A pyramidal number of the form n(n + 1)(5n - 2)/6, The first few are 1, 8, 26, 60, 115, ... (OEIS A002413). The generating function for the heptagonal pyramidal numbers is (x(4x + 1))/(x - 1)^4 = x + 8x^2 + 26x^3 + 60x^4 + ....