pub(crate) fn hankel_below(order: u32, z: Complex) -> Result<[Complex; 2]>Expand description
Outgoing cylindrical functions [H_(m-1)(z), H_m(z)] from one AMOS sequence, with
H_-1 = -H_1: the derivative u^m H_(m-1)(u) of u^m H_m(u) without the
cancelling product rule.