On the log-concavity of the composite Bessel function $x^αJ_{ν}\left( βx^γ\right) $

Abstract

For a twice differentiable function $f:\left( a,b\right) \rightarrow \mathbb{R}$ define $v\left( f\right) =f^{\prime}f^{\prime}-f^{\prime\prime }f.$ It is well known that the positivity of $v\left( f\right) $ implies that the function $\left\vert f\right\vert $ is strictly log-concave on each subinterval which does not contain zeros of $f.$ In this paper we provide criteria for the positivity of $v\left( F\right) $ for the composite Bessel function $F\left( x\right) =J_{α,β,γ,ν}\left( x\right) :=x^αJ_ν\left( βx^γ\right) $ for positive numbers $β$ and $γ$ and real numbers $α$ and $ν.$