Module

DerivedLocalCohomologyTopFinite.lean

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

Finite top cohomology and a regular scalar

A regular scalar in the maximal ideal lowers the support dimension of finite coefficients. The top cohomology of its quotient vanishes, so the scalar acts surjectively on top cohomology. If that cohomology were finite, Nakayama's lemma would force it to be zero.

Imports12
Imported by2

Declarations

EtaleFundamentalGroups.derivedLocalCohomology_maximalIdeal_isZero_succ_of_finite_of_regular

Finite top local cohomology vanishes when the finite coefficient module admits a regular scalar in the maximal ideal.

Source context 5 commands

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

namespace EtaleFundamentalGroups
universe u
open CategoryTheory Limits IsLocalRing
open scoped Pointwise
variable {R : Type u} [CommRing R] [IsLocalRing R] [IsNoetherianRing R]
theorem derivedLocalCohomology_maximalIdeal_isZero_succ_of_finite_of_regular
    (M : ModuleCat.{u} R) [Module.Finite R M] (d : ℕ)
    (hdim : Module.supportDim R M = ((d + 1 : ℕ) : WithBot ℕ∞))
    (x : R) (hx : x ∈ maximalIdeal R) (hreg : IsSMulRegular M x)
    [Module.Finite R (derivedLocalCohomologyModule (maximalIdeal R) M (d + 1))] :
    IsZero (derivedLocalCohomology (maximalIdeal R) M (d + 1))