Digital Root -- from Wolfram MathWorld

Consider the process of taking a number, taking its digit sum, then adding the digits of numbers derived from it, etc., until the remaining number has only one digit. The number of additions required to obtain a single digit from a number n in a given base is called the additive persistence of n, and the digit obtained is called the digital root of n. For example, the sequence obtained from the starting number 9876 in base 10 is (9876, 30, 3), so 9876 has an additive persistence of 2 and a...