higher geometry / derived geometry
geometric little (∞,1)-toposes
geometric big (∞,1)-toposes
derived smooth geometry
=–
A space $X$ is called formally smooth if every morphisms $Y \to X$ into it has all possible infinitesimal extensions.
(If there is at most one extension per infinitesimal extension of $Y$ with no guarantee of existence it is called a formally unramified morphism. If the thickenings exist uniquely, it is called a formally etale morphism).
Traditionally this has considered in the context of geometry over formal duals of rings and associative algebras. This we discuss in the section (Concrete notion). But generally the notion makes sense in any context of infinitesimal cohesion. This we discuss in the section General abstract notion.
Let
be an adjoint triple of functors with $u^*$ a full and faithful functor that preserves the terminal object.
We may think of this as exhibiting infinitesimal cohesion (see there for details, but notice that in the notation used there we have $u^* = i_!$, $u_* = i^*$ and $u^! = i_*$).
We think of the objects of $\mathbf{H}$ as cohesive spaces and of the objects of $\mathbf{H}_{th}$ as such cohesive spaces possibly equipped with infinitesimal extension.
As a class of examples that is useful to keep in mind consider a Q-category $(cod \dashv \epsilon \dashv dom) : \bar A \to A$ of infinitesimal thickening of rings and let
be the corresponding Q-category of copresheaves.
For any such setup there is a canonical natural transformation
Details of this are in the section Adjoint quadruples at cohesive topos.
From this we get for every morphism $f : X \to Y$ in $\mathbf{H}$ a canonical morphism
A morphism $f : X \to Y$ in $\mathbf{H}$ is called formally smooth if (1) is an effective epimorphism.
This appears as (KontsevichRosenberg, def. 5.1, prop. 5.3.1.1).
The dual notion, where the above morphism is a monomorphism is that of formally unramified morphism. If both conditions hold, hence if the above morphism is an isomorphism, one speaks of a formally étale morphism.
An object $X \in \mathbf{H}$ is called formally smooth if the morphism $X \to *$ to the terminal object is formally smooth.
The object $X$ is formally smooth precisely if
is an effective epimorphism.
This appears as (KontsevichRosenberg, def. 5.3.2).
Formally smooth morphisms are closed under composition.
This appears as (KontsevichRosenberg, prop. 5.4).
Let $k$ be a field and let $CAlk_k$ be the category of commutative associative algebras over $k$. Write
for the presheaf topos over the opposite category $CAlg_k^{op}$. This is the context in which schemes and algebraic spaces over $k$ live.
A morphism $f :X\to Y$ in $\mathbf{H} = [CAlg_k, Set]$ is formally smooth if it satisfies the infinitesimal lifting property: for every algebra $A$ and nilpotent ideal $I\subset A$ and morphism $Spec(A)\to Y$ the induced map
is surjective.
This is due to (EGAIV${}_4$ 17.1.1)
An object $X \in [CAlg_k, Set]$ is formally smooth in the concrete sense of def. 3 precisely if it is so in the abstract sense of def. 1.
This appears as (KontsevichRosenbergSpaces, 4.1).
For a morphism $f:X\to Y$ of schemes, and $x$ a point of $X$, the following are equivalent
(i) $f$ is a smooth morphism of schemes at $x$
(ii) $f$ is locally of finite presentation at $x$ and there is an open neighborhood $U\subset X$ of $x$ such that $f|_U: U\to Y$ is formally smooth
(iii) $f$ is flat at $x$, locally of finite presentation at $x$ and the sheaf of Kähler differentials $\Omega_{X/Y}$ is locally free in a neighborhood of $x$
The relative dimension of $f$ at $x$ will equal the rank of the module of Kähler differentials.
This is (EGAIV${}_4$ 17.5.2 and 17.15.15)
A scheme $S$, i.e. a scheme over the ground ring $k$, is a formally smooth scheme if the corresponding morphism $S \to Spec(k)$ is a formally smooth morphism.
There is also an interpretation of formal smoothness via the formalism of Q-categories.
Let $k$ be a field and let $Alg_k$ be the category of associative algebras over $k$ (not necessarily commutative). Let
be the Q-category of infinitesimal thickenings of $k$-algebras (whose objects are surjective $k$-algebra morphisms with nilpotent kernel). Notice that the presheaf topos
is the context in which noncommutative schemes live. Let $\mathbf{H}_{th} \to \mathbf{Q}$ be the copresheaf Q-category over $Alg_k^{inf}$.
Let $f : R \to S$ be a morphism in $Alg_k$ such that $R$ is a separable algebra. Write $Spec f : Spec S \to Spec R$ for the corresponding morphism in $\mathbf{H} = [Alg_k, Set]$.
This $Spec f$ is formally smooth in the sense of def. 1 precisely if the $S \otimes_k S^{op}$-module
is a projective object in $S \otimes_k S^{op}$Mod.
In particular, setting $R = k$ we have that an object of the form $Spec S$ is formally smooth according to def. 2 precisely if $\Omega^1(S|k)$ is projective. This is what in (CuntzQuillen) is called the condition for a quasi-free algebra.
formally smooth morphism and formally unramified morphism $\Rightarrow$ formally étale morphism.
(shape modality $\dashv$ flat modality $\dashv$ sharp modality)
$(ʃ \dashv \flat \dashv \sharp )$
dR-shape modality $\dashv$ dR-flat modality
$ʃ_{dR} \dashv \flat_{dR}$
(reduction modality $\dashv$ infinitesimal shape modality $\dashv$ infinitesimal flat modality)
$(\Re \dashv \Im \dashv \&)$
fermionic modality $\dashv$ bosonic modality $\dashv$ rheonomy modality
$(\rightrightarrows \dashv \rightsquigarrow \dashv Rh)$
The definition over commutative rings is in
The definition over noncommutative algebras is in
The general abstract definition and its relation to the standard definitions is in
See also
A. Ardizzoni, Separable functors and formal smoothness, J. K-Theory 1 (2008), no. 3, 535–582, math.QA/0407095, doi, MR2009k:16069
T. Brzeziński, Notes on formal smoothness, in: Modules and Comodules (series Trends in Mathematics). T Brzeziński, JL Gomez Pardo, I Shestakov, PF Smith (eds), Birkhäuser, Basel, 2008, pp. 113-124 (doi, arXiv:0710.5527)
Guillermo Cortiñas, The structure of smooth algebras in Kapranov’s framework for noncommutative geometry, J. of Algebra 281 (2004) 679-694, math.RA/0002177