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
IMO number using its check digit. This is done by multiplying each of the first six digits by a factor of "2" to "7" corresponding to their position from right to left. The rightmost digit of this sum is the check digit. For example, for IMO 9074729: (9×"7") + (0×"6") + (7×"5") + (4×