In the context of homological algebra, the -functor is the derived tensor product: the left derived functor of the tensor product of -modules, for a commutative ring.
Together with the Ext-functor it constitutes one of the central operations of interest in homological algebra.
Given a ring the bifunctor from two copies of -Mod to Ab is a right exact functor. Its left derived functors are the Tor-functors
with respect to one argument with fixed another, if they exist, are parts of a bifunctor
Existence and balancing
Given a right -module
and a left -module
there are in principle three different ways to compute their derived tensor product :
keeping fixed and deriving the functor
keeping fixed and deriving the functor
deriving the functor
in both arguments
If both and have enough projectives, then all these three derived functors exist and all give the same result.
Existence is clear from the very definition of derived functor in homological algebra. So we show that deriving in the left argument gives the same result as deriving in the right argument.
Let and be projective resolutions of and , respectively. The corresponding tensor product of chain complexes , hence the total complex of the degreewise tensor product of modules double complex carries the filtration by horizontal degree as well as that by vertical degree.
Accordingly there are the corresponding two spectral sequences of a double complex, to be denoted here (for the filtering by -degree) and (for the filtering by -degree). By the discussion there, both converge to the chain homology of the total complex.
We find the value of both spectral sequences on low degree pages according to the general discussion at spectral sequence of a double complex - low degree pages.
The 0th page for both is
For the first page we have
Now using the universal coefficient theorem in homology and the fact that and is a resolution by projective objects, by construction, hence of tensor acyclic objects for which all Tor-modules vanish, this simplifies to
It follows for the second pages that
Now both of these second pages are concentrated in a single row and hence have converged on that page already. Therefore, since they both converge to the same value:
Respect for direct sums and filtered colimits
Each respects direct sums.
Let Set and let be an -family of -modules. Observe that
if is an family of projective resolutions, then their degreewise direct sum is a projective resolution of .
the tensor product functor distributes over direct sums (this is discussed at tensor product of modules – monoidal category structure)
the chain homology functor preserves direct sums (this is discussed at chain homology - respect for direct sums).
Using this we have
Let hence be a filtered diagram of modules. For each , we may find a projective resolution and in fact a free resolution . Since chain homology commutes with filtered colimits (this is discussed at chain homology - respect for filtered colimits), this means that
is still a quasi-isomorphism. Moreover, by Lazard's criterion the degreewise filtered colimits of free modules for each are flat modules. This means that is flat resolution of . By the very definition or else by the basic properties of flat modules, this means that it is a -acyclic resolution. By the discussion there it follows that
Now the tensor product of modules is a left adjoint functor (the right adjoint being the internal hom of modules) and so it commutes over the filtered colimit to yield, using again that chain homology commutes with filtered colimits,
Relation to torsion groups
An abelian group is called torsion if its elements are “nilpotent”, hence if all its elements have finite order.
For Ab and , write
for the -torsion subgroup consisting of all those elements whose -fold sum with themselves gives 0.
For with , write for the cyclic group of order , as usual.
For , , and Ab Mod any abelian group, we have an isomorphism
of the -group with the -torsion subgroup of .
For we have
For the first statement, the short exact sequence
constitutes a projective resolution (even a free resolution) of . Accordingly we have
Here in the last step we use that acts as
For the second statement, is already free hence is already a projective resolution and hence .
By a fundamental fact about finite abelian groups (see this theorem), is a direct sum of cyclic group . By prop. 1 respects this direct sum, so that
By prop. 3 every direct summand on the right is a torsion group and hence so is the whole direct sum.
More generally we have:
Let and be abelian groups. Write for the left derived functor of tensoring over . Then
is a torsion group. Specifically it is a filtered colimit of torsion subgroups of .
is the torsion subgroup of .
is a torsion-free group precisely if , equivalently if .
For instance (Weibel, prop. 3.1.2, prop. 3.1.3, cor. 3.1.5).
The group may be expressed as a filtered colimit
of finitely generated subgroups (this is discussed at Mod - Limits and colimits). Each of these is a direct sum of cyclic groups.
By prop. 2 preserves these colimits. By prop. 3 every cyclic group is sent to a torsion group (of either or Tor_1(A,B)$ is a filtered colimit of direct sums of torsion groups. This is itself a torsion group.
See at flat module - Examples for more.
Symmetry in the two arguments
For and there is a natural isomorphism
We first give a proof for a principal ideal domain such as .
Let be a principal ideal domain such as (in the latter case Mod Ab). Then by the discussion at projective resolution – length-1 resolutions there is always a short exact sequence
exhibiting a projective resolution of any module . It follows that .
Let then be such a short resolution for . Then by the long exact sequence of a derived functor this induces an exact sequence of the form
Since by construction and are already projective modules themselves this collapses to an exact sequence
To the last three terms we apply the natural symmetric braiding isomorphism in to get
This exhibits a morphism as the morphism induced on kernels from an isomorphism between two morphisms. Hence this is itself an isomorphism. (This is just by the universal property of the kernel, but one may also think of it as a simple application of the the four lemma/five lemma.)
For instance (Weibel, cor. 3.2.13).
Standard textbook accounts include the following:
Lecture notes include
section 3 of
and specifically for symmetric model categories of spectra
Original articles include
- Patrick Keef, On the Tor functor and some classes of abelian groups, Pacific J. Math. Volume 132, Number 1 (1988), 63-84. (Euclid)