# Derived tensor product

In algebra, given a differential graded algebra A over a commutative ring R, the derived tensor product functor is

$-\otimes _{A}^{\textbf {L}}-:D({\mathsf {M}}_{A})\times D({}_{A}{\mathsf {M}})\to D({}_{R}{\mathsf {M}})$ where ${\mathsf {M}}_{A}$ and ${}_{A}{\mathsf {M}}$ are the categories of right A-modules and left A-modules and D refers to the homotopy category (i.e., derived category). By definition, it is the left derived functor of the tensor product functor $-\otimes _{A}-:{\mathsf {M}}_{A}\times {}_{A}{\mathsf {M}}\to {}_{R}{\mathsf {M}}$ .

## Derived tensor product in derived ring theory

If R is an ordinary ring and M, N right and left modules over it, then, regarding them as discrete spectra, one can form the smash product of them:

$M\otimes _{R}^{L}N$

whose i-th homotopy is the i-th Tor:

$\pi _{i}(M\otimes _{R}^{L}N)=\operatorname {Tor} _{i}^{R}(M,N)$ .

It is called the derived tensor product of M and N. In particular, $\pi _{0}(M\otimes _{R}^{L}N)$  is the usual tensor product of modules M and N over R.

Geometrically, the derived tensor product corresponds to the intersection product (of derived schemes).

Example: Let R be a simplicial commutative ring, Q(R) → R be a cofibrant replacement, and $\Omega _{Q(R)}^{1}$  be the module of Kähler differentials. Then

$\mathbb {L} _{R}=\Omega _{Q(R)}^{1}\otimes _{Q(R)}^{L}R$

is an R-module called the cotangent complex of R. It is functorial in R: each RS gives rise to $\mathbb {L} _{R}\to \mathbb {L} _{S}$ . Then, for each RS, there is the cofiber sequence of S-modules

$\mathbb {L} _{S/R}\to \mathbb {L} _{R}\otimes _{R}^{L}S\to \mathbb {L} _{S}.$

The cofiber $\mathbb {L} _{S/R}$  is called the relative cotangent complex.