A family of entire functions connecting the Bessel function $J_1$ and the Lambert $W$ function

Abstract

Motivated by the problem of determining the values of $\alpha>0$ for which $f_\alpha(x)=e^\alpha - (1+1/x)^{\alpha x},\ x>0$ is a completely monotonic function, we combine Fourier analysis with complex analysis to find a family $\varphi_\alpha$, $\alpha>0$, of entire functions such that $f_\alpha(x) =\int_0^\infty e^{-sx}\varphi_\alpha(s)\,ds, \ x>0.$ We show that each function $\varphi_\alpha$ has an expansion in power series, whose coefficients are determined in terms of Bell polynomials. This expansion leads to several properties of the functions $\varphi_\alpha$, which turn out to be related to the well known Bessel function $J_1$ and the Lambert $W$ function. On the other hand, by numerically evaluating the series expansion, we are able to show the behavior of $\varphi_\alpha$ as $\alpha$ increases from $0$ to $\infty$ and to obtain a very precise approximation of the largest $\alpha>0$ such that $\varphi_\alpha(s)\geq0,\, s>0$, or equivalently, such that $f_\alpha$ is completely monotonic.