dm³ 101 · Week 06 · Ladder: ρ_n → τ

The Recurrence Ladder — First Rung

φ, η, Δ, Σ, Ω — seven constants, one convergent sequence
dm³ 101 · Week 06 · Ladder: ρ_n → τ
The Recurrence Ladder — First Rung
Course: dm³ 101  ·  Ladder: ρ_n → τ  ·  Source: Book 3, “The Recurrence Ladder — From π to Hexabonacci”, Zenodo 10.5281/zenodo.19117400

Week 5 fixed \(T^*=2\pi\) as a period constant standing outside the ladder. This week introduces the ladder itself: a sequence of seven named constants, each the dominant root \(\rho_n\) of an n-bonacci characteristic polynomial \(x^n - x^{{n-1}} - \cdots - x - 1 = 0\), climbing from Fibonacci toward the embodiment threshold \(\tau = 2\).

The seven rungs
\(n{{=}}2\): \(\varphi \approx 1.618\) (Fibonacci)  ·  \(n{{=}}3\): \(\eta \approx 1.839\) (Tribonacci)  ·  \(n{{=}}4\): \(\Delta \approx 1.928\) (Tetranacci)  ·  \(n{{=}}5\): \(\Sigma \approx 1.966\) (Pentanacci)  ·  \(n{{=}}6\): \(\Omega \approx 1.984\) (Hexabonacci)  ·  \(n\to\infty\): \(\rho_\infty = 2 = \tau\).

Why the limit is exactly 2: write the n-bonacci polynomial as \(x^n(1 - 1/x - 1/x^2 - \cdots - 1/x^n) = 0\). As \(n \to \infty\), the bracketed sum becomes the geometric series \(1/(x-1)\) for \(x>1\), so the equation becomes \(1 - 1/(x-1) = 0 \Rightarrow x = 2\). This is Theorem Ω.1 in the source chapter: \(\lim_{{n\to\infty}} \rho_n = 2\), and that limit is identified with the same \(\tau=2\) that appears in the canonical triple from Week 4 and in the noise-tolerance identity \(\tau \cdot \varepsilon_0 = 2/3\).

Notice the spacing as you climb: \(\eta - \varphi \approx 0.221\), \(\Delta - \eta \approx 0.089\), \(\Sigma - \Delta \approx 0.038\), \(\Omega - \Sigma \approx 0.018\). Each rung closes roughly half the remaining gap to \(\tau=2\) — a geometric convergence rate, not a linear one. Hexabonacci (\(\Omega \approx 1.984\)) is already within 1% of the limit after only six terms.

The criticality threshold, previewed
The ladder is not just decorative. The dm³ potential \(V(q) = q^3 - cq\) has a Whitney \(A_1\) fold exactly at \(q=1\) when \(c = 3\) — not related numerically to any single rung, but the regime each rung sits in matters: \(\varphi\) (subcritical, fold left of \(q=1\)) never triggers a dm³ transition on its own; \(\eta\), remarkably, is the constant tied to the Collatz map \(c=3\) discussion in Week 15. This bridge is developed properly in dm³ 102.
This week’s content is grounded in Principia Orthogona, Book 3 (“The Recurrence Ladder”) and AXLE_v6.lean — theorem names and Lean identifiers above point at the actual source files.
-- dm³ 101 · Week 06 · n-bonacci ladder, from AXLE_v6.lean + Book 3

-- g6 = τ^5 + 1 = 33  (crystal arithmetic, already proved, Week 4-adjacent)
-- theorem tau_embodiment : tau^6 = 2^6  -- proved (AXLE_v6.lean)

-- The ladder itself (ρ_n for n=2..6, and the n→∞ limit) is stated
-- and numerically verified in the Book 3 chapter (chapters-pi-phi-
-- mu-eta-delta-sigma-omega.html, Theorem Ω.1); a general Lean proof
-- of lim ρ_n = 2 for arbitrary n is not yet in AXLE — an open
-- formalization task, not a closed theorem, distinct from the
-- already-proved specific facts about τ=2 itself.
example : (2:ℝ)^6 = 64 := by norm_num