Gross's conjecture on the Brumer–Stark reciprocity law

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Let FF, HH, SS, TT, and p\mathfrak{p} be as above, and let L/FL/F be a finite abelian CM extension containing HH and unramified outside Sp=S∪{p}S_{\mathfrak{p}}=S\cup\{\mathfrak{p}\}. Write g=Gal⁡(L/F)\mathfrak{g}=\operatorname{Gal}(L/F), Γ=Gal⁡(L/H)\Gamma=\operatorname{Gal}(L/H), G=Gal⁡(H/F)G=\operatorname{Gal}(H/F), and let II be the relative augmentation ideal, the kernel of Z[g]↠Z[G]\mathbf{Z}[\mathfrak{g}]\twoheadrightarrow\mathbf{Z}[G]. For the global reciprocity map rec⁡P:HP∗→Γ\operatorname{rec}_{\mathfrak{P}}:H_{\mathfrak{P}}^*\to\Gamma, define

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

where σ~\widetilde{\sigma} is any lift of σ\sigma to g\mathfrak{g}. Gross's conjecture. One has

rec⁡G(up)≡ΘSp,TL/F(modI2)\operatorname{rec}_G(u_{\mathfrak{p}})\equiv\Theta_{S_{\mathfrak{p}},T}^{L/F}\pmod {I^2}

in I/I2I/I^2. The source proves this congruence in (I/I2)⊗Zp(I/I^2)\otimes\mathbf{Z}_p when the rational prime below p\mathfrak{p} is odd; the integral statement is therefore the remaining conjectural assertion.

References

Primary source

Samit Dasgupta and Mahesh Kakde, “Brumer-Stark Units and Explicit Class Field Theory”, arXiv:2103.02516 (2023).

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