Heatley originally defined the function in terms of the integral
(2)
where is a modified Bessel function of the first kind, which is similar to an integral of Watson (1966, p. 394), with Watson's changed to and a few other minor changes of variables. In terms of this function,
(3)
(Heatley 1943). Heatley (1943) also gives a number of recurrences and other identities satisfied by .