Gross's integral Gross–Stark conjecture

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Let L/H/FL/H/F be a tower in which FF is totally real and HH and LL are finite abelian CM extensions of FF, with LL containing HH. Write G=Gal⁡(L/F)\mathcal{G}=\operatorname{Gal}(L/F), G=Gal⁡(H/F)G=\operatorname{Gal}(H/F), and let S=R∪{p}S=R\cup\{\mathfrak{p}\}, where p\mathfrak{p} splits completely in H/FH/F. Define

ΘS,TL/F=∑σ∈GζS,T(L/F,σ,0)⊗σ−1∈C[G],\Theta_{S,T}^{L/F}=\sum_{\sigma\in G}\zeta_{S,T}(L/F,\sigma,0)\otimes\sigma^{-1}\in\mathbb{C}[\mathcal{G}],

let II be the relative augmentation ideal, and let rec⁡p:Fp∗→G\operatorname{rec}_{\mathfrak{p}}:F_{\mathfrak{p}}^*\to\mathcal{G} be the local reciprocity map. For up=∑σ∈Gup(σ)⊗σ−1u_{\mathfrak{p}}=\sum_{\sigma\in G}u_{\mathfrak{p}}(\sigma)\otimes\sigma^{-1}, define

rec⁡G(up)=∑σ∈G(rec⁡p(up(σ))−1)σ~−1∈I/I2,\operatorname{rec}_G(u_{\mathfrak{p}})=\sum_{\sigma\in G}\bigl(\operatorname{rec}_{\mathfrak{p}}(u_{\mathfrak{p}}(\sigma))-1\bigr)\widetilde{\sigma}^{-1}\in I/I^2,

where σ~∈G\widetilde{\sigma}\in\mathcal{G} is any lift of σ\sigma. Gross's integral Gross–Stark conjecture. One has

rec⁡G(up)≡ΘS,TL/F\operatorname{rec}_G(u_{\mathfrak{p}})\equiv\Theta_{S,T}^{L/F}

in I/I2I/I^2. This conjecture is an integral refinement of the Gross–Stark conjecture, relating reciprocity images of Brumer–Stark units to Stickelberger elements. The source gives no resolution status for this conjecture.

References

Primary source

Matthew H. L. Honnor, “On the Root of Unity Ambiguity in a Formula for the Brumer–Stark Units”, arXiv:2302.12332 (2025).

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