One of three "magic numbers" referred to by Robert Anton Wilson in the Illuminatus! trilogy: the others are 5 and 23. Cunningly, these three account for all the single-digit prime numbers, allowing for almost any piece of numerical data to be related to them, since any number you can name has either 1, 2, 3, 5, or 7 in it, or 6 (2*3), 9 (3^2), 8 (1+7), 4 ((5+7)-(2^3)*1), and so on.