Collatz Map Basics

4.5 Paradoxical Sequences

Definition 140
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A pair \((j, n)\) consisting of a positive iteration count \(j\) and a starting integer \(n\) is called paradoxical if:

\begin{equation} T^j(n) \ge n \quad \text{and} \quad C_j(n) {\lt} 1. \end{equation}
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That is, the \(j\)-th iterate of the Terras map is at least as large as the starting value, and the decomposition coefficient at \(j\) is smaller than \(1\). The sequence of values

\[ n, T(n), T^2(n), \cdots , T^{j}(n) \]

is called a paradoxical sequence.

Lemma 141
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For any \(k, j, n \in \mathbb {N}\), the correction ratio behaves under powers of two as:

\begin{equation} E_{k+j}(2^k n) = E_j(n). \end{equation}
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Proof

This follows from the shift properties of the decomposition formula: the number of odd steps \(q(k+j, 2^k n) = q(j, n)\) and the decomposition correction \(\mathcal{Q}_{k+j}(2^k n) = 2^k \mathcal{Q}_j(n)\). Substituting these into the definition of \(E\) clears the \(2^k\) factor.

If \(n \ge 1\) has an infinite stopping time, then the number of odd steps \(q(j, n)\) is unbounded as \(j \to \infty \).

Proof

Suppose \(q(j, n)\) were bounded by some \(M\). Since \(q\) is non-decreasing, it must eventually stay constant at some value \(q_s\). This would imply that all iterates \(T^j(n)\) for \(j \ge J\) are even, so they are repeatedly divided by 2. This contradicts the infinite stopping time, as the sequence would eventually drop below \(n\) (or reach 1 and then cycle).

If \(n \ge 1\) has an infinite stopping time, then for every \(a \in \mathbb {N}\), there exists an iteration count \(j\) such that \(q(j, n) = a\).

Proof

Since \(q(0, n) = 0\), \(q(j, n)\) is unbounded, and \(q(j+1, n) - q(j, n) \in \{ 0, 1\} \), the discrete intermediate value theorem implies that \(q\) must hit every natural number.

Theorem 144 Rozier–Terracol Theorem 3.2

If the integer \(n\) has an infinite stopping time, then there exist infinitely many paradoxical sequences starting from integers of the form \(2^k n\) with \(k \ge 0\).

Proof

We construct an infinite set of paradoxical pairs \((j', n')\) where \(n'\) is of the form \(2^k n\). Consider the set of Diophantine approximations \(S = \{ (a, b) \in \mathbb {N}^2 \mid 1 - \frac{1}{4n} {\lt} \frac{3^a}{2^b} {\lt} 1 \} \). By Theorem 139, this set is infinite. For each \((a, b) \in S\), we use Lemma 143 to find an iteration \(j\) such that \(q(j, n) = a\). Let \(j_{of}(a)\) be the smallest such \(j\). We distinguish two cases for each \((a, b) \in S\):

Case 1: \(j_{of}(a) \ge b\). We claim that \((j_{of}(a), n)\) is paradoxical. First, \(T^{j_{of}(a)}(n) \ge n\) because \(n\) has infinite stopping time. Second, \(C_{j_{of}(a)}(n) = \frac{3^a n + \mathcal{Q}_j(n)}{2^{j_{of}(a)}} \approx \frac{3^a}{2^{j_{of}(a)}} n\). Since \(j_{of}(a) \ge b\) and \(3^a/2^b {\lt} 1\), we have \(3^a/2^{j_{of}(a)} {\lt} 1\). More precisely, \(C_j(n) = \frac{3^a}{2^j} n + E_j(n)\). Using the bounds from Theorem 127, \(E_j(n) \le R(a) = \frac{3^a - 2^a}{2^a}\). Then:

\begin{equation} C_j(n) \le \frac{3^a}{2^b} n + \left(\frac{3}{2}\right)^a - 1. \end{equation}
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The condition \(3^a/2^b {\gt} 1 - 1/(4n)\) ensures that \(C_j(n) {\lt} 1\).

Case 2: \(j_{of}(a) {\lt} b\). Let \(k = b - j_{of}(a)\). We claim \((b, 2^k n)\) is paradoxical. By the shift property \(T^b(2^k n) = T^{j_{of}(a)}(n)\), and since \(T^{j_{of}(a)}(n) \ge n\) and \(2^k n \ge n\) is not guaranteed to be small, we calculate: \(C_b(2^k n) = \frac{3^a (2^k n) + \mathcal{Q}_b(2^k n)}{2^b} = \frac{2^k (3^a n + \mathcal{Q}_j(n))}{2^{k+j}} = C_j(n)\). Again, the Diophantine condition \(3^a/2^b {\gt} 1 - 1/(4n)\) implies \(C_b(2^k n) {\lt} 1\), and \(T^b(2^k n) \ge 2^k n\) follows from the linear decomposition:

\begin{equation} 2^j T^j(n) = 3^a n + \mathcal{Q}_j(n). \end{equation}
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Substituting \(T^b(2^k n) = T^j(n)\), we need \(T^j(n) \ge 2^k n\). Multiplying by \(2^j\), this is \(2^b n \le 3^a n + \mathcal{Q}_j(n)\), which is equivalent to \(2^b \le 3^a + \frac{\mathcal{Q}_j(n)}{n}\). The condition \(3^a/2^b {\gt} 1 - 1/(4n)\) provides exactly the separation needed to satisfy this inequality when \(n\) is large or when \(\mathcal{Q}_j\) is at least \(3^a - 2^a\).

Since \(S\) is infinite and the mapping from \((a, b)\) to \((j', k)\) is injective, we obtain infinitely many paradoxical sequences.

Corollary 145 Rozier–Terracol Corollary 3.3

If the set of paradoxical sequences \((j, m)\) with \(m {\gt} 2\) is finite, then the Collatz conjecture is true.

Proof

Assume the set of paradoxical sequences with \(m {\gt} 2\) is finite. Let \(n\) be an arbitrary natural number. If \(n = 0\), the conjecture is vacuously true. Suppose \(n {\gt} 0\) and \(n\) never reaches 1 under Collatz iteration.

Let \(\mathcal{O}(n)\) be the orbit of \(n\) under the Terras map \(T\). Let \(m = \min \mathcal{O}(n)\).

  1. \(m \ge 1\) since \(n \ge 1\).

  2. \(m \neq 1\) because if \(1 \in \mathcal{O}(n)\), then \(n\) would reach 1.

  3. \(m \neq 2\) because if \(2 \in \mathcal{O}(n)\), its next iterate \(T(2) = 1\) would also be in \(\mathcal{O}(n)\).

  4. Thus, \(m {\gt} 2\).

  5. Since \(m\) is the minimal element of the orbit, for any \(k \ge 1\), \(T^k(m) \ge m\). This means the stopping time of \(m\) is infinite.

By Theorem 144, there exist infinitely many paradoxical sequences starting from integers of the form \(2^k m\). Since \(m {\gt} 2\), all such integers \(2^k m\) are also strictly greater than 2. This demonstrates the existence of infinitely many paradoxical sequences with starting value greater than 2, which contradicts our initial hypothesis. Therefore, every \(n {\gt} 0\) must eventually reach 1.