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Integral line-bundle translation changes the infinitesimal character

Example

For G=SL2, start at the singular twist κ=0 with the nonzero D0-module M=OP1(−1). Tensor by the integral line bundle OX(ρ)=OP1(−1). The result is the Dρ-module OP1(−2), since integral line-bundle translation changes the twist from κ to κ+ρ. The Casimir characters change from χ0(C)=−12toχρ(C)=0. The translated parameter ρ is not antidominant, and the translated module still has zero global sections (indeed dim⁡H1=1); the twist equivalence on sheaf modules does not assert that global sections is faithful at the translated parameter.

Facts & Assumptions

Given: The type A1 root datum, ρ(h)=1, the line-bundle convention and parameter-shifting functor, and the Casimir C=ef+fe+12h2.

[F1]

Tensoring by an integral line bundle OX(η) identifies Dκ-modules with Dκ+η-modules (PBW and integral line-bundle twisting for flag TDOs).

[F2]

For sl2, the Harish–Chandra projection sends C to h+12h2, and χκ(C)=pr⁡(C)(κ−ρ) (Flag-variety twisted differential operators and their modules, The Harish-Chandra projection computes the highest-weight scalar).

[F3]

The Cech complex of the two standard affine charts computes Hq(P1,O(d)) (Cech cohomology computes quasi-coherent cohomology on a separated scheme).

[F5]

Quasi-coherence is the affine-local associated-module condition (Quasi-coherent module on a scheme).

Verification

1.1F1given

In type A1, ρ is the fundamental weight and OX(ρ)=O(−1). Applying [F1] with κ=0 and η=ρ sends M=O(−1) to M⊗OX(ρ)=O(−2) at the new twist κ+η=ρ.

2.1F2step 1.1algebra

The Harish–Chandra polynomial from [F2] is p(t)=t+12t2 in the h-coordinate. At κ=0, the evaluation point is κ−ρ=−1, so p(−1)=−12. At κ′=ρ, the evaluation point is 0, so p(0)=0. Thus the central characters differ; also ⟨ρ,α∨⟩=1, so the target parameter is non-antidominant.

3.1F3F5step 1.1constructalgebra∎

Put z=y/x on U0 and w=x/y on U1. For d=−2, the Cech complex has C0=C[z]e0⊕C[w]e1 and C1=C[z,z−1]e0, with transition e1=z−2e0. The image contains powers zn for n≥0 and z−2−n for n≥0, leaving the single quotient basis z−1. Hence H0(O(−2))=0 and dim⁡H1(O(−2))=1 by [F3]. The nonzero identity of M=O(−2) is killed by global sections, so that functor is not faithful. For the standard chart inclusions j0,j1,j01, put E=j0∗(M∣U0)⊕j1∗(M∣U1) and F=j01∗(M∣U0∩U1). Restriction and difference give a Dρ-linear exact sequence 0→M→E→F→0: differential operators act on direct images by restriction, the kernel consists of sections that glue, and on Ui the other direct-image summand maps isomorphically onto F∣Ui. These sheaves are quasi-coherent by [F5]: on U0=Spec⁡C[z] their distinguished-open sections are the localizations of C[z]e0, (C[z]⊕C[z,z−1])e0 and C[z,z−1]e0, respectively; the same calculation holds with w,e1 on U1. The map Γ(E)→Γ(F) is exactly the computed Cech differential with nonzero cokernel. Thus global sections is not exact on quasi-coherent Dρ-modules. This computation makes no claim that line-bundle translation preserves global-section faithfulness.

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