The central theorem establishes r β₯ r: the spectral radius (hence average growth rate) of the branching operator with ancestral memory, acting on the space SN of infinite memory sequences, is never smaller than the Perron-Frobenius eigenvalue r of the memoryless mean matrix m . Memory activation is "fair" (law of the backward generation T is memory-independent), so the advantage is intrinsic, not driven by selection. The proof chain: many-to-one formula β biased Markov chain on symbol space β unique ergodicity via asymptotic coupling β Birkhoff ergodic theorem. Equality holds when the matrix is balanced (h β‘ 1); strictness is demonstrated explicitly in Example 1.2 .
Simulated/approximate figure: the shaded band is read from the axis range of the paper's plot of r(u) in Example 1.2; exact values of r(u) are not stated in the text. The dashed line is the exactly reported memoryless value.
| Claim | Status | Method |
|---|---|---|
| r β₯ r (memory never hurts average growth) | Proven (Theorem 1.1) | Many-to-one formula + ergodic theorem |
| Strict inequality in general | Shown by example | Explicit 2-type spectral calculation |
| Equality for balanced matrices | Proven (Example 3.1) | Perron-Frobenius, h β‘ 1 |
| Unique ergodicity of biased chain Q | Proven (Prop. 3.2) | Asymptotic coupling (ultimately successful Markovian coupling) |
| Elementary bounds min h/max h β€ r/r β€ max h/min h | Proven (3.4) | Operator norm estimates |
Strengths: rigorous, self-contained; the coupling argument (bounded-expectation entry into a consolidation phase, geometric control of exits) is elegant and handles the fact that classical tools (irreducibility, strong Feller, compactness) fail for unbounded memory support. The Feller property and failure of the strong Feller property are cleanly characterized. Limitations the paper acknowledges: full Perron-Frobenius extension (strictly positive eigenfunction, stability/contraction) remains unresolved for unbounded memory support; Champagnat-Strickler-Villemonais quasi-stationary criteria (Assumption (A), Harnack (H)) could not be verified here . Blind spots: purely theoretical β no simulation or empirical biological validation; no natural selection or environmental variation modeled, so the evolutionary interpretation (atavism beneficial) is a heuristic analogy, not a demonstrated biological prediction. The generalization to countable/infinite type spaces is not addressed. Comparative memory-benefit findings in other dynamical systems are mixed β e.g., in rock-paper-scissors games longer memory helps only up to a critical threshold before inducing oscillatory instability β underscoring that Bertoin's monotonicity result depends on the specific "fair reactivation" mechanism and superlinear branching setting. Confidence: high for the mathematical claims ( proofs are explicit); low-to-moderate for the evolutionary-biology interpretation.
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