“Passing then to augmented series... and diminished... or altered... constituted from sums or differences of two or more other series. ...[I]t was not too difficult to relate everything to series of equals... I have continued the investigation with the same success not only for these series, ...but also for those which are as the squares, cubes, or any higher power... Where at the same time we made use of the figurate numbers, thus triangular, pyramidal, etc... and their distinguishing features were unexpectedly uncovered.”