nLab
schematic homotopy type

Idea

A schematic homotopy type is a geometric ∞-stack over a site of formal duals of $k$-algebras that models a homotopy type in generalization to how a dg-algebra models a rational space in rational homotopy theory (via the fundamental theorem of dg-algebraic rational homotopy theory): schematic homotopy types in particular model more general fundamental groups

(…)

Definition

Let $k$ be a commutative ring, $T$ the Lawvere theory of commutative $k$-associative algebras. Let $\mathbb{U} \subset \mathbb{V}$ be an inclusion of universes. Let

$$T \hookrightarrow C = T Alg_{\mathbb{U}} \hookrightarrow T Alg_{\mathbb{V}}$$

be the site on formal duals of small $k$-algebras equipped with the fpqc-topology.

By the general discussion at function algebras on ∞-stacks we have then the Isbell duality pair of adjoint (∞,1)-functors

$$(\mathcal{O} \dashv Spec) : (T Alg_{\mathbb{V}}^{\Delta})^{op} \stackrel{\overset{\mathcal{O}}{\leftarrow}}{\underset{Spec}{\to}} Sh_\infty(C) =: \mathbf{H}$$

(due to Toën) where the (∞,1)-topos $\mathbf{H}$ is the (∞,1)-category of (∞,1)-sheaves on $C$.

Definition

(…) Let $Perf \in \mathbf{H}$ be the stack of perfect complexes of modules on $C$. (…)

Write $P \subset Mor(\mathbf{H})$ for the class of morphisms such that for all $p \in P$ we have that $\mathbf{H}(p,Perf)$ is an equivalence.

This is discussed in (HirschowitzSimpson, paragraph 21).

Definition

A pointed schematic homtopy type is the delooping $\mathbf{B}G \in \mathbf{H}$ of an ∞-group $G \in \mathbf{H}$ such that

• $G$ is in the image of $Spec$, in that there is $A \in T Alg^\Delta$ such that $G \simeq Spec A$;

• $\mathbf{B}G$ is a $P$-local object.

This appears as (Toën, def 3.1.2)

Properties

Proposition

A schematic homotopy type is in particular a geometric ∞-stack over $C$.

Examples

de Rham schematic homotopy type

For a connected scheme $X$ let $X_{dR}$ be its de Rham space. According to Toën, sect. 3.5.1 one finds that the functor

$$Ho(SchHoType/\mathbb{C}) \to Set$$

$$F \mapsto Ho_{Sh_{(\infty,1)}(Alg_\mathbb{C}^{op})}(X_{dR}, F)$$

is co-representable by a schematic homotopy type $X^{dR}$. This is the de Rham schematic homotopy type. The cohomology of $X^{dR} \in Sh_{(\infty,1)}$ is the algebraic de Rham cohomology of $X$.

Remark

A similar construction exists in every cohesive (∞,1)-topos. See the discussion in the section cohesive (∞,1)-topos – de Rham cohomology.

References

An introduction to the general theory

• Ludmil Katzarkov, Tony Pantev, Bertrand Toën, Schematic homotopy types and non-abelian Hodge theory, math.AG/0107129

• Bertrand Toën, Champs affines, Selecta Math. new series 12 (2006), no. 1, 39-135 (arXiv:math/0012219, doi:10.1007/s00029-006-0019-z)

The stack $Perf$ of perfect complexes is discussed for instance in section 21 of

• Andre Hirschowicz, Carlos Simpson, Descente pour les n-champs (arXiv:math/9807049)