There are two types of functions known as Hankel functions. The more common one is a complex function (also called a Bessel function of the third kind, or Weber Function) which is a linear combination of Bessel functions of the first and second kinds.
Another type of Hankel function is defined by the contour integral
for ,
,
, where
is a Hankel contour.
The Riemann zeta function can be expressed
in terms of
as
for
and
,
where
is the gamma function (Krantz 1999, p. 160).