15

15 = 3 * 5

15 = 1 + 2 + 3 + 4 + 5 (5th triangular number)

2¹⁵ - 1 = 7 * 31 * 151

10¹⁵ - 1 = 3 * 3 * 3 * 31 * 37 * 41 * 271 * 2906161

14 - 15 - 16