*(English)*Zbl 0703.33002

The main objective of the author is to give some formulas of practical importance connected with certain particular solutions of Besselâ€™s equation. The particular solutions considered in the paper are defined in terms of classical ones as follows: ${F}_{\mu}\left(z\right)=\frac{1}{2}\{{e}^{\mu \pi i/2}{H}_{\mu}^{\left(1\right)}\left(z\right)+{e}^{-\mu \pi i/2}{H}_{\mu}^{\left(2\right)}\left(z\right)\},{G}_{\mu}\left(z\right)=\frac{1}{2i}\{{e}^{\mu \pi i/2}{H}_{\mu}^{\left(1\right)}\left(z\right)-{e}^{-\mu \pi i/2}{H}_{\mu}^{\left(2\right)}\left(z\right)\},{L}_{\mu}\left(z\right)=\frac{\pi i}{2sin\mu \pi}\{{I}_{\mu}\left(z\right)+{I}_{-\mu}\left(z\right)\},\phantom{\rule{1.em}{0ex}}(\mu \ne 0)\xb7$For these functions recurrence relations, analytic continuation formulas, power series and integral representations, asymptotic expressions for the zeros as well as some uniformly valid asymptotic expansions are derived. The derivations are achieved, in general, by a direct use of known results related to the classical solutions. A particular emphasis is given to the case of purely imaginary order where $\mu =i\nu $ with positive $\nu $. By using these latter a rather general equation of the form

is examined for large u.

##### MSC:

33C10 | Bessel and Airy functions, cylinder functions, ${}_{0}{F}_{1}$ |

34E20 | Asymptotic singular perturbations, turning point theory, WKB methods (ODE) |