ConceptComplete

Derived Functors - Key Properties

Understanding the properties of derived functors is essential for practical computations and theoretical applications.

Theorem4.5Long Exact Sequence of Derived Functors

Let FF be a left exact functor and 0→A→B→C→00 \to A \to B \to C \to 0 a short exact sequence. Then there exists a long exact sequence:

0→F(A)→F(B)→F(C)→δR1F(A)→R1F(B)→R1F(C)→δR2F(A)→⋯0 \to F(A) \to F(B) \to F(C) \xrightarrow{\delta} R^1F(A) \to R^1F(B) \to R^1F(C) \xrightarrow{\delta} R^2F(A) \to \cdots

The connecting homomorphisms δ\delta are natural in the short exact sequence.

Proof

Use the Horseshoe Lemma to lift the short exact sequence to a short exact sequence of injective resolutions: 0→IA∙→IB∙→IC∙→00 \to I^\bullet_A \to I^\bullet_B \to I^\bullet_C \to 0

Applying FF gives a short exact sequence of cochain complexes (since each InI^n is injective). The Snake Lemma provides connecting homomorphisms in the cohomology sequence.

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Definition4.6Effaceable Functor

A functor T:A→BT: \mathcal{A} \to \mathcal{B} is effaceable if for every object MM, there exists a monomorphism M↪EM \hookrightarrow E such that T(E)=0T(E) = 0.

For cohomological functors, this property ensures uniqueness via dimension shifting.

Theorem4.7Dimension Shifting

For any short exact sequence 0→A→E→B→00 \to A \to E \to B \to 0 where EE is FF-acyclic (i.e., RiF(E)=0R^iF(E) = 0 for i>0i > 0):

Rn+1F(B)≅RnF(A)R^{n+1}F(B) \cong R^nF(A)

for all n≥1n \geq 1. This allows computation of higher derived functors from lower ones.

ExampleComputing Ext via Dimension Shifting

To compute ExtZn(Z/2Z,Z/3Z)\text{Ext}^n_{\mathbb{Z}}(\mathbb{Z}/2\mathbb{Z}, \mathbb{Z}/3\mathbb{Z}), use the short exact sequence: 0→Z→×2Z→Z/2Z→00 \to \mathbb{Z} \xrightarrow{\times 2} \mathbb{Z} \to \mathbb{Z}/2\mathbb{Z} \to 0

Since Z\mathbb{Z} is projective (free), the long exact sequence gives: Extn(Z/2Z,Z/3Z)≅Extn−1(Z,Z/3Z)=0\text{Ext}^n(\mathbb{Z}/2\mathbb{Z}, \mathbb{Z}/3\mathbb{Z}) \cong \text{Ext}^{n-1}(\mathbb{Z}, \mathbb{Z}/3\mathbb{Z}) = 0

for n≥2n \geq 2. Thus Extn=0\text{Ext}^n = 0 for n≥1n \geq 1.

Theorem4.8Grothendieck Spectral Sequence

Let F:A→BF: \mathcal{A} \to \mathcal{B} and G:B→CG: \mathcal{B} \to \mathcal{C} be left exact functors. If FF sends injectives to GG-acyclic objects, there is a spectral sequence:

E2p,q=RpG(RqF(M))⇒Rp+q(G∘F)(M)E_2^{p,q} = R^pG(R^qF(M)) \Rightarrow R^{p+q}(G \circ F)(M)

Remark

The Grothendieck spectral sequence is a fundamental tool for computing derived functors of compositions. It relates the derived functors of G∘FG \circ F to those of GG and FF separately.

Definition4.9Acyclic Resolution

An FF-acyclic resolution of MM is an exact sequence: 0→M→A0→A1→⋯0 \to M \to A^0 \to A^1 \to \cdots

where each AiA^i is FF-acyclic. Derived functors can be computed using any FF-acyclic resolution, not just injective resolutions.