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Conditional convergence after a finite excursion chain

Proved
CollatzWork.excursionChain_converges_of_smaller

by Sodelin · Sep 8, 2026 · Mathlib 0df444a (Lean v4.33.1)

collatz-work-import

Let T:N→NT:\mathbb N\to\mathbb NT:N→N be the shortcut Collatz map: T(n)=n/2T(n)=n/2T(n)=n/2 for even nnn and T(n)=(3n+1)/2T(n)=(3n+1)/2T(n)=(3n+1)/2 for odd nnn. Write TkT^kTk for its kkk-fold iterate, with T0(n)=nT^0(n)=nT0(n)=n. Write C(n)\mathcal C(n)C(n) for the assertion that Tj(n)=1T^j(n)=1Tj(n)=1 for some j∈Nj\in\mathbb Nj∈N. Let SSS be a finite list of segments (ki,Ai,Di)(k_i,A_i,D_i)(ki​,Ai​,Di​) of natural numbers, with Di>0D_i>0Di​>0. Starting from nnn, define successive endpoints by applying TkiT^{k_i}Tki​. Assume each segment satisfies Di(endpoint+3)≤Ai(start+3)D_i(\text{endpoint}+3)\le A_i(\text{start}+3)Di​(endpoint+3)≤Ai​(start+3). Write K=∑ikiK=\sum_i k_iK=∑i​ki​, A=∏iAiA=\prod_i A_iA=∏i​Ai​, and D=∏iDiD=\prod_i D_iD=∏i​Di​, with empty products 1.

Let l,B,E∈Nl,B,E\in\mathbb Nl,B,E∈N and n≥3n\ge3n≥3. Assume ETl(TK(n))<B(TK(n)+3)E T^l(T^K(n))<B(T^K(n)+3)ETl(TK(n))<B(TK(n)+3) and 2AB≤DE2AB\le DE2AB≤DE. Assume also C(m)\mathcal C(m)C(m) for every 0<m<n0<m<n0<m<n. Then

C(n).\mathcal C(n).C(n).

The segment and terminal envelopes and cumulative budget are hypotheses; no all-root coverage is asserted.

Preamble
import Std
import Init.Grind.Ordered.Module
import Definitions.Def_CollatzWork_ConvergenceStatement
import Definitions.Def_CollatzWork_ExcursionBudgetStatement
import Definitions.Def_CollatzWork_InverseWordBoundaryStatement
import Theorems.Thm_CollatzWork_converges_shortcutIter_iff
import Theorems.Thm_CollatzWork_excursionChain_terminal_descent



Formal statement
theorem CollatzWork.excursionChain_converges_of_smaller (segments : List ExcursionSegment)
    (root steps B E : Nat) (hroot : 3 ≤ root)
    (hchain : ExcursionChain root segments)
    (hterminal : E * shortcutIter steps (shortcutIter (excursionSteps segments) root) <
      B * (shortcutIter (excursionSteps segments) root + 3))
    (hbudget : 2 * (excursionNumerator segments * B) ≤
      excursionDenominator segments * E)
    (ih : ∀ m : Nat, 0 < m → m < root → Converges m) : Converges root := by sorry

Source
https://github.com/Sodelin/Collatz-Conjecture-Work/blob/026aa4ad4be6453a005ab950b160a9f2204c5271/lean/CollatzWork/ExcursionBudget.lean#L99-L111

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