The Asymptotic Expansion of Kummer Functions for Large Values of the   $a$-Parameter, and Remarks on a Paper by Olver
  It is shown that a known asymptotic expansion of the Kummer function $U(a,b,z)$ as $a$ tends to infinity is valid for $z$ on the full Riemann surface of the logarithm. A corresponding result is also proved in a more general setting considered by Olver (1956). 
Hans Volkmer
math.CA