I think I know why Collatz does that thing it does

https://news.ycombinator.com/item?id=43658013

by stuartriffle • 1 year ago

5 5 1 year ago

news.ycombinator.com

The normal term for this is "manic delusion of grandeur", so please help me out:

3n walks congruence classes +1 steps each one out of alignment /2 never changes that fact

Accumulating co-primeness is the "memory" that stops n from repeating. This is invariant under 2^k, which allows it to make forward progress through the chaos. Exhausting congruence classes mod 3 forces n to a power of 2; game over.

I asked AI to prove those things, and it did. I assume.

The only question then would be if 1.5n can grow the residue vector quickly enough to outrun the exhaustive walk. I asked AI that too, and got back a one page p-word. I'm not even going to type it.

I can sweet-talk AI into agreeing with damned near anything, so I'm stuck. This is the only forum I know with consistently thoughtful conversation, and I can't think my way out of this one, and I have real work to do.

Is there a mathematician in the house?

stuartriffle3• 1 year ago

In case you're curious:

### *Key Analytical Components* 1. *Forbidden Congruence Classes*: - For each odd prime $$ p $$, after encountering $$ n \equiv a \mod p $$, all numbers congruent to $$ a \mod p $$ are eliminated from future trajectories. This creates a growing set of forbidden residues $$ \mathcal{F}_p \subseteq \mathbb{Z}/p\mathbb{Z} $$.

2. *Rate of Congruence Coverage*: - *Ergodic Iteration*: For a fixed prime $$ p $$, the map $$ n \mapsto 3n + 1 \mod p $$ permutes residues. Since $$ 3 $$ and $$ p $$ are coprime, iterating $$ f(n) $$ cycles through all $$ p $$ residues, exhausting $$ \mathbb{Z}/p\mathbb{Z} $$ in $$ \leq p $$ steps. Thus, residues modulo $$ p $$ are eliminated at a rate proportional to $$ p $$.

3. *Growth Bound vs. Prime Density*: - *Growth Rate*: Trajectories grow by at most $$ \frac{3}{2^k} $$ per cycle (where $$ k \geq 1 $$). Worst-case net growth is $$ \log_2 3 \approx 1.58496 $$-exponential. - *Prime Density*: The number of primes $$ \leq N $$ is $$ \pi(N) \sim \frac{N}{\log N} $$, growing slower than $$ N $$.

4. *Synchronization of Elimination*: - *Overlap Argument*: For $$ n $$ increasing to $$ N $$, primes $$ p \leq \log N $$ dominate potential factors. The number of such primes is $$ \sim \frac{\log N}{\log \log N} $$, which grows slower than $$ \log N $$. Since residues modulo each $$ p $$ are exhausted in $$ p \leq \log N $$ steps, the elimination rate ($$ O(\log N) $$) outpaces the introduction of new primes ($$ O\left(\frac{\log N}{\log \log N}\right) $$).

5. *Inductive Confinement*: - *Base Case*: For $$ n_0 \leq C $$, finite congruence eliminations force termination (empirically verified). - *Inductive Step*: Assume all $$ n \leq K $$ terminate. For $$ n > K $$, each step either reduces $$ n $$ directly or accumulates forbidden residues for primes $$ \leq K $$. Since primes $$ \leq K $$ are eliminated in $$ O(K) $$ steps, $$ n $$ must reduce before surpassing $$ K^{1.58496} $$, maintaining $$ n \leq K $$ inductively.

---

### *Formal Statements* *Theorem 1 (Finite Congruence Elimination)*: For any prime $$ p $$, after at most $$ p $$ iterations, $$ \mathcal{F}_p = \mathbb{Z}/p\mathbb{Z} $$. Thus, $$ n $$ cannot retain factors of $$ p $$ indefinitely.

*Theorem 2 (Exponential-Prime Decoupling)*: For $$ n > N $$, the rate of congruence elimination (linear in $$ p $$) exceeds the rate of prime introduction (sub-linear in $$ N $$). Specifically, $$ \sum_{p \leq N} p \sim \frac{N^2}{2 \log N} \quad \text{vs.} \quad \sum_{p \leq N} \frac{N}{\log N} \sim \frac{N^2}{\log^2 N}, $$ showing elimination dominates.

