# Contents

## Idea

This entry describes the higher geometry/derived geometry modeled on (∞,1)-sites of formal duals of dg-algebras, bounded or unbounded, over a field of characteristic 0.

The corresponding (∞,1)-topos is the context for classical rational homotopy theory, which arises by forming function algebras on ∞-stacks over constant ∞-stacks. It is also the context in which classical and higher order Hochschild homology of algebras and dg-algebras arises naturally as the function $\infty$-algebra on free loop space objects.

## The $(\infty,1)$-toposes

We discuss some basic aspects of the (∞,1)-toposes over (∞,1)-sites of formal duals of cdg-algebras and of cdg-algebras of functions on its objects.

### Over formal duals of non-positively graded cdg-algebras

Let $k$ be a field of characteristic 0, or more generally a commutative $\mathbb{Q}$-algebra.

#### Definition

###### Proposition/Definition

Write

• $cdgAlg_k$ for the category of graded-commutative cochain dg-algebras (meaning: with differential of degree +1) in arbitrary degree;

• $cdgAlk_k^-$ for the full subcategory on objects with vanishing cochain cohomology in positive degree, $H^{\bullet \geq 1}(-) = 0$.

There are the standard projective model structures on dg-algebras on these categories, whose weak equivalences are the quasi-isomorphisms and whose fibrations are the degreewise surjections.

This is considered in (Toën-Vezzosi, 2.3.1)

###### Proposition/Definition

Let

$C \hookrightarrow ((cdgAlg_k^-)^{op})^\circ$

be a small full sub-(∞,1)-category of the (∞,1)-category presented by this model structure, and let $C$ be equipped with the structure of a subcanonical (∞,1)-site.

Write

$\mathbf{H} := Sh_{(\infty,1)}(C)$

for the (∞,1)-category of (∞,1)-sheaves on $C$. We have a derived Isbell duality

$(\mathcal{O} \dashv j) : (cdgAlg_k^{op})^\circ \stackrel{\overset{\mathcal{O}}{\leftarrow}}{\underset{j}{\to}} \mathbf{H}$

where the left adjoint (∞,1)-functor $\mathcal{O}$ is the Yoneda extension of the inclusion $cdgAlg^+_k \hookrightarrow cdgAlg_k$.

This is considered in (Ben-ZviNadler). See function algebras on ∞-stacks for details.

#### Properties

###### Proposition

The inclusion

$cdgAlg^-_k \hookrightarrow cdgAlg_k$

is a homotopical context in the sense of (ToënVezzosi, def. 1.1.0.11).

This is (ToënVezzosi, lemma 2.3.11). We record the following implications of this statement

###### Corollary

$(cdgAlg_k, \otimes_k)$ is a symmetric monoidal model category.

###### Corollary

For $B \in (dgcAlg_k)_{proj}$ a cofibrant object, the tensor product with $B$ preserves weak equivalences.

This follows from (ToënVezzosi, assumption 1.1.0.4).

###### Corollary

The inclusion

$(cdgAlg_k^-)^{op} \hookrightarrow (cdgAlg_k)^{op}$

preserves homotopy limits, hence the induced inclusion

$((cdgAlg_k^-)^{op})^\circ \hookrightarrow ((cdgAlg_k)^{op})^\circ$

preserves (∞,1)-limits.

This follows from (ToënVezzosi, assumption 1.1.0.6).

###### Definition

For $A \in cdgAlg_k$ a dg-algebra, write

###### Corollary

For any $A\in cdgAlg_k$ say a morphism in $cdgAlg_A$ is

• a weak equivalence precisely if it is a quasi-isomorphism;

• a fibration precisely if it is degreewise surjective.

This makes $cdgAlg_A$ into a model category that is

There is an equivalence of categories with the under category of cdg-algebras under $A$

$cdgAlg_A \simeq A/cdgAlg_k$

which is a Quillen equivalence with respect to the standard model structure on an under category on the right.

###### Corollary

For $B \in cdgAlg_A$ cofibrant with respect to the model structure in cor 4, the tensor product (base change) functor

$B \otimes_A (-) : A Mod \to B Mod$

preserves weak equivalences.

This is (ToënVezzosi, assumption 1.1.0.4).

###### Proposition

The monoidal Dold-Kan correspondence provides a Quillen equivalence

$(\Gamma^{cmon} \dashv N_\bullet) : cAlg_k^{\Delta^{op}} \stackrel{\overset{\Gamma^{cmon}}{\leftarrow}}{\underset{N_\bullet}{\to}} cdgAlg_k^+$

(since $k$ is assumed to be of characteristic 0). Under this equivalence we have that $U \in cAlg_k \hookrightarrow cAlg_k^{\Delta^{op}} \hookrightarrow \mathbf{H}$ is $\mathcal{O}$-perfect:

$\mathcal{O} (X^{K}) \simeq K \cdot \mathcal{O}(X)$

and this recovers the constructions discussed above in The Hochschild chain complex of an associative algebra.

###### Proof

Since the (∞,1)-Yoneda embedding $y$ commutes with (∞,1)-limits we have that the powering $(y(U))^{K} \simeq y(U^K)$ is still representable. Therefore

$\mathcal{O} ((y(U))^K) \simeq \mathcal{O}(U^K) \;\; \in (cdgAlg_k^-)^{op} \hookrightarrow (cdgAlg_k)^{op}$

is simply the formal dual of $U^K$, which is $K \cdot \mathcal{O}(U)$ formed in $cdgAlg_k$ by formal duality. By the above proposition the inclusion $cdgAlg_k^- \hookrightarrow cdgAlg_k$ preserves this $(\infty,1)$-colimit.

(…)

## References

Various model category presentations of dg-geometry are presented in

The geometric ∞-function theory of perfect ∞-stacks in dg-geometry, and the corresponding Hochschild cohomology is considered in

The $(\mathcal{O} \dashv Spec)$-adjunction for dg-geometry is studied in

The basic reference for the model structure on dg-algebras (see there for more details) for the commutative case over a field of characteristic 0 is

Details on the use of this model category structure for modelling dg-spaces are in

Revised on October 17, 2011 13:27:35 by Urs Schreiber (82.113.99.57)