The perturbed Bessel equation and Duality Theorem

The Euler-Gauss linear transformation formula for the
hypergeometric function was extended by Goursat for the case of
logarithmic singularities. By replacing the perturbed Bessel
differential equation by a monodromic functional equation, and
studying this equation separately from the differential equation by
an appropriate Laplace-Borel technique, we associate with the latter
equation another monodromic relation in the dual complex plane. This
enables us to prove a duality theorem and to extend Goursat's
formula to much larger classes of functions.