Other notations and names for include
(Kölbig et al. (1970)), Spence function
(’t Hooft and Veltman (1979)), and (Maximon (2003)).
In the complex plane has a branch point at . The
principal branch has a cut along the interval and agrees with
(25.12.1) when ; see also §4.2(i).
The remainder of the equations in this
subsection apply to principal branches.