# Derived scheme

In algebraic geometry, a **derived scheme** is a pair consisting of a topological space *X* and a sheaf of commutative ring spectra [1] on *X* such that (1) the pair is a scheme and (2) is a quasi-coherent -module. The notion gives a homotopy-theoretic generalization of a scheme.

A derived stack is a stacky generalization of a derived scheme.

## Differential graded scheme

Over a field of characteristic zero, the theory is equivalent to that of a differential graded scheme. By definition, a **differential graded scheme** is obtained by gluing affine differential graded schemes, with respect to étale topology.[2] It was introduced by Maxim Kontsevich[3] "as the first approach to derived algebraic geometry."[4] and was developed further by Mikhail Kapranov and Ionut Ciocan-Fontanine.

### Connection with differential graded rings and examples

Just as affine algebraic geometry is equivalent (in categorical sense) to the theory of commutative rings (commonly called commutative algebra), affine derived algebraic geometry over characteristic zero is equivalent to the theory of commutative differential graded rings. One of the main example of derived schemes comes from the derived intersection of subschemes of a scheme, giving the Koszul complex. For example, let , then we can get a derived scheme

where

is the étale spectrum. Since we can construct a resolution

the derived ring is the koszul complex . The truncation of this derived scheme to amplitude provides a classical model motivating derived algebraic geometry. Notice that if we have a projective scheme

where we can construct the derived scheme where

with amplitude

## Cotangent Complex

### Construction

Let be a fixed differential graded algebra defined over a field of characteristic . Then a -differential graded algebra is called **semi-free** if the following conditions hold:

- The underlying graded algebra is a polynomial algebra over , meaning it is isomorphic to
- There exists a filtration on the indexing set where and for any .

It turns out that every differential graded algebra admits a surjective quasi-isomorphism from a semi-free differential graded algebra, called a semi-free resolution. These are unique up to homotopy equivalence in a suitable model category. The (relative) **cotangent complex** of an -differential graded algebra can be constructed using a semi-free resolution : it is defined as

Many examples can be constructed by taking the algebra representing a variety over a field of characteristic 0, finding a presentation of as a quotient of a polynomial algebra and taking the Koszul complex associated to this presentation. The Koszul complex acts as a semi-free resolution of the differential graded algebra where is the graded algebra with the non-trivial graded piece in degree 0.

### Examples

The cotangent complex of a hypersurface can easily be computed: since we have the dga representing the **derived enhancement** of , we can compute the cotangent complex as

where and is the usual universal derivation. If we take a complete intersection, then the koszul complex

is quasi-isomorphic to the complex

This implies we can construct the cotangent complex of the derived ring as the tensor product of the cotangent complex above for each .

### Remarks

Please note that the cotangent complex in the context of derived geometry differs from the cotangent complex of classical schemes. Namely, if there was a singularity in the hypersurface defined by then the cotangent complex would have infinite amplitude. These observations provide motivation for the **hidden smoothness** philosophy of derived geometry since we are now working with a complex of finite length.

## Tangent Complexes

### Polynomial Functions

Given a polynomial function then consider the (homotopy) pullback diagram

where the bottom arrow is the inclusion of a point at the origin. Then, the derived scheme has tangent complex at is given by the morphism

where the complex is of amplitude . Notice that the tangent space can be recovered using and the measures how far away is from being a smooth point.

### Stack Quotients

Given a stack there is a nice description for the tangent complex:

If the morphism is not injective, the measures again how singular the space is. In addition, the euler characteristic of this complex yields the correct (virtual) dimension of the quotient stack. In particular, if we look at the moduli stack of principal -bundles, then the tangent complex is just .

## Derived Schemes in Complex Morse Theory

Derived schemes can be used for analyzing topological properties of affine varieties. For example, consider a smooth affine variety . If we take a regular function and consider the section of

Then, we can take the derived pullback diagram

where is the zero section, constructing a **derived critical locus** of the regular function .

### Example

Consider the affine variety

and the regular function given by . Then,

where we treat the last two coordinates as . The derived critical locus is then the derived scheme

Note that since the left term in the derived intersection is a complete intersection, we can compute a complex representing the derived ring as

where is the koszul complex.

## Derived Critical Locus

Consider a smooth function where is smooth. The derived enhancement of , the **derived critical locus**, is given by the differential graded scheme where the underlying graded ring are the polyvector fields

and the differential is defined by contraction by .

### Example

For example, if

we have the complex

representing the derived enhancement of .

## Notes

- also often called -ring spectra
- Behrend, Kai (2002-12-16). "Differential Graded Schemes I: Perfect Resolving Algebras". arXiv:math/0212225. Bibcode:2002math.....12225B. Cite journal requires
`|journal=`

(help) - Kontsevich, M. (1994-05-05). "Enumeration of rational curves via torus actions". arXiv:hep-th/9405035.
- http://ncatlab.org/nlab/show/dg-scheme

## References

- Reaching Derived Algebraic Geometry - Mathoverflow
- M. Anel, The Geometry of Ambiguity
- K. Behrend, On the Virtual Fundamental Class
- P. Goerss, Topological Modular Forms [after Hopkins, Miller, and Lurie]
- B. Toën, Introduction to derived algebraic geometry
- M. Manetti, The cotangent complex in characteristic 0
- G. Vezzosi, The derived critical locus I - basics