as a special case of the following more general statement:

  • If SS, TT are finitary monads and f:STf: S \to T is a monad morphism, then the relative forgetful functor
    f *:Alg TAlg S,f^*: Alg_T \to Alg_S,

    which pulls back a TT-algebra ξ:TXX\xi: T X \to X to the SS-algebra ξfX:SXX\xi \circ f X: S X \to X, admits a left adjoint.

(In the case under discussion, SS is the free algebra monad on SetSet, TT is the free smooth algebra monad, and f:STf: S \to T is induced from the obvious inclusion f(n):S(n)T(n)f(n): S(n) \to T(n) which interprets an nn-ary algebra operation (in the theory Th STh_S) as a smooth operation in the theory Th TTh_T. See finitary monad for discussion on the connection between finitary monads TT and Lawvere theories Th TTh_T.)

The desired left adjoint f !f_! takes an algebra θ:SXX\theta: S X \to X to the reflexive coequalizer exhibited as a diagram

TSXTθ(μ TTf)XTXπT SXT S X \stackrel{\overset{(\mu_T \circ T f)X}{\to}}{\underset{T\theta}{\to}} T X \stackrel{\pi}{\to} T \otimes_S X

in the category of TT-algebras, where μ T:TTT\mu_T: T T \to T is the monad multiplication. The coequalizer is denoted T SXT \otimes_S X to emphasize the analogy with pushing forward SS-modules XX along a ring homomorphism f:STf: S \to T to get TT-modules; the proof below is an arrow-theoretic transcription of the usual proof of the adjunction between pushing forward and pulling back in the context of rings and modules.

A finitary monad TT preserves reflexive coequalizers, so that there is a canonical isomorphism

T(T SX)(TT) SXT(T \otimes_S X) \cong (T T) \otimes_S X

It follows that TT-algebra (TT-module) maps g¯:T SXY\bar{g}: T \otimes_S X \to Y, i.e., maps that render commutative the diagram

TT SX Tg¯ TY μ SX ξ T SX g¯ Y\array{ T T \otimes_S X & \stackrel{T\bar{g}}{\to} & T Y \\ \mu \otimes_S X \downarrow & & \downarrow \xi \\ T \otimes_S X & \underset{\bar{g}}{\to} & Y }

are in bijection with maps g:TXYg: T X \to Y that render commutative

TTX Tg TY μ ξ TX g Y\array{ T T X & \stackrel{T g}{\to} & T Y \\ \mu \downarrow & & \downarrow \xi \\ T X & \underset{g}{\to} & Y }

(that is to say, TT-algebra maps g:TXYg: T X \to Y) which additionally coequalize the parallel pair in the diagram

TSXTθ(μ TTf)XTXgYT S X \stackrel{\overset{(\mu_T \circ T f)X}{\to}}{\underset{T\theta}{\to}} T X \stackrel{g}{\to} Y

Since f:STf: S \to T is a monad morphism, we have commutativity of parallel squares in

TSX Tθ(μ TTf)X TX g Y fSX fX SSX Sθμ SX SX\array{ T S X & \stackrel{\overset{(\mu_T \circ T f)X}{\to}}{\underset{T\theta}{\to}} & T X & \stackrel{g}{\to} & Y \\ f S X \uparrow & & \uparrow f X & & \\ S S X & \stackrel{\overset{\mu_S X}{\to}}{\underset{S \theta}{\to}} & S X }

so that gfXg \circ f X coequalizes the bottom pair. However, because g:TXYg: T X \to Y is a TT-algebra map, its pullback gfX:SXYg \circ f X: S X \to Y defines an SS-algebra map SXf *YS X \to f^* Y. This SS-algebra map gfXg \circ f X factors through the coequalizer of the bottom pair of maps in Alg SAlg_S, i.e., factors uniquely through an SS-algebra map Xf *YX \to f^*Y. This establishes the adjunction f !f *f_! \dashv f^*.

Finitely presented C C^\infty-rings

Proposition (MSIA, prop. 1.1)

C ( n)C^\infty(\mathbb{R}^n) is the free smooth algebra on nn generators, in that for every nn \in \mathbb{N} and every smooth algebra AA there is an adjunction isomorphism

Hom C Alg(C ( n),A)Hom Alg([x 1,...,x n],A()). Hom_{C^\infty Alg}(C^\infty(\mathbb{R}^n), A) \simeq Hom_{Alg}(\mathbb{R}[x_1,...,x_n], A(\mathbb{R})) \,.
  • Every finitely presented C C^\infty-ring is fair/germ determined.

We have a chain of inclusions

  • finitely presented C C^\infty-rings

  • \subset “good” C C^\infty-rings

  • \subset fair C C^\infty-rings

  • \subset finitely generated C C^\infty-rings

Points of smooth loci

An \mathbb{R}-point of a C C^\infty-ring CC is a point *𝕃(C)* \to \mathbb{L}(C) of the corresponding smooth locus, i.e. a morphism CC (*)C \to \mathbb{R} \cong C^\infty(*).


