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Counterexample (§1, p. 2) — Heisenberg group

Proved
LocalConjugacy.counterexample_heisenberg

by burkh4rt · Sep 30, 2026 · Mathlib 0df444a (Lean v4.33.1)

group-theorylocal-conjugacynonabelian-cohomologyprofinite-groups

We stress that the hypothesis N∩H⊴NN\cap H\trianglelefteq NN∩H⊴N in Cor. 1.3 is necessary. Consider the cyclic group J=C6J=C_6J=C6​ acting on the Heisenberg group NNN of order 272727 as in C3≀S3C_3\wr S_3C3​≀S3​. Then JJJ and NNN are nilpotent, G≅N⋊JG\cong N\rtimes JG≅N⋊J is supersolvable of order 162162162, and there exists a subgroup H≅C3×S3H\cong C_3\times S_3H≅C3​×S3​ of order 181818 that contains a conjugate of some Sylow ppp-subgroup of JJJ for each prime ppp but not a conjugate of JJJ.

Preamble
import Definitions.Def_LocalConjugacy_Examples

/-
Second unnumbered counterexample, §1, p. 2. The Heisenberg and wreath-product
identifications are included explicitly, along with every stated local/global property.

This is an open draft target. The deliberate `sorry` is the target proof hole;
all definitions and the structural proofs on which the statement rests compile
without admitted proofs.
-/
universe u v
open LocalConjugacy
Formal statement
theorem LocalConjugacy.counterexample_heisenberg :
    ∃ (G : ProfiniteGrp.{0}) (N J H : Subgroup G),
      -- All concrete group identifications in the source are part of the target.
      Finite G ∧ Nonempty (G ≃* WreathC3S3) ∧
      Nonempty (N ≃* Heisenberg3) ∧
      Nonempty (J ≃* Multiplicative (ZMod 6)) ∧
      Nonempty (H ≃* C3 × S3) ∧
      Nat.card G = 162 ∧ Nat.card N = 27 ∧ Nat.card J = 6 ∧ Nat.card H = 18 ∧
      -- N and J form the specified internal semidirect product.
      Splits N J ∧ Group.IsNilpotent N ∧ Group.IsNilpotent J ∧ Supersolvable G ∧
      IsClosed (N : Set G) ∧ IsClosed (J : Set G) ∧ IsClosed (H : Set G) ∧
      -- The local inclusion test succeeds, but global inclusion fails.
      LocallyContains H J ∧ (¬ ∃ g : G, conjugate g J ≤ H) ∧
      ¬ IntersectionNormal N H := by sorry
Source
Michael C. Burkhart, Local conjugacy in prosolvable groups, arXiv:2609.37678v1 (29 September 2026), https://arxiv.org/pdf/2609.37678v1, p. 2, Counterexample (§1, p. 2) — Heisenberg group; standing conventions in §1.2, pp. 2–3.
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What the Lean code literally says, in plain math · GPT-6 family (exact model variant not exposed)

There exist a profinite group GGG whose underlying type lies in universe 000 and subgroups N,J,H≤GN,J,H\le GN,J,H≤G with all the following properties. The group GGG is finite and, as an abstract group, is isomorphic to (C3{0,1,2})⋊S3(C_3^{\{0,1,2\}})\rtimes S_3(C3{0,1,2}​)⋊S3​, where C3C_3C3​ is the additive group of Z/3Z\mathbb Z/3\mathbb ZZ/3Z written multiplicatively, the function group has pointwise multiplication, S3S_3S3​ permutes {0,1,2}\{0,1,2\}{0,1,2}, and the action on functions is (s⋅b)(i)=b(s−1(i))(s\cdot b)(i)=b(s^{-1}(i))(s⋅b)(i)=b(s−1(i)); the semidirect multiplication is (b,s)(b′,s′)=(b(s⋅b′),ss′)(b,s)(b',s')=(b(s\cdot b'),ss')(b,s)(b′,s′)=(b(s⋅b′),ss′). The subgroup NNN is isomorphic to the group of triples (a,b,c)∈(Z/3Z)3(a,b,c)\in(\mathbb Z/3\mathbb Z)^3(a,b,c)∈(Z/3Z)3 with multiplication (a,b,c)(a′,b′,c′)=(a+a′,b+b′,c+c′+ab′)(a,b,c)(a',b',c')=(a+a',b+b',c+c'+ab')(a,b,c)(a′,b′,c′)=(a+a′,b+b′,c+c′+ab′), identity (0,0,0)(0,0,0)(0,0,0), and inverse (−a,−b,−c+ab)(-a,-b,-c+ab)(−a,−b,−c+ab). The subgroup JJJ is isomorphic to the additive group of Z/6Z\mathbb Z/6\mathbb ZZ/6Z written multiplicatively, and HHH is isomorphic to C3×S3C_3\times S_3C3​×S3​. Their cardinalities are exactly ∣G∣=162|G|=162∣G∣=162, ∣N∣=27|N|=27∣N∣=27, ∣J∣=6|J|=6∣J∣=6, and ∣H∣=18|H|=18∣H∣=18. The subgroup NNN is normal in GGG, every element of GGG has a unique expression njnjnj with n∈N,j∈Jn\in N,j\in Jn∈N,j∈J, both NNN and JJJ are nilpotent, and GGG satisfies the following series condition. For a group RRR, the series condition used here means that there exist m∈Nm\in\mathbb Nm∈N and a nondecreasing sequence (Si)i∈N(S_i)_{i\in\mathbb N}(Si​)i∈N​ of normal subgroups of RRR such that S0={1}S_0=\{1\}S0​={1}, Sm=RS_m=RSm​=R, and, for each i<mi<mi<m, some ri∈Rr_i\in Rri​∈R satisfies Si+1=⟨Si,ri⟩S_{i+1}=\langle S_i,r_i\rangleSi+1​=⟨Si​,ri​⟩. Repeated terms are allowed, and m=0m=0m=0 is allowed precisely when RRR is trivial. Each of N,J,HN,J,HN,J,H is closed in GGG. For every natural prime ppp there exist a Sylow pro-ppp subgroup PpP_pPp​ of JJJ and an element gp∈Gg_p\in Ggp​∈G with gpPpgp−1≤Hg_pP_pg_p^{-1}\le Hgp​Pp​gp−1​≤H, yet there is no g∈Gg\in Gg∈G with gJg−1≤HgJg^{-1}\le HgJg−1≤H, and N∩HN\cap HN∩H is not normal as a subgroup of NNN. A Sylow pro-ppp subgroup PPP of a subgroup A≤GA\le GA≤G means a subgroup P≤AP\le AP≤A that is closed in GGG, for which every quotient P/UP/UP/U by an open normal subgroup of PPP has the property that every element is killed by some power pkp^kpk with k∈Nk\in\mathbb Nk∈N, and that is maximal under inclusion among the closed subgroups of GGG contained in AAA with this quotient property. The local witnesses may depend on the prime, and the prime quantifier includes primes other than 222 and 333, with trivial Sylow subgroups of JJJ. All isomorphisms asserted are group isomorphisms, with no specified compatibility among them. The explicit finite cardinalities exclude trivial groups in this existential assertion, and none of these cardinalities uses the infinite-cardinality value 000.

Human review
  • Endorsed by Shuze Chen · Sep 30, 2026

    Confirmed by the moderator at approval.

  • Endorsed by burkh4rt · Sep 30, 2026

    Confirmed by the mission captain (proposal self-audit).

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