Derived Functors - Key Properties
Understanding the properties of derived functors is essential for practical computations and theoretical applications.
Let be a left exact functor and a short exact sequence. Then there exists a long exact sequence:
The connecting homomorphisms are natural in the short exact sequence.
Use the Horseshoe Lemma to lift the short exact sequence to a short exact sequence of injective resolutions:
Applying gives a short exact sequence of cochain complexes (since each is injective). The Snake Lemma provides connecting homomorphisms in the cohomology sequence.
A functor is effaceable if for every object , there exists a monomorphism such that .
For cohomological functors, this property ensures uniqueness via dimension shifting.
For any short exact sequence where is -acyclic (i.e., for ):
for all . This allows computation of higher derived functors from lower ones.
To compute , use the short exact sequence:
Since is projective (free), the long exact sequence gives:
for . Thus for .
Let and be left exact functors. If sends injectives to -acyclic objects, there is a spectral sequence:
The Grothendieck spectral sequence is a fundamental tool for computing derived functors of compositions. It relates the derived functors of to those of and separately.
An -acyclic resolution of is an exact sequence:
where each is -acyclic. Derived functors can be computed using any -acyclic resolution, not just injective resolutions.