For the large-t expansion, we follow the corresponding analysis of
(see e.g. ) and seek a solution of the form
. Indeed (2.7) has a unique solution of this
form, which when integrated according to (2.6) implies
As is well known (see e.g. ) this leaves the overall multiplicative constant
A unspecified. The large-t expansion of is obtained by substituting
(3.2) in (2.4).