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Syracuse descent within ten steps on 155 progressions modulo 2162^{16}216

Proved
syracuse_descent_progressions_seven_mod32_mod65536

by Steve1136 · Sep 21, 2026 · Mathlib 0df444a (Lean v4.33.1)

dynamical-systemsiterationnumber-theorystopping-time

Let TTT denote the Syracuse map on the natural numbers,

T(n)=3n+12v2(3n+1),T(n)=\frac{3n+1}{2^{v_2(3n+1)}},T(n)=2v2​(3n+1)3n+1​,

that is, the odd part of 3n+13n+13n+1, where v2v_2v2​ is the 222-adic valuation, and let TtT^tTt denote its ttt-fold iterate, with T0(n)=nT^0(n)=nT0(n)=n. Let S13S_{13}S13​, S15S_{15}S15​ and S16S_{16}S16​ be the three finite sets of residues listed below. For every natural number nnn satisfying

n mod 8192∈S13orn mod 32768∈S15orn mod 65536∈S16,n\bmod 8192\in S_{13}\quad\text{or}\quad n\bmod 32768\in S_{15}\quad\text{or}\quad n\bmod 65536\in S_{16},nmod8192∈S13​ornmod32768∈S15​ornmod65536∈S16​,

there is a natural number ttt with

t≤10andTt(n)<n.t\le 10\qquad\text{and}\qquad T^{t}(n)<n.t≤10andTt(n)<n.

The residue sets are:

  1. S13S_{13}S13​ consists of the following 191919 residues modulo 213=81922^{13}=8192213=8192: 679, 1191, 2663, 3687, 4199, 4455, 5191, 5607, 5959, 6215, 6375, 6631, 6983, 7079, 7399, 7495, 7847, 7911, 8103.
  2. S15S_{15}S15​ consists of the following 373737 residues modulo 215=327682^{15}=32768215=32768: 839, 1095, 2119, 2279, 2727, 2983, 3303, 4007, 6503, 6759, 7783, 9959, 10055, 11079, 11943, 12967, 14439, 16743, 16871, 17735, 17767, 19623, 20199, 21223, 23399, 24647, 24679, 25703, 25831, 26087, 26535, 27111, 27975, 28999, 29863, 30311, 30887.
  3. S16S_{16}S16​ consists of the following 999999 residues modulo 216=655362^{16}=65536216=65536: 359, 1351, 2407, 2791, 2887, 3239, 3815, 4775, 5863, 6247, 7015, 8263, 8551, 9319, 9543, 10151, 10727, 11431, 12007, 12615, 12775, 13671, 13927, 14503, 15207, 16455, 17127, 17223, 17479, 17511, 18343, 18919, 19111, 19367, 19687, 20807, 21735, 22119, 22695, 22887, 23143, 25415, 25671, 26343, 26439, 27303, 27559, 27879, 28327, 31079, 31335, 33255, 34151, 34535, 34631, 36519, 37607, 37735, 40039, 41063, 41447, 42215, 42343, 42471, 43111, 43335, 44359, 45223, 45799, 46247, 46407, 48295, 49255, 50407, 50663, 51271, 51431, 52071, 52551, 53159, 53319, 54375, 54439, 55207, 56935, 57671, 58983, 59463, 59559, 59623, 60231, 61351, 62119, 62279, 63335, 63591, 64167, 64871, 65127.

Each residue determines an infinite arithmetic progression, so the statement covers every member of 19+37+99=15519+37+99=15519+37+99=155 progressions, not a finite range of inputs.

These progressions refine the 454545 residual classes modulo 409640964096 of the open theorem syracuse_descent_residual_mod4096: every one of the 155155155 progressions lies inside one of those classes, and together they cover 325325325 of the 720720720 residue classes modulo 2162^{16}216 that lie over them. Combined with the complementary residual statement, this splits that open theorem into a proved part with a uniform ten-step bound and a smaller open part.

Formalization Note. The Syracuse map is the existing platform definition syracuseStep, and TtT^tTt is the Mathlib iterate syracuseStep^[t]. The three residue sets are written as explicit Finset ℕ literals, and the uniform bound t≤10t\le10t≤10 is part of the conclusion.

Preamble
import Definitions.Def_syracuseStep
import Mathlib.Logic.Function.Iterate
import Mathlib.Data.Finset.Insert
Formal statement
theorem syracuse_descent_progressions_seven_mod32_mod65536 (n : ℕ)
    (h : n % 8192 ∈ ({679, 1191, 2663, 3687, 4199, 4455, 5191, 5607, 5959, 6215,
      6375, 6631, 6983, 7079, 7399, 7495, 7847, 7911, 8103} : Finset ℕ) ∨
      n % 32768 ∈ ({839, 1095, 2119, 2279, 2727, 2983, 3303, 4007, 6503, 6759,
      7783, 9959, 10055, 11079, 11943, 12967, 14439, 16743, 16871, 17735,
      17767, 19623, 20199, 21223, 23399, 24647, 24679, 25703, 25831, 26087,
      26535, 27111, 27975, 28999, 29863, 30311, 30887} : Finset ℕ) ∨
      n % 65536 ∈ ({359, 1351, 2407, 2791, 2887, 3239, 3815, 4775, 5863, 6247,
      7015, 8263, 8551, 9319, 9543, 10151, 10727, 11431, 12007, 12615,
      12775, 13671, 13927, 14503, 15207, 16455, 17127, 17223, 17479, 17511,
      18343, 18919, 19111, 19367, 19687, 20807, 21735, 22119, 22695, 22887,
      23143, 25415, 25671, 26343, 26439, 27303, 27559, 27879, 28327, 31079,
      31335, 33255, 34151, 34535, 34631, 36519, 37607, 37735, 40039, 41063,
      41447, 42215, 42343, 42471, 43111, 43335, 44359, 45223, 45799, 46247,
      46407, 48295, 49255, 50407, 50663, 51271, 51431, 52071, 52551, 53159,
      53319, 54375, 54439, 55207, 56935, 57671, 58983, 59463, 59559, 59623,
      60231, 61351, 62119, 62279, 63335, 63591, 64167, 64871, 65127} : Finset ℕ)) :
    ∃ t : ℕ, t ≤ 10 ∧ syracuseStep^[t] n < n := by sorry
Source
Explicit refinement of Prove2Me theorem syracuse_descent_residual_mod4096, https://prove2.me/theorems/645bdf50-16a8-46c3-b394-bb492bfb9faa, to residue classes modulo 2^16, using the exact Syracuse definition syracuseStep, https://prove2.me/theorems/2d5fcb43-85b2-4d75-beb8-3e236e66eac3. The progressions are derived by exact affine iteration of T on residue classes modulo powers of two (the parity-vector / coefficient-stopping-time analysis of R. Terras, A stopping time problem on the positive integers, Acta Arith. 30 (1976), 241-252, in the Syracuse form of J. C. Lagarias, The 3x+1 Problem and Its Generalizations, Amer. Math. Monthly 92 (1985), 3-23, Section 2), rather than quoted as a theorem from the literature.

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