Module

LocalCohomologyRegularSequence.lean

Nyll.EtaleFundamentalGroups.Purity.LocalCohomologyRegularSequence
1 declaration indexed from sourceView source ↗

Local-cohomology vanishing from regular sequences

A regular sequence in an ideal annihilates the low positive-degree Ext groups by the Rees theorem. Combining that result with the comparison between the two Ext implementations gives the corresponding local-cohomology vanishing.

Declarations

EtaleFundamentalGroups.localCohomology_isZero_of_isRegular

If an ideal contains a regular sequence of length greater than i, then local cohomology in degree i vanishes.

Source context 5 commands

Surrounding namespaces, section variables, and local commands at this declaration.

noncomputable section
open CategoryTheory Limits Opposite
open RingTheory.Sequence
namespace EtaleFundamentalGroups
universe u
theorem localCohomology_isZero_of_isRegular
    {R : Type u} [CommRing R] [IsNoetherianRing R]
    (I : Ideal R) (M : ModuleCat.{u} R) [Module.Finite R M]
    (smul_lt : I • (⊤ : Submodule R M) < ⊤)
    (rs : List R) (mem : ∀ r ∈ rs, r ∈ I) (reg : IsRegular M rs)
    (i : ℕ) (hi : i < rs.length) :
    IsZero ((localCohomology I i).obj M)