A trick on PDE?? Hi all. I am reading a text in mathematical wave theory. I saw and am confused by a manipulation of a PDE, as shown in the attached figure. I don't really undertand how the equation (1.9) is transformed by "introducing the charcteristic variables). (as indicated by the red arrow and the question mark) I guess there are some missing steps? Could somone fill in the missing links so that I could know what's going on there.