English

A sharp inequality involving hyperbolic and inverse hyperbolic functions

Classical Analysis and ODEs 2018-09-25 v1

Abstract

We prove that the inequality cosh(arcosh(2coshu)tanhu)<exp(utanhu)\cosh \left( \mathrm{arcosh}(2 \cosh u) \cdot \tanh u \right) < \exp \left( u \cdot \tanh u \right) holds for all u>0u > 0. We check with the computation program Mathematica that the ratio between the left-hand and the right-hand side is greater than 0,97 for all u0u \ge 0, so this is a quite sharp inequality. It is also equivalent to any of the two inequalities: cosh(11t2arcosh2t)<exp(11t2arcosht) \cosh \left( \sqrt{1 - \frac{1}{t^2}} \cdot \mathrm{arcosh}\,{2t} \right) < \exp \left( \sqrt{1 - \frac{1}{t^2}} \cdot \mathrm{arcosh}\,{t} \right) for all t>1t > 1, and cosh(carcosh21c2)<exp(carcosh11c2) \cosh \left( c \cdot \mathrm{arcosh}{\frac{2}{\sqrt{1-c^2}}} \right) < \exp \left( c \cdot \mathrm{arcosh}{\frac{1}{\sqrt{1-c^2}}} \right) for all c(0,1)c \in (0,1).

Cite

@article{arxiv.1809.08974,
  title  = {A sharp inequality involving hyperbolic and inverse hyperbolic functions},
  author = {Roman Drnovšek},
  journal= {arXiv preprint arXiv:1809.08974},
  year   = {2018}
}

Comments

4 pages

R2 v1 2026-06-23T04:16:29.816Z