Digital root s(x) represents the result of adding the various digits of x. When base > 1 and the digits are greater than 2, obviously s(x) < x, so that x will inevitably become a unit number and denoted as s*( x), the digital root of x.
Consider that for a k-ary number x (mod k-1), then a2 * k^2 + a1 * k + a0 = a2 + a1 + a0 (mod k-1), thus s(x) = x (mod k -1), resulting in the final digital root = x (mod k-1)
And only the root of 0 is 0, others > 0, so that the digital root can be solved quickly.
https://en.wikipedia.org/wiki/Digital_root