Misc,

On the remarkable properties of the pentagonal numbers

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(May 2005)

Abstract

In this paper Euler considers the properties of the pentagonal numbers, those numbers of the form \$3n^2 n2\$. He recalls that the infinite product \$(1-x)(1-x^2)(1-x^3)...\$ expands into an infinite series with exponents the pentagonal numbers, and tries substituting the roots of this infinite product into this infinite series. I am not sure what he is doing in some parts: in particular, he does some complicated calculations about the roots of unity and sums of them, their squares, reciprocals, etc., and also sums some divergent series such as 1-1-1+1+1-1-1+1+..., and I would appreciate any suggestions or corrections about these parts.

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