Higher unramified cohomology extension conjecture

Let kk be the base field, let X=(X,D)\mathcal{X}=(X,D) be an object of \categorymSm\category{mSm}, and let q0q\geq 0. Write X\mathcal{X}^{\circ} for the complement of the boundary, MH\etq+1\underline{\mathrm{M}}\mathrm{H}^{q+1}_{\et} for the corresponding unramified étale cohomology sheaf, and ωexcHurq+1(X)\omega^{\mathrm{exc}}\mathrm{H}^{q+1}_{\mathrm{ur}}(\mathcal{X}) for the exceptional pullback subgroup of Hurq+1(X)\mathrm{H}^{q+1}_{\mathrm{ur}}(\mathcal{X}^{\circ}). The sheaf MHurq+1\underline{\mathrm{M}}\mathrm{H}^{q+1}_{\mathrm{ur}} is the resulting unramified cohomology sheaf on \categorymSm\category{mSm}. Higher unramified cohomology extension conjecture. For every q0q\geq 0 and X=(X,D)\categorymSm\mathcal{X}=(X,D)\in\category{mSm}, one has

MH\etq+1(X)=ωexcHurq+1(X)\underline{\mathrm{M}}\mathrm{H}^{q+1}_{\et}(\mathcal{X})=\omega^{\mathrm{exc}}\mathrm{H}^{q+1}_{\mathrm{ur}}(\mathcal{X})

as subgroups of Hurq+1(X)\mathrm{H}^{q+1}_{\mathrm{ur}}(\mathcal{X}^{\circ}). Moreover, MHurq+1\underline{\mathrm{M}}\mathrm{H}^{q+1}_{\mathrm{ur}} is (CIBI)(\mathrm{CI}\cup\mathrm{BI})-local and defines an object mHurq+1\mathrm{mH}^{q+1}_{\mathrm{ur}} of \categorymDA\eff(k)\category{mDA}^{\eff}(k), with an equivalence

ΩSt1(mHurq+1)mHurq.\Omega_{S^1_t}(\mathrm{mH}^{q+1}_{\mathrm{ur}})\simeq\mathrm{mH}^{q}_{\mathrm{ur}}.

The statement is proposed as an extension of the preceding theorem from degree one to all higher unramified cohomological degrees; its resolution is not supplied in the given text.

Sources & referencesView supporting material

Primary source

Junnosuke Koizumi, Hiroyasu Miyazaki and Shuji Saito, “Motivic homotopy theory with ramification filtrations”, arXiv:2504.02223 (2025).

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