15.57 Computing Tor
Let R be a ring. We denote D(R) the derived category of the abelian category \text{Mod}_ R of R-modules. Note that \text{Mod}_ R has enough projectives as every free R-module is projective. Thus we can define the left derived functors of any additive functor from \text{Mod}_ R to any abelian category.
This applies in particular to the functor - \otimes _ R M : \text{Mod}_ R \to \text{Mod}_ R whose left derived functors are the Tor functors \text{Tor}_ i^ R(-, M), see Algebra, Section 10.75. There is also a total left derived functor
which is denoted -\otimes _ R^{\mathbf{L}} M. Its satellites are the Tor modules, i.e., we have
A special situation occurs when we consider the tensor product with an R-algebra A. In this case we think of - \otimes _ R A as a functor from \text{Mod}_ R to \text{Mod}_ A. Hence the total right derived functor
which is denoted -\otimes _ R^{\mathbf{L}} A. Its satellites are the tor groups, i.e., we have
In particular these Tor groups naturally have the structure of A-modules.
We will generalize the material in this section to unbounded complexes in the next few sections.
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