Throughout this subsection δ is an arbitrary small positive constant.
To obtain approximations for U(a,-z) and V(a,-z) as z→∞ combine the results above
with (12.2.15) and (12.2.16). See also Temme (2015, Chapter 11).
Bounds and re-expansions for the error term in (12.9.1) can be
obtained by use of (12.7.14) and §§13.7(ii),
13.7(iii). Corresponding results for (12.9.2) can be
obtained via (12.2.20).