" Hence 8197. He liked 97 because it was the largest two-digit prime number, and he loved 81 because it was absolutely the only number out of all the literally infinite possibilities whose square root was also the sum of its digits. Square root of eighty-one was nine, and eight and one made nine. No other nontrivial number in the cosmos had that kind of sweet symmetry. Perfect. "

― Lee Child , Bad Luck and Trouble (Jack Reacher, #11)