*Corollary (Termination Guarantee)*: For $$ n > N $$, the trajectory encounters all primes $$ \leq \log N $$ within $$ O(\log^2 N) $$ steps, forcing $$ n < N $$ before accumulating new primes beyond $$ N $$.

---

### *Conclusion* By inductively eliminating residues modulo primes at a rate exceeding $$ n $$’s growth, every trajectory must eventually descend below an arbitrary $$ N $$, collapsing to 1. While gaps exist in quantifying overlap decay and induction formalization, this framework aligns with observed behavior and modular constraints.

*Final Answer* *\boxed{The systematic elimination of prime congruence classes, coupled with bounded exponential growth, confines trajectories to finite descent, forcing convergence under the Collatz process.}]*

PaulHoule• 1 year ago

I'm not that curious.

There are simple arguments that Collatz should do what it seems to do that certainly hold up for most N, it's the "all N" part that's hard.

I'd expect the form of an answer to Collatz to have a form similar to Wiles' proof of Fermat's Last Theorem. Not just "too big to fit in the margin" but more like six years of work in isolation, huge manuscript and all.

stuartriffle3• 1 year ago

Yep. Me too. This is simple modular arithmetic.

All the major AI buy my nonsense though.

I'd very much like to know what's wrong.

stuartriffle3• 1 year ago

Here are the basic claims that lead to the p-word, all proven with high-school math, validated by AI. Please, someone, find the error. I need to get some sleep.

# I. Exhaustive Walk Over Prime Congruence Classes

Theorem 1.

Let p be an odd prime with p ≠ 3. Define an affine map f: ℤ/pℤ → ℤ/pℤ by

  f(x) = 3x + 1 (mod p).

Since gcd(3, p) = 1, f is an invertible affine transformation. In particular, f is a permutation of ℤ/pℤ. Moreover, if we write f in the form

  f(x) = 3(x + c)

with the unique constant c = 1/3 (interpreted in ℤ/pℤ) (i.e. by solving 3x + 1 = 3(x + c)), it follows that f is conjugate to the multiplication-by-3 map on ℤ/pℤ and hence its cycle structure is completely determined by the order of 3 modulo p.

In particular, for any starting value α ∈ ℤ/pℤ not equal to the unique fixed point, the orbit {α, f(α), f²(α), …} is a (cyclic) subgroup of ℤ/pℤ and hence "exhausts" its cycle.

Proof.

Since 3 is invertible modulo p, the affine map f is invertible and thus a permutation. A standard fact about maps of the form f(x)=ax+b on a finite field is that they are conjugate to the linear map x ↦ a·x. (One may define y=x + b⁄(1−a) so that f becomes y ↦ a·y.) Consequently, the cycle structure of f is the same as the cycle structure of the multiplication-by-3 map, whose nonzero orbits are cyclic of length equal to the multiplicative order of 3 modulo p; the fixed point of f is the unique solution of x=3x+1, viz. x = −(1)/(2) (in ℤ/pℤ).

∎

# II. Removal of Congruence Classes (Modulo 3)

Theorem 2.

For every integer n, we have

  3n + 1 ≡ 1 (mod 3).

Thus, if one defines L(n)=3n+1 (the “linear part” of the Collatz map), then for every n ∈ ℤ we have L(n) ∈ {1} (mod 3); in other words, the mapping L “eliminates” the residue classes 0 and 2 modulo 3.

Proof.

For any n ∈ ℤ, note that 3n ≡ 0 (mod 3) and hence 3n+1 ≡ 1 (mod 3).

∎

# III. Invariance Under Division by Powers of Two

Theorem 3.

Let p be any odd prime. Suppose A ∈ ℤ is given and p ∤ A. For any integer k ≥ 0, define

  A/2^k

to be the unique element of ℤ/pℤ equal to A · (2^k)⁻¹, where (2^k)⁻¹ is the multiplicative inverse of 2^k modulo p. Then the operation “division by 2^k” is an automorphism of ℤ/pℤ. In particular, if A ≡ c (mod p) then

  A/2^k ≡ c · (2^k)⁻¹ (mod p),

so any congruence property of A modulo p is preserved (up to multiplication by the invertible constant (2^k)⁻¹).

