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Elements of ⋂nι(E)Upn\bigcap_n \iota(E)U^{p^n}⋂n​ι(E)Upn: closure of ι(E)\iota(E)ι(E) times prime-to-ppp torsion

Proved
Leopoldt.mem_unitClosure_iff

by xuanji · Sep 27, 2026 · Mathlib 0df444a (Lean v4.33.1)

leopoldtnumber-theory

Let ppp be a prime, KKK a number field, UUU the semilocal units at ppp, ι:E→U\iota:E\to Uι:E→U the diagonal embedding of the global units, and Eˉ=⋂nι(E) Upn\bar E=\bigcap_{n}\iota(E)\,U^{p^n}Eˉ=⋂n​ι(E)Upn (the platform's unitClosure). Then u∈Eˉu\in \bar Eu∈Eˉ if and only if u=c tu=c\,tu=ct where ccc lies in the topological closure ι(E)‾\overline{\iota(E)}ι(E)​ of ι(E)\iota(E)ι(E) in UUU and t∈Ut\in Ut∈U is a torsion element of order prime to ppp. In other words

⋂n≥1ι(E) Upn  =  ι(E)‾⋅μ′(U),\bigcap_{n\ge1}\iota(E)\,U^{p^n}\;=\;\overline{\iota(E)}\cdot\mu'(U),n≥1⋂​ι(E)Upn=ι(E)​⋅μ′(U),

where μ′(U)\mu'(U)μ′(U) denotes the prime-to-ppp torsion of UUU. This corrects the identification Eˉ=ι(E)‾\bar E=\overline{\iota(E)}Eˉ=ι(E)​ in the source: the intersection may be strictly larger (e.g. K=QK=\mathbb QK=Q, p=7p=7p=7 gives μ6\mu_6μ6​ versus {±1}\{\pm1\}{±1}), though only by a finite group, so the Zp\mathbb Z_pZp​-ranks agree.

Preamble
import Definitions.Def_LeopoldtDefect
Formal statement
namespace Leopoldt
theorem mem_unitClosure_iff (p : ℕ) [Fact p.Prime] (K : Type*) [Field K] [NumberField K]
    (u : SemilocalUnits p K) :
    u ∈ unitClosure p K ↔ ∃ c ∈ closure (Set.range (diagonalUnits p K)),
      ∃ t : SemilocalUnits p K, (∃ m : ℕ, m.Coprime p ∧ t ^ m = 1) ∧ c * t = u := by sorry
end Leopoldt
Source
P. Mihăilescu, On CM Z_p-extensions and the Leopoldt conjecture for CM fields, arXiv:1105.4544, §1.1, p.3 (definition of Ē = ⋂_n ι(E)·U^{p^n}); J. Neukirch, Algebraic Number Theory, Ch. II, Prop. 5.7 (structure U ≅ μ_{q-1} × U^{(1)} of local units, U^{(1)} a finitely generated ℤ_p-module).

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