I have followed this discussion with some interest, as I find the Vortex method intriguing. Barry has shown a mathematical difference between the methods that is real. Conrad claims the methods are mathematically equivalent, but they are only equivalent in intent.
Take the two equations [1] S * (pc-y) (Conrad) and [2] pc - y * (1+s) (Lichello). It is clear that [1] can only be negative with y > pc, while [2] can still be negative when y < pc, or, put differently, only becomes negative when y > pc * (1+s). Obviously, a harmonization of these cases would require the equality pc = pc * (1+s), which requires s=0, or zero safe.
But more interesting than the differences would be the respective behaviors, IMO! Perhaps we could study that?
Regards,
karel
