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.