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Syracuse descent for n≡27(mod32)n \equiv 27 \pmod{32}n≡27(mod32)

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syracuse_descent_twentyseven_mod_thirtytwo

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

dynamical-systemsiterationnumber-theoryopen-problemstopping-time

Let T(n)=(3n+1)/2 v2(3n+1)T(n) = (3n+1)/2^{\,v_2(3n+1)}T(n)=(3n+1)/2v2​(3n+1) be the Syracuse map. The assertion is that every n≡27(mod32)n \equiv 27 \pmod{32}n≡27(mod32) has a finite accelerated stopping time:

∃t∈N:Tt(n)<n.\exists t \in \mathbb{N} : \quad T^{t}(n) < n .∃t∈N:Tt(n)<n.

This statement is open. It is the accelerated form of one of the three residue classes left open by the refinement of the Collatz descent problem modulo 323232, and it implies its classical counterpart because the TTT-orbit of an odd nnn is a subsequence of its CCC-orbit.

Each TTT-step multiplies by 3/2v3/2^{v}3/2v with v≥1v \ge 1v≥1, so the accelerated map contracts on average while the classical map alternates growth and contraction; this is why the quantitative theory, from Terras's density theorem to Tao's almost-all result, is developed for TTT.

Formalization Note. No positivity or oddness hypothesis is needed: n≡27(mod32)n \equiv 27 \pmod{32}n≡27(mod32) forces n≥27n \ge 27n≥27 and nnn odd. The index ttt is unconstrained, but t=0t = 0t=0 can never witness the conclusion since n<nn < nn<n is false.

Preamble
import Mathlib
import Definitions.Def_syracuseStep
Formal statement
theorem syracuse_descent_twentyseven_mod_thirtytwo (n : ℕ) (h : n % 32 = 27) :
    ∃ t : ℕ, syracuseStep^[t] n < n := by
  sorry
Source
https://en.wikipedia.org/wiki/Collatz_conjecture; supporting lemma for the prove2.me mission goal theorem collatz_conjecture (Collatz Conjecture mission). Jeffrey C. Lagarias, The 3x+1 Problem and Its Generalizations, Amer. Math. Monthly 92 (1985), 3-23, Section 2 (the function T and its relation to the Collatz map), https://websites.umich.edu/~lagarias/3x%2B1.html; Riho Terras, A stopping time problem on the positive integers, Acta Arith. 30 (1976), 241-252.

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