The monotonicity rules for the ratio of two Laplace transforms with applications

Let $f$ and $g$ be both continuous functions on $(0,\mathrm{\infty })$ with $g\left(t\right)>0$ for $t\in (0,\mathrm{\infty })$ and let $F\left(x\right)=\mathcal{L}\left(f\right)$, $G\left( x\right) =\mathcal{L }\left( g\right) $berespectivelytheLaplacetransformsof$f$and$g$convergingfor$x>0$\mathrm{.}Weprovethatifthereisa$t^{\ast }\in \left( 0,\infty \right) $suchthat$f/g$isstrictlyincreasingon$\left( 0,t^{\ast }\right) $andstrictlydecreasingon$\left( t^{\ast },\infty \right) $,thentheratio$F/G$isdecreasingon$\left( 0,\infty \right) $ if and only if \begin{equation*} H_{F,G}\left( 0^{+}\right) =\lim_{x\rightarrow 0^{+}}\left( \frac{F^{\prime }\left( x\right) }{G^{\prime }\left( x\right) }G\left( x\right) -F\left( x\right) \right) \geq 0, \end{equation*} with \begin{equation*} \lim_{x\rightarrow 0^{+}}\frac{F\left( x\right) }{G\left( x\right) } =\lim_{t\rightarrow \infty }\frac{f\left( t\right) }{g\left( t\right) }\text{
and \ }\lim_{x\rightarrow \infty }\frac{F\left( x\right) }{G\left( x\right) }=\lim_{t\rightarrow 0^{+}}\frac{f\left( t\right) }{g\left( t\right) } \end{equation*} provide the indicated limits exist. While $H_{F,G}\left( 0^{+}\right) <0$,thereisatleasone$x^{\ast }>0$suchthat$F/G$ is increasing on $(0,x^{\ast })$ and decreasing on $\left( x^{\ast },\infty \right) $. As applications of this monotonicity rule, a unified treatment for certain bounds of psi function is presented, and some properties of the modified Bessel functions of the second are established. These show that the monotonicity rules in this paper may contribute to study for certain special functions because many special functions can be expressed as corresponding Laplace transforms.