Persistence of a number assumed. The single-digit final state reached in the process of calculating an integer's additive persistence is its digital root. Put another way, a number's additive persistence counts how many times we must sum its digits to arrive at its digital root. The additive persistence of 2718 is 2: first we find that 2 + 7 + 1 + 8 = 18, and then that 1 + 8 = 9. The multiplicative persistence of 39 is 3, because it takes three steps to reduce 39 to a single digit: 39 → 27 → 14 → 4. Also, 39 is the smallest
Missing-digit sum Missing-digit sum Missing-digit sums are integer numbers that are equal to the sum of numbers created by deleting one or more digits at a time from the original number. For example, the OEIS lists these two integers as missing-digit sums in base ten: Missing-digit sums are therefore a subset of narcissistic numbers, when these are defined as numbers that are equal to some manipulation of their own digits (for example, 153 and 132 are narcissistic numbers in base ten because 153 = 1 + 5 + 3 and 132 = 13 + 32 + 12 + 31 + 23 +