giving an abundant but rather uninteresting supply of solutions to equation (2). It is natural to restrict ones attention to the primitive solutions, which is what we will do from now on. Equations (1), (2) and (3) also have certain obvious “trivial” solutions: a solution is called trivial if xyz = 0 or ±1, and is called non-trivial otherwise. The work of Hellegouarch, Frey [12], Serre [27], an...