Hi Carl and Acer,

Thank you very much!

The three of your solutions have worked for me!

Extra thanks to Carl for the MultiSeries:-asympt

Extra thanks to Acer for noting my mistake, the correct expression is

Gamma(x)*(y/2)^(-x)/2, it can be found here https://dlmf.nist.gov/10.30 as well as a lot of information on special functions

Best regards