Points of a C C^\infty-ring are in bijection with points of the underlying \mathbb{R}-algebra U(C)U(C), i.e. with ordinary \mathbb{R}-algebra morphisms U(C)U(C) \to \mathbb{R}.

In particular every Weil algebra WW has a unique point *𝕃(W)* \to \mathbb{L}(W): every Weil algebra is the algebra of functions on an infinitesimal thickening of an ordinary point.

By the properties of C (X)C^\infty(X) for XX a smooth manifold discussed below, the \mathbb{R}-points of C (X)C^\infty(X) are precisely the ordinary points of the manifold XX.

Smooth function algebras on smooth manifolds

Proposition (MSIA, prop. 2.5, 2.6 )

Let f:XZf : X \to Z and g:YZg : Y \to Z be transversal maps of smooth manifolds. Then the functor C ()C^\infty(-) sends the pullback

X× ZY X f Y g Z \array{ X \times_Z Y &\to& X \\ \downarrow && \downarrow^f \\ Y &\stackrel{g}{\to}& Z }

to the pushout

C (X) C (Z)C (Y)=: C (X× ZY) C (X) f * C (Y) g * C (Z) \array{ C^\infty(X) \otimes_{C^\infty(Z)} C^\infty(Y) =: & C^\infty(X \times_Z Y) &\leftarrow& C^\infty(X) \\ & \uparrow && \uparrow^{f^*} \\ & C^\infty(Y) &\stackrel{g^*}{\leftarrow}& C^\infty(Z) }

In particular this implies (for Z=*Z = {*})that the the smooth tensor product of functions on XX and YY is the algebra of functions on the product X×YX \times Y:

C (X×Y)C (X) C (Y). C^\infty(X \times Y) \simeq C^\infty(X) \otimes_\infty C^\infty(Y) \,.

The ordinary algebraic tensor product of C (X)()C^\infty(X)(\mathbb{R}) and C (Y)()C^\infty(Y)(\mathbb{R}) regarded as ordinary algebras does not in general satisfy this property. Rather one has an inclusion

C (X)()C (Y)()C (X×Y)(). C^\infty(X)(\mathbb{R}) \otimes C^\infty(Y)(\mathbb{R}) \subset C^\infty(X \times Y)(\mathbb{R}) \,.

In the context of geometric function theory the corresponding general statement (without the transversality condition) says that C (X)C^\infty(X) is a “good” kind of function. The above equation is one sub-aspect of the one of the fundamental theorems of geometric infinity-function theory.

Turning this inclusion into an equivalence is usually called a completion of the algebraic tensor product. Therefore we see:

The smooth tensor product is automatically the completed tensor product.

In summary this yields the following characterization of smooth function algebras on manifolds.

Theorem (MSIA, theorem 2.8)

The functor C ()=Hom Diff(,):DiffC AlgC^\infty(-) = Hom_{Diff}(-,-) : Diff \to C^\infty Alg

Deformation theory of smooth algebras

under construction

For CC any category whose objects we think of as “functions algebras on test spaces”, such as C=C RingC = C^\infty Ring, there is a general intrinsic notion of tangent complex and deformation theory of such objects.

As describe there, the key structure of interest from which all the other structure here is induced is the tangent category

TCC. T C \to C \,.

This is TC=Ab(Arr(C))T C = Ab(Arr(C)) the codomain fibration of CC “fiberwise stabilized”, meaning that in each fiber one takes it to consist of Ab(C/A)Ab(C/A), the abelian group objects in the overcategory.

We now first recall what this means for ordinary rings and how it induces the ordinary notion of derivations and modules for ordinary rings by setting C=C = CRing, and then look at what it implies for C C^\infty-rings by setting C=C RingsC = C^\infty Rings.

By an old argument by Quillen, for C=C = CRing we have that TC=ModT C = Mod is the bifibration of modules over rings, there is a natural equivalence

Mod AAb(C/A). Mod_A \stackrel{\simeq}{\to} Ab(C/A) \,.

This is induced by the functor that sends an AA-module NN to the corresponding object in the square-0-extension RNRR \oplus N \to R. (See module).

From this structure alone a lot of further structure follows:

  • a derivation δAN\delta A \to N is precisely a section of the corresponding morphism ANAA \oplus N \to A in C/AC/A, in the category CC namely a ring homomorphism

    A δ AN = A. \array{ A &&\stackrel{\delta}{\to}&& A \oplus N \\ & {}_ {=}\searrow && \swarrow \\ && A } \,.
  • The forgetful functor TRingModRingT Ring \simeq Mod \to Ring has a left adjoint

    Ω K 1:RingMod \Omega_K^1 : Ring \to Mod

    that sends each ring to its module of Kähler differentials.

