A Størmer number is a positive integer n for which the greatest prime factor p of n^2 + 1 is at least 2n. Every Gregory number t_x can be expressed uniquely as a sum of t_ns where the ns are Størmer numbers. The first few Størmer numbers are given by Conway and Guy and Todd and are given by n = 1, 2, 4, 5, 6, 9, 10, 11, 12, 14, 15, 16, 19, 20, ... (OEIS A005528), corresponding to greatest prime factors 2, 5, 17, 13, 37, 41, 101, 61, 29, ... (OEIS A005529).

greatest prime factor | Gregory number | inverse tangent