Threshold Phenomena and Bounds in Normalized Remainders of Degenerate Exponential Functions

Abstract

In this work, we study a normalized remainder $T_{n,λ}[\e_λ]$ for the degenerate exponential $\e_λ(u)=(1+λu)^{1/λ}$ ($λ>0$). We establish an integral representation, an exact monotonicity threshold at $λ=1/(n+1)$, and rigorous conditions for the local failure of logarithmic convexity at the origin. We then prove a sharp asymptotic result: for every $λ$ in the increasing regime $(0,1/(n+1))$, the second logarithmic derivative satisfies $u^2L(u)\to -α<0$ as $u\to\infty$, showing that global logarithmic convexity on $(0,\infty)$ fails throughout this regime. We further give a necessary and sufficient condition for absolute monotonicity, showing it holds only on a countable, measure-zero set of parameters, and we derive explicit two-sided truncation-error bounds that are pointwise sharp at the origin.