On a Neumann-type series for modified Bessel functions of the first kind

In this paper, we are interested in a Neumann-type series for modified Bessel functions of the first kind which arises in the study of Dunkl operators associated with dihedral groups and as an instance of the Laguerre semigroup constructed by Ben Said-Kobayashi-Orsted. We first revisit the particular case corresponding to the group of square-preserving symmetries for which we give two new and different proofs other than the existing ones. The first proof uses the expansion of powers in a Neumann series of Bessel functions while the second one is based on a quadratic transformation for the Gauss hypergeometric function and opens the way to derive further expressions when the orders of the underlying dihedral groups are powers of two. More generally, we give another proof of De Bie \& al formula expressing this series as a $\Phi_2$-Horn confluent hypergeometric function. In the course of proving, we shed the light on the occurrence of multiple angles in their formula through elementary symmetric functions, and get a new representation of Gegenbauer polynomials.