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Inequalities for the gamma function
Authors:Horst Alzer
Affiliation:Morsbacher Str. 10, 51545 Waldbröl, Germany
Abstract:We prove the following two theorems:

(i) Let $M_r(a,b)$ be the $r$th power mean of $a$ and $b$. The inequality

begin{displaymath}M_r(Gamma(x),Gamma(1/x))ge 1 end{displaymath}

holds for all $xin(0,infty)$ if and only if $rge 1/C-pi^2/(6C^2)$, where $C$ denotes Euler's constant. This refines results established by W. Gautschi (1974) and the author (1997).

(ii) The inequalities

begin{equation*}x^{alpha(x-1)-C}<Gamma(x)<x^{beta(x-1)-C}tag{$*$} end{equation*}

are valid for all $xin(0,1)$ if and only if $alphale 1-C$ and $betage (pi^2/6-C)/2$, while $(*)$ holds for all $xin (1,infty)$ if and only if $alphale (pi^2/6-C)/2$ and $betage 1$. These bounds for $Gamma(x)$ improve those given by G. D. Anderson an S.-L. Qiu (1997).

Keywords:Gamma function   psi function   power mean   inequalities
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