Numbers n such that (n, sigma(n)) lies on the hyperbola y^2 - x^2 = m^2, for some natural number m, i.e., sigma(n)^2 - n^2 = m^2.

A066784

Numbers n such that (n, sigma(n)) lies on the hyperbola y^2 - x^2 = m^2, for some natural number m, i.e., sigma(n)^2 - n^2 = m^2.