Ihara lemma, indefinite case

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Assume the indefinite Shimura-variety setup of the source. Let S0∘\mathcal{S}_0^\circ and S0∙\mathcal{S}_0^\bullet be the relevant Shimura varieties, let Ωλ\Omega_\lambda be the local system, and let n0\mathfrak{n}_0 denote the localization ideal. Ihara lemma, indefinite case. If ℓ∤p∏i=1n0(1−(−p)i)\ell\nmid p\prod_{i=1}^{n_0}(1-(-p)^i), if Hi(S0∘⊗FQ‾,Zλ)n0=0\mathrm{H}^i(\mathcal{S}_0^\circ\otimes_F\overline{\mathbb{Q}},\mathbb{Z}_\lambda)_{\mathfrak{n}_0}=0 for i≠n−1i\neq n-1 and Hn−1(S0∘⊗FQ‾,Zλ)n0\mathrm{H}^{n-1}(\mathcal{S}_0^\circ\otimes_F\overline{\mathbb{Q}},\mathbb{Z}_\lambda)_{\mathfrak{n}_0} is finite free over Zλ\mathbb{Z}_\lambda, if V0,λΠ\mathrm{V}^\Pi_{0,\lambda} is residually absolutely irreducible, and if the Satake parameter of Π0,p\Pi_{0,\mathfrak{p}} modulo λ\lambda contains pp at most once, then

H2r−1(S0∘⊗FQ‾,Ωλ)n0⟶H2r−1(S0∙⊗FQ‾,Zλ)n0\mathrm{H}^{2r-1}(\mathcal{S}_0^\circ\otimes_F\overline{\mathbb{Q}},\Omega_\lambda)_{\mathfrak{n}_0}\longrightarrow \mathrm{H}^{2r-1}(\mathcal{S}_0^\bullet\otimes_F\overline{\mathbb{Q}},\mathbb{Z}_\lambda)_{\mathfrak{n}_0}

is surjective. This is stated as a generalization of the Ihara lemma for modular curves, or unitary Shimura curves, in the source.

References

Primary source

Yifeng Liu, Yichao Tian, Liang Xiao, Wei Zhang and Xinwen Zhu, “Survey on bounding Selmer groups for Rankin–Selberg motives”, arXiv:2509.16881 (2025).

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