15.4 Digital root sequences 429
Project: Sum of squares of digits
Letf(n)be sum of the digits ofn. For example,f(1) = 1,f(2) = 4,
f(10) = 1,f(12) = 5,f(123) = 14etc.
Study the iterations off.
For example, the iterations offbeginning with 4 leads to the cycle
4 → 16 → 37 → 58 → 89 → 145 → 42 → 20 → 4.