Alphabeta Math
DefinitionDefinition: Literature-sourcedProof: Not applicablePipeline-generatedjudge pass (gpt-6-sol)audited 2026-09-27
How statement and proof provenance work

The first chip identifies the source of the statement or construction; the second identifies the source of its local proof or verification.

  • Literature-sourced: the exact statement appears in a cited source; only wording and notation differ.
  • AI-adapted: a semantically identical restatement of literature-sourced material, modulo indexing, notation, and boundary cases adopted by the library.
  • AI-generated: a genuinely novel statement formulated by AI, with no source for the claim itself.

These labels describe origin, not correctness: citations and verification chips remain separate evidence.

Cohomological dimension relative to a sheaf class

Definition

Assume the Axiom of Choice (The Axiom of Choice). Let X be a topological space, let C be a class of sheaves of abelian groups on X (that is, a subclass of the objects of Ab(X), for example the class of all abelian sheaves, or a class of sheaves of modules over a fixed sheaf of rings), and let Hq(X,−) be sheaf cohomology relative to the supplied functorial injective resolution datum (Sheaf cohomology as right derived global sections). The cohomological dimension of X relative to C is cd⁡C(X):=inf⁡{d∈Z: Hq(X,F)=0 for every F∈C and every q>d}, the infimum of the empty set of integers being +∞. Thus:

  1. cd⁡C(X)≤d if and only if Hq(X,F)=0 for every F∈C and every q>d; the value +∞ means that for every integer d the class C contains a sheaf with Hq(X,F)≠0 for some q>d;
  2. if H0(X,F)≠0 for some F∈C, then cd⁡C(X)∈Z≥0∪{+∞}. If this value is finite, the infimum is attained and cd⁡C(X)=min⁡{d≥0: Hq(X,F)=0 for all F∈C, q>d}, the least nonnegative integer with the vanishing property. If there is no finite uniform bound, then cd⁡C(X)=+∞ and the infimum is not attained;
  3. cd⁡C(X)=−∞ exactly when Hq(X,F)=0 for every F∈C and every q≥0; in particular cd⁡C(∅)=−∞ for every class C on the empty space, since F(∅) is the one-element group and Γ(∅,−) is exact (A set-valued sheaf has a unique section over the empty open set, An exact functor has vanishing positive derived functors);
  4. cd⁡C(X)≤cd⁡C′(X) whenever C⊆C′; when C is the class of all abelian sheaves on X one writes cd⁡(X) for cd⁡C(X) and calls it the cohomological dimension of X;
  5. the definition is independent of the supplied resolution datum up to the canonical isomorphisms of Sheaf cohomology as right derived global sections, so cd⁡C(X) depends on X and C only.

In degree zero the vanishing hypothesis in item 2 is satisfied by any class containing a sheaf with a nonzero global section, by Degree-zero sheaf cohomology is global sections.

Depends on

Used by

Dependency tree · two levels

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Sources