Problem Statement

The digital root,

X

{\displaystyle X}

, of a number,

n

{\displaystyle n}

, is calculated: The additive persistence is the number of summations required to obtain the single digit. The task is to calculate the additive persistence and the digital root of a number, e.g.: The digital root may be calculated in bases other than 10.

Let's start with the solution:

(for N (627615 39390 588225 393900588225)
   (for ((A . I) N  T  (sum format (chop I)))
      (T (> 10 I)
         (prinl N " has additive persistance " (dec A) " and digital root of " I ";") ) ) )