Tao 2019 Collatz Blueprint

3 The Collatz–Syracuse dictionary and logarithmic density

Definition 3.1 Collatz and Syracuse maps; the reduction (1.2)
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The Collatz map \(\mathrm{col}(N)=3N+1\) if \(N\) is odd and \(N/2\) otherwise; its orbit minimum \(\mathrm{Colmin}(N)=\inf _k \mathrm{col}^{[k]}(N)\). The odd part \(\mathrm{oddPart}(N)=N/2^{v_2(N)}\), the Syracuse map \(\mathrm{syr}(N)=\mathrm{oddPart}(3N+1)\) (odd \(\to \) odd), and its orbit minimum \(\mathrm{Syrmin}\). The elementary reduction (paper (1.2)) that lets the entire problem be studied on the odd numbers:

\[ 0{\lt}N \; \Longrightarrow \; \mathrm{Colmin}(N) = \mathrm{Syrmin}(\mathrm{oddPart}\, N). \]

Also the cheap odd-preservation and positivity facts (\(\mathrm{syr}\) maps odds to odds, etc.).

Campaign estimate: 3–6 treadmill laps; risk low (95% confidence the node completes as stated).

Lemma 3.2 Valuation vector, integer offset, and the iteration identity (1.7)

The \(n\)-step valuation vector (paper (1.8)) \(\vec a^{(n)}(N)_i = v_2\bigl(3\, \mathrm{syr}^{[i]}(N)+1\bigr)\), the integerified offset (design D2) \(\texttt{fnat}\, n\, a = \sum _{m{\lt}n} 3^{\, n-1-m}\, 2^{\, a_{[1,m]}}\), and the load-bearing iteration identity, the paper’s (1.7) multiplied through by \(2^{|a|}\) so it holds in \(\mathbb {N}\): for odd \(N\), with \(a=\vec a^{(n)}(N)\),

\[ 2^{\, a_{[1,n]}}\cdot \mathrm{syr}^{[n]}(N) = 3^n N + \texttt{fnat}\, n\, a . \]

(By induction on \(n\); the step is \(2^{a_{n+1}}\mathrm{syr}(\mathrm{syr}^{[n]}N) = 3\, \mathrm{syr}^{[n]}N + 1\).) Together with Lemma 2.1: the valuation vector is the unique tuple with all entries \(\ge 1\) making the affine quotient odd. COMPLETE (judge pass 21, 2026-07-13). valVec_unique (Lemma 2.1 as a hypothesis-guarded iff: divisibility \(\wedge \) odd quotient \(\leftrightarrow \) \(a = \texttt{valVec}\), for tuples with entries \(\ge 1\)) was proved by an external Codex session and judge-verified: statement RATIFIED vs Lemma 2.1 p.14 (RATIFY-2 resolved — the iff matches the paper’s uniqueness claim; the paper’s membership half, \(\vec a^{(n)} \in (\mathbb {N}+1)^n\) itself, is an unformalized one-line companion, consumer-safe since candidate tuples carry the hypothesis by construction); proof route is the paper’s own last-entry induction; dated judge runs 2026-07-13 on syr_iterate_key, valVec_unique, syr_iterate_odd all exactly \([\texttt{propext}, \texttt{Classical.choice}, \texttt{Quot.sound}]\).

Proof

Forward: induction on \(n\) peeling the last entry (the paper’s own route) — the truncated vector satisfies the \(n\)-step condition (recursion identities for pre/fnat under truncation), the quotient is identified with \(\mathrm{syr}^{[n]}(N)\) via the iteration identity, and the last entry is extracted as \(\nu _2(3\, \mathrm{syr}^{[n]}(N)+1)\). Backward: the iteration identity gives divisibility, and the quotient is \(\mathrm{syr}^{[n]}(N)\), odd since Syracuse iterates of odd numbers stay odd (syr_iterate_odd).

Definition 3.3 Logarithmic density and the reduction Thm 1.6 \(\Rightarrow \) Thm 1.3

The logarithmic-density apparatus: logSum, logProb, the sampling window Finset.Icc 1 x, the predicate HasLogDensity, and the “almost all” quantifiers AlmostAllPos and its odd-window form AlmostAllOdd (logarithmic density restricted to \(2\mathbb {N}+1\)). With the harmonic-sum integral tests (paper (5.25)(5.26)) and the elementary splitting reducing the even-index statement Thm 1.6 to the main theorem Thm 1.3 (node 5.1) by passing to odd parts.

Campaign estimate: 6–12 treadmill laps; risk low (85% confidence the node completes as stated).