Breakdown of Perturbative Expansions and Exact Algebraic Absorption of Finite-Size Fluctuations in Statistical Mechanics

Abstract

In statistical mechanics, evaluating finite-size macroscopic fluctuations typically relies on Edgeworth expansions. However, these perturbative methods append additive polynomial corrections that break down in the large deviation regime, yielding unphysical negative probabilities. We propose a structural resolution: rather than relying on additive polynomials, we absorb finite-size skewness using a globally stable $q$-deformed framework. By introducing a dynamic scaling law $1-q_n=O(n^{-1})$ for the nonextensivity parameter, we prove this $q$-deformed framework captures macroscopic higher-order fluctuations in independent and identically distributed (i.i.d.) systems. Specifically, this algebraic tuning absorbs third-order skewness while guaranteeing probability density nonnegativity across the entire domain. Furthermore, the $k$-th degree term of this $q$-logarithmic expansion corresponds to the $O(n^{1-k/2})$ asymptotic order of classical $(k+1)$-th moment Edgeworth corrections. This correspondence functions as a stable resummation of divergent asymptotic expansions, establishing a mathematical bridge between finite-size i.i.d. fluctuations and the Tsallis statistics governing complex systems.

22 pages, 4 figures. Accepted for publication in Physica A: Statistical Mechanics and its Applications