Approximations for the Bessel and Airy functions with an explicit error term

Abstract We show how one can obtain an asymptotic expression for some special functions with a very explicit error term starting from appropriate upper bounds. We will work out the details for the Bessel function J_ (x) and the Airy function Ai (x). In particular, we answer a question raised by Olenko and find a sharp bound on the difference between J_ (x) and its standard asymptotics. We also give a very simple and surprisingly precise approximation for the zeros Ai (x).