ABSTRACT
Let X, Y be the functions on R2 defined by X(x, y) = x, Y(x ,y) = y. So the identity Z(x,y) = (x,y) on R2 is Z = (X, Y) and dZ — (dX, dY). For F = (p , q) any mapping of R2 into R2, F · dZ = p d X + q d Y . In conformity with traditional notation we shall frequently use “x” , “y” in place of “X ” , “Y”.
