Deformability in towers for cyclic extensions

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Let n≥m∈Z≥0n\geq m\in\mathbb{Z}_{\geq 0}, and consider a tower of Galois extensions

k[[x]]→ϕmk[[ym]]→ϕn/mk[[yn]],ϕn=ϕn/m∘ϕm.k[[x]]\xrightarrow{\phi_m}k[[y_m]]\xrightarrow{\phi_{n/m}}k[[y_n]],\qquad \phi_n=\phi_{n/m}\circ\phi_m.

Write RϕmR_{\phi_m}, Rϕn/mR_{\phi_{n/m}}, and RϕnR_{\phi_n} for the corresponding deformation rings, and let G:=Gal⁡(k[[yn]]/k[[x]])G:=\operatorname{Gal}(k[[y_n]]/k[[x]]).

Deformability in towers conjecture. If GG is cyclic, then the natural morphism

ind⁡:Spec⁡Rϕn⟶Spec⁡Rϕm\operatorname{ind}:\operatorname{Spec}R_{\phi_n}\longrightarrow\operatorname{Spec}R_{\phi_m}

is surjective. Equivalently, for every complete discrete valuation ring RR with residue field kk, after a finite extension R′R' of RR one can fill in the commutative deformation diagram in the source, and in this case ϕn\phi_n is called deformable in towers.

This conjecture asks whether every deformation of the lower-level cyclic cover can be lifted to the upper level after a finite extension of the base DVR. It is a proposed generalization of known descriptions of deformation rings in certain wildly ramified cases.

References

Primary source

Huy Dang, “Deforming cyclic covers in towers”, arXiv:2010.13614 (2023).

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