I am currently reading this paper Physics paper
please have a look at the definition of (20) and then (36). In (36) they investigate the limit of the hypergeometric function $_2F_1(\alpha,\beta;\gamma;\xi)$ for $\xi \rightarrow - \infty$ and they argue to get an actual result. I mean how is this possible?
I always thought that this function is only defined for $|\xi| \le 1$ like here enter link description here