An Ancient Diophantine Equation with applications to Numerical Curios and Geometric Series

In this paper we examine the diophantine equation $x^k-y^k=x-y$ where $k$ is a positive integer $\geq 2$, and consider its applications. While the complete solution of the equation $x^k-y^k=x-y$ in positive rational numbers is already known when $k=2$ or $3$, till now only one numerical solution of the equation in positive rational numbers has been published when $k=4$, and no nontrivial solution is known when $k \geq 5$. We describe a method of generating infinitely many positive rational solutions of the equation when $k=4$. We use the positive rational solutions of the equation with $k=2,\, 3$ or 4 to produce numerical curios involving square roots, cube roots and fourth roots, and as another application of these solutions, we show how to construct examples of geometric series with an interesting property.