Gross's integral Gross–Stark conjecture

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=σGζS,T(L/F,σ,0)σ1C[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 recp:FpG\operatorname{rec}_{\mathfrak{p}}:F_{\mathfrak{p}}^*\to\mathcal{G} be the local reciprocity map. For up=σGup(σ)σ1u_{\mathfrak{p}}=\sum_{\sigma\in G}u_{\mathfrak{p}}(\sigma)\otimes\sigma^{-1}, define

recG(up)=σG(recp(up(σ))1)σ~1I/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

recG(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.

Sources & referencesView supporting material

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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