  Most of theoretical physics is based on the mathematics of functions of a
real or a complex variable; yet we frequently are drawn to try extending our
reach to include quaternions. The non-commutativity of the quaternion algebra
poses obstacles for the usual manipulations of calculus; but we show in this
paper how many of those obstacles can be overcome. The surprising result is
that the first order term in the expansion of F(x+delta) is a compact formula
involving both F'(x) and [F(x) - F(x*)]/(x-x*). This advance in the
differential calculus for quaternionic variables also leads us to some progress
in studying integration.
