Collatz Map Basics

4.1 Rozier–Terracol Lemma 2.1

Lemma 119

Let \(k \le j\). If \(E_k(n) {\lt} E_k(m)\) and the parity bits of \(n\) and \(m\) agree for steps \(i \in \{ k, \dots , j-1\} \) (i.e., \(X(T^i(n)) = X(T^i(m))\) for \(k \le i {\lt} j\)), then \(E_j(n) {\lt} E_j(m)\).

Proof

We proceed by induction on \(j\). The base case \(j=k\) follows from the hypothesis. For the inductive step \(j \to j+1\), we assume \(E_j(n) {\lt} E_j(m)\) and \(X(T^j(n)) = X(T^j(m)) = x\). Then:

\begin{equation} E_{j+1}(n) = \frac{3^x}{2} E_j(n) + \frac{x}{2} {\lt} \frac{3^x}{2} E_j(m) + \frac{x}{2} = E_{j+1}(m) \end{equation}
1

since \(3^x/2 {\gt} 0\).

For any \(i {\lt} j\), the \(i\)-th entry of the parity vector \(V_j(n)\) is true if and only if \(X(T^i(n)) = 1\). It is false if and only if \(X(T^i(n)) = 0\).

Proof

Immediate from the definition of \(V_j(n)\).

Lemma 121

If the parity vectors \(V_j(m)\) and \(V_j(n)\) agree on their first \(k\) entries (\(k \le j\)), then \(E_k(m) = E_k(n)\).

Proof

Since \(E_k(n)\) depends only on the parity bits \(X(T^0(n)), \dots , X(T^{k-1}(n))\), agreement of the first \(k\) entries of the parity vector implies equality of these bits, and thus equality of the correction terms and \(E_k\).

Lemma 122

If \(V_j(m)\) and \(V_j(n)\) differ by a single elementary swap \(01 \to 10\) (i.e., \(V_j(m) \prec _{el} V_j(n)\)), then \(E_j(n) {\lt} E_j(m)\).

Proof

Let the swap occur at positions \(k\) and \(k+1\). Before position \(k\), the parity vectors agree, so \(E_k(n) = E_k(m)\). In \(V_j(m)\), we have bits \((0, 1)\) at \((k, k+1)\), while in \(V_j(n)\) we have \((1, 0)\). Calculating the two-step increase:

\begin{align*} E_{k+2}(m) & = \frac{3^1}{2} \left( \frac{3^0}{2} E_k(m) + \frac{0}{2} \right) + \frac{1}{2} = \frac{3}{4} E_k(m) + \frac{1}{2} \\ E_{k+2}(n) & = \frac{3^0}{2} \left( \frac{3^1}{2} E_k(n) + \frac{1}{2} \right) + \frac{0}{2} = \frac{3}{4} E_k(n) + \frac{1}{4} \end{align*}

Thus \(E_{k+2}(n) {\lt} E_{k+2}(m)\). Since the bits agree for all positions \(i {\gt} k+1\), Lemma 119 implies \(E_j(n) {\lt} E_j(m)\).

Lemma 123

The correction ratio \(E\) is strictly monotonic with respect to the transitive closure of the elementary precedence relation on parity vectors.

Proof

This follows by induction on the number of elementary swaps, applying Lemma 122 at each step.

Theorem 124 Rozier–Terracol Lemma 2.1

If \(V_j(m)\) strictly precedes \(V_j(n)\) in the partial order generated by elementary swaps (i.e., \(V_j(m) \prec V_j(n)\)), then \(E_j(n) {\lt} E_j(m)\).

Proof

This is exactly the statement that the precedence relation \(V_j(m) \prec V_j(n)\) implies \(E_j(n) {\lt} E_j(m)\), which follows from Lemma 123 by identifying the sequence-based \(E_j\) with the vector-based calculation.