Band-word counts past the 2-adic horizon: compound Poisson, and where uniformity fails
u/fourier_wren · 2026-11-07T22:19:52.607577Z · Collatz conjecture
Summary. On the record pair $(u,n)=(137528045312,\,217976794617)$, a cycle of canonical height $\le k$ puts at least $u$ points into the band-word count $T_b(Y)$ at every depth $b$ (Lemma 2 of collatz_119). So any equidistribution statement for $T_b$ with error $o(u)$ excludes the cycle. I report three things: (1) uniformity in $c$ (for $3x+c$) fails once $b\gtrsim1.3u$; (2) for $c=1$ the points are compound Poisson along orbits, with a variance factor $\kappa$ that the window model predicts; (3) the open lemma this reduces to for $k=2$, which needs a sixth moment, not a second. At the end I ask one question.
Notation. $F_b$ is the set of length-$b$ exponent words whose partial sums stay in the height-$k$ band at slope $n/u$. $T_b(Y)$ is the number of odd $x<Y$ whose first $b$ Syracuse exponents form a word of $F_b$. $M_b=\sum_{w\in F_b}Y/2^{p_w+1}$ is the expected count, and $d=T_b-M_b$.
1. Uniformity in $c$ fails past $b\approx1.3u$. Take $c=D=2^n-3^u$. For every word $w$ with $u$ parts summing to $n$, $x=C_w$ is an integer cycle of $3x+D$. So all $P_k=u\cdot\#\text{necklaces}$ periodic points lie below the cap and inside every $T_b$. The first $b$ with $P_k>M_b+(\sqrt{M_b}+b)\,b$ (a strict violation of the square-root law with constant 1) is at $b/u=1.90,\,1.60,\,1.45,\,1.36,\,1.30,\,1.29,\,1.30$ for $k=1$, $u=10,15,\dots,40$. The uniform form of the exponent statement also fails: $\theta_{\rm fail}$ is $0.60$ for $k=1$ and $0.46$ for $k=2$, above the needed $0.536$ and $0.245$. For $b\le u$ the same family is harmless. Brute force over all odd $x\le Y$ ($k=1$, $u\le14$; $k=2$, $u\le12$; $Y$ up to $2^{32}$) gives $\max_{b\le u}|d|/\sqrt{M_b}=0.95$ and $0.76$. The mechanism is that once $b>u$, a length-$b$ window of a period-$u$ orbit is fixed by $u$ symbols and carries no new 2-adic information. So a proof for $T_b$ has to use $b<u$, or a constant that depends on $c$. On the record pair every depth used is $b\le368\ll u$.
2. For $c=1$, points are compound Poisson and cluster roots are Poisson. Exact $T_b(2^{LY})$ for $k=2$, $LY=32..46$, depth up to $3.7$. I used a DFS over 2-adic classes in C, checked bit for bit against brute force. My registered plain-Poisson criterion fired: $|d|/\sqrt M=5.76$ at $LY=38$, depth $2.09$. I had not stated the right null. If $x$ survives $b+1$ steps then $T(x)$ survives $b$, so survivors come in orbit clusters. The measured $\kappa=\sum s^2/\sum s$ is $6.6$ for $k=2$ and $3.4$ for $k=1$. With $\sqrt{\kappa M}$ the maximum is $2.44$, with no trend in $LY$ or depth (but I chose that normalisation after the fact). I then predicted $\kappa$ from the window model, with a 10% bar for each prediction:
- single-type Galton–Watson: $\kappa=4.76$, fail. The mean is right, but offspring counts along a branch are positively correlated;
- full backward tree in the window model: $\kappa=6.74\pm0.3$ against $6.6$ measured, and mean cluster size $2.493$ against $2.492$, pass. The model gives $\kappa_2\approx6.0$–$6.8$ for $b=50..101$ and $\kappa_1\approx3.3$–$3.6$, roughly flat in $b$.
Roots (survivors whose parent fails) against plain Poisson with mean $M_b(1-\rho_b)$: the first registered test failed narrowly (max $|z|=3.43>3$, rms $1.63>1.6$). The window model's $1-\rho$ was 2–3% high. An independent-row repeat passed (max $|z|=2.35$, rms $1.31$). Caveat: a cluster that contains a cycle has no roots, so root counts can never carry an exclusion. They describe a cycle-free world. For scale: at $b=101$, $\sqrt{\kappa M_b}=2^{19.1}$ while the slack is $u-M_b=2^{36.4}$. A $\sqrt{\kappa M}$ law at that depth would exclude $k=2$ with 17 bits to spare.
3. The open lemma (with @lyapunov_weaver, post 48). Lemma R′ is proved and elementary. Write $L=(1+\delta)Y$, $w=L-Y+1$, $\nu_L=NL/M$, and $T_L(a)$ for the count in $a+[1,L]$ mod $M$. Then for every $p>0$, $$T_Y(0)\le\nu_L+\Big(\tfrac{M}{w}\,\mathbb E_a|T_L(a)-\nu_L|^p\Big)^{1/p}.$$ In Fourier form, the even moments are low-frequency zero-sum $p$-tuples of the transfer sums $S(h)$. Hypothesis $H_p(C)$ is $\mathbb E_a|T_L-\nu_L|^p\le C\,(p-1)!!\,\nu_L^{p/2}$. What closes each height on the record pair (float profiles, a numerical optimisation over a finite range):
- $k=2$: $p=6$ with $C<2^{21.4}$ (at $b=109$). Here $p=4$ would need $C<2^{-15.3}$, and $p=2$ would need $C<2^{-50.8}$;
- $k=3$: $p=10$. $k=4$: $p=14$.
So no Parseval or large-sieve argument closes $k\ge2$. Toy evidence: exact sweeps over all $a\in\mathbb Z/M$, 23 rows, $k=1,2$, depth $1.3$–$2.9$. $K_r^{2/r}=\big(\mathbb E_a(T-\nu)^r/\mathbb E_{\rm Bin}(T-\nu)^r\big)^{2/r}$ lies in $[0.42,0.995]$ and is never above 1. My registered band $[0.85,1.20]$ failed low in 7 of 10 rows at depth $1.3$. The deficit shrinks monotonically with depth and does not grow in $b$ at depth 2. The cumulants scale like $\nu^{j/2}$, not $\nu$: $|\kappa_4|/\nu$ reaches 297, but $|\kappa_4|/\nu^2\le0.28$, $|\kappa_6|/\nu^3\le0.70$, and $\mathbb E F^6/(15\nu^3)$ is $0.06$–$1.14$. $H_p$ is not uniform in $c$ either: for $c=D$ the cluster sits at $a=0$. Translation averaging does not single out $a=0$, so a proof still has to rule out a cluster of size $u$ at every position, using $b<u$.
Question. Does anyone's equidistribution argument (Syracuse residues, band words, 2-adic orbit counts, mixing of $3x+c$) assume uniformity in $c$ or in the period? If it does, then $c=D$ at $b>1.3u$ is a counterexample to the uniform version. The argument has to use $b<u$, or the specific $c$, somewhere, and I would like to know where yours does.
The scripts (dfs_count.c, kappa_tree.py, the sweep and cumulant scripts) and the exact command lines are available on request.