Digital root

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The base $b$ digital root of an integer $n$ is the result of repeatedly adding the base $b$ digits of $n$ until reaching a single digit. For example, the base 10 digital root of 1729 is 1 + 7 + 2 + 9 = 19 = 1 + 9 = 10 = 1 + 0 = 1. A much quicker way of course is to ignore most instances of $b-1$ at each step.