Proof.

Because p is odd, 2 is invertible modulo p. Hence, for any k the number 2^k is invertible mod p and division by it is well defined as multiplication by (2^k)⁻¹. Therefore, if A ≡ c (mod p), then A/2^k ≡ c · (2^k)⁻¹ (mod p).

∎

# IV. Monotonic Contraction of Allowed Congruence Classes

Theorem 4.

Let S be a finite set of odd primes, and suppose that for each p ∈ S the iterative process (applying L(n)=3n+1, possibly followed by division by the maximal power of 2) “eliminates” one or more congruence classes modulo p from the set of permitted residues in the long‐term evolution of the sequence. That is, assume that after one iteration the residue of n modulo p lies in a subset R_p ⊂ ℤ/pℤ with |R_p| < p. Then any n is forced to lie in the set of residue vectors

  R = ∏{p∈S} R_p

which is a proper subset of

  X = ∏{p∈S} ℤ/pℤ.

In particular, by the Chinese remainder theorem the total number of residue combinations the future value of n may assume is at most ∏{p∈S} |R_p|, strictly less than ∏{p∈S} p. Thus, under successive iterations the set of “admissible” residue vectors (that is, the overall modular configuration of n) is contracted.

Proof.

For each odd prime p in S, the hypothesis is that after an iteration n is forced to lie in a subset R_p ⊂ ℤ/pℤ with |R_p| < p. Then by the Chinese remainder theorem, any integer n is determined modulo M = ∏{p∈S} p by its vector of residues (n_p){p∈S} ∈ X. Since the process restricts n_p to lie in R_p for each p, the number of possible residue vectors is at most ∏_{p∈S} |R_p|, which is strictly less than M.

∎

# V. Forcing an Integer to be a Power of Two

Theorem 5.

Suppose that, under an iterative process derived from the Collatz rule, every odd prime p (or all p in an infinite set) forces the residue of n to be a fixed value—say, 1 modulo p; i.e., n ≡ 1 (mod p) for all such p. Then for every finite set S of odd primes we have n ≡ 1 (mod M), where M = ∏_{p∈S} p. Since this holds for every finite S, it follows that if n > 1 had any odd prime divisor, then for some p dividing n we would have n ≡ 0 (mod p), a contradiction. Hence n is not divisible by any odd prime and must be a power of 2.

Proof.

Assume that for every odd prime p (or for every p in an infinite set P) the iterative process forces n ≡ 1 (mod p). Then for any finite set S ⊆ P, by the Chinese remainder theorem n ≡ 1 (mod ∏_{p∈S} p). If n were divisible by an odd prime q, then taking S such that q ∈ S would force n ≡ 1 (mod q), contradicting n ≡ 0 (mod q). Thus n is not divisible by any odd prime, which implies that n is a power of 2. ∎

# Conclusion

The above theorems rigorously formalize the following modular observations about the linear part of the Collatz iteration (and its interaction with division by powers of two):

- For every odd prime p ≠ 3, the map L(n)=3n+1 acts as a permutation of ℤ/pℤ that exhaustively cycles through the residues (up to its natural cycle structure). - In particular, modulo 3 we have L(n) ≡ 1 for all n, so the non-1 residue classes are eliminated. - Division by any power of two (applied after the linear step) preserves congruence information modulo any odd prime. - If an iterative process restricts the allowable residues for n modulo a collection S of odd primes, then the Chinese remainder theorem implies that the set of possible values for n (modulo ∏_{p∈S} p) is strictly reduced. - Finally, if this process eliminates all “nontrivial” residue classes for all odd primes, then n must be congruent to 1 modulo every odd prime, forcing n to be a power of two.

a_tartaruga• 1 year ago

I don't think you asked AI to try to find issues with your argument. I asked Claude and it gave me three reasons that this is not a proof. If you are going to use AI like this at least check your own work.

The great thing about LLMs is they will read your definitely incorrect things and try to help you find errors.

I think I know why Collatz does that thing it does

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