    The fact that it is left adjoint is the universal property of the Kähler differentials as the objects co-representing derivations

    Hom Ab(Ring/R)(Ω K 1(A),N)Hom Ring/A(A,AN)Hom_{Ab(Ring/R)}(\Omega_K^1(A),N) \simeq Hom_{Ring/A}(A,A \oplus N).

    So every derivation δ:AN\delta : A \to N uniquely corresponds to a module morphism Ω K 1(A)N\Omega^1_K(A) \to N, namely the one that sends daδ(a)d a \mapsto \delta(a).

This abstract story remains precisely the same for C C^\infty-rings (and in fact for everything else!) but what it means concretely changes.

The crucial observation is (as one can show) that an abelian group object in C Ring/AC^\infty Ring/A is a square-0 extension (AN)(A \oplus N) for NN an (ordinary) module of the underlying \mathbb{R}-algebra AA. This square-0-extension happens to be uniquely equipped with the C C^\infty-Ring-structure given by

(fC ( n))((a 1,n 1),,(a n,n n))f(a 1,,a n)+ i=1 nfx i(a 1,,a n)n i). (f \in C^\infty(\mathbb{R}^n) \to \mathbb{R}) \mapsto \left( (a_1, n_1), \cdots, (a_n, n_n)) \mapsto f(a_1, \cdots, a_n) + \sum_{i = 1^n} \frac{\partial f}{\partial x_i}(a_1, \cdots, a_n) n_i \right) \,.

This uniquely induced smooth structure on objects in Ab(C Ring/A)Ab(C^\infty Ring/A) then in turn affects the nature of the notion of derivation and of Kähler differentials, as those are defined by general abstract reasoning from the former.

First of all it follows that a derivation – by general abstract definition a morphism of C C^\infty-rings Idδ:AANId \oplus \delta : A \to A \oplus N – is a morphism that satisfies for all fC ( n,)f \in C^\infty(\mathbb{R}^n, \mathbb{R}) that

δ:f(a 1,,a n) ifx iδa i. \delta : f(a_1, \cdots, a_n) \mapsto \sum_i \frac{\partial f}{\partial x_i} \delta a_i \,.

For ordinary rings only the compatibility δ(a 1a 2)=δ(a 1)a 2+a 1δ(a 2)\delta (a_1 \cdot a_2) = \delta (a_1) a_2 + a_1 \delta(a_2) with the single product operation is required. Here, however, compatibility with infinitely more operations fC ( n,)f \in C^\infty(\mathbb{R}^n, \mathbb{R}) is demanded.

Accordingly, then, the Kähler differentials as defined with respect to such derivations are different from the purely ring-theoretic ones: they produce the right notion of smooth 1-forms here, whereas the ring-theoretic one does not.


The generalization of the notion of smooth algebra for (∞,1)-category theory is

The generalization to supergeometry is


Textbook account

The concept of C C^\infty-rings in particular and that of synthetic differential geometry in general was introduced in

  • Bill Lawvere, Categorical dynamics

    in Anders Kock (eds.) Topos theoretic methods in geometry, volume 30 of Various Publ. Ser., pages 1-28, Aarhus Univ. (1979)

but examples of the concept are older. A discussion from the point of view of functional analysis is in

  • G. Kainz, A. Kriegl, Peter Michor, C C^\infty-algebras from the functional analytic view point Journal of pure and applied algebra 46 (1987) (pdf)

A characterization of those C C^\infty-rings that are algebras of smooth functions on some smooth manifold is given in

Lawvere’s ideas were later developed by Eduardo Dubuc, Anders Kock, Ieke Moerdijk, Gonzalo Reyes, and Gavin Wraith.

Studies of the properties of C C^\infty-rings include

The notion of the spectrum of a C C^\infty-ring and that of C C^\infty-schemes is discussed in

and more generally in

  • Ieke Moerdijk, Gonzalo Reyes, Rings of smooth functions and their localization II , in Mathematical logic and theoretical computer science page 275 (Google books)

  • Marta Bunge, Eduardo Dubuc, Archimedian local C C^\infty-rings and models of synthetic differential geometry Cahiers de Topologie et Géométrie Différentielle Catégoriques, 27 no. 3 (1986), p. 3-22 (numdam).

More recent developments along these lines are in

The higher geometry generalization to a theory of derived smooth manifolds – spaces with structure sheaf taking values in simplicial C∞-rings – was initiated in

based on the general machinery of structured (∞,1)-toposes in

where this is briefly mentioned in the very last paragraph.

See also the references at Fermat theory, of which C C^\infty-rings are a special case. And the references at smooth locus, the formal dual of a C C^\infty-ring. And the references at super smooth topos, which involves generalizations of C C^\infty-rings to supergeometry.

Last revised on July 16, 2023 at 19:22:37. See the history of this page for a list of all contributions to it.