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The ppp-adic closure of the global units lies in ⋂nι(E)Upn\bigcap_n \iota(E)U^{p^n}⋂n​ι(E)Upn

Proved
Leopoldt.closure_range_diagonalUnits_subset_unitClosure

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

leopoldtnumber-theoryp-adic

Let KKK be a number field, ppp a prime, U=∏p∣pOp×U=\prod_{\mathfrak p\mid p}\mathcal O_{\mathfrak p}^\timesU=∏p∣p​Op×​ the semilocal units at ppp and ι:E=OK×→U\iota:E=\mathcal O_K^\times\to Uι:E=OK×​→U the diagonal embedding. Then the topological closure of ι(E)\iota(E)ι(E) in UUU is contained in

Eˉ=⋂n≥0ι(E)⋅Upn+1.\bar E=\bigcap_{n\ge 0}\iota(E)\cdot U^{p^{n+1}}.Eˉ=n≥0⋂​ι(E)⋅Upn+1.

This is the inclusion ι(E)‾⊆⋂nι(E)Upn\overline{\iota(E)}\subseteq\bigcap_n\iota(E)U^{p^n}ι(E)​⊆⋂n​ι(E)Upn of the identity Eˉ=ι(E)‾=⋂nι(E)Upn\bar E=\overline{\iota(E)}=\bigcap_n\iota(E)U^{p^n}Eˉ=ι(E)​=⋂n​ι(E)Upn in §1.1 of the source; it follows because the right-hand side is a closed subgroup containing ι(E)\iota(E)ι(E). (The reverse inclusion can fail by a finite group of roots of unity of order prime to ppp — e.g. for K=QK=\mathbb QK=Q, p=7p=7p=7 the intersection is μ6\mu_6μ6​ while ι(E)‾={±1}\overline{\iota(E)}=\{\pm1\}ι(E)​={±1} — which does not affect Zp\mathbb Z_pZp​-ranks.)

Preamble
import Definitions.Def_LeopoldtDefect
Formal statement
namespace Leopoldt

theorem closure_range_diagonalUnits_subset_unitClosure (p : ℕ) [Fact p.Prime] (K : Type*)
    [Field K] [NumberField K] :
    closure (Set.range (diagonalUnits p K)) ⊆ (unitClosure p K : Set (SemilocalUnits p K)) := by sorry

end Leopoldt
Source
P. Mihăilescu, On CM ℤ_p-extensions and the Leopoldt conjecture for CM fields, arXiv:1105.4544, §1.1 (definition Ē = closure(ι(E)) = ⋂_{n>0} ι(E)·U^{p^n})

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