Ihara lemma, indefinite case

From papers

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 pi=1n0(1(p)i)\ell\nmid p\prod_{i=1}^{n_0}(1-(-p)^i), if Hi(S0FQ,Zλ)n0=0\mathrm{H}^i(\mathcal{S}_0^\circ\otimes_F\overline{\mathbb{Q}},\mathbb{Z}_\lambda)_{\mathfrak{n}_0}=0 for in1i\neq n-1 and Hn1(S0FQ,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

H2r1(S0FQ,Ωλ)n0H2r1(S0FQ,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.

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Sources & referencesView supporting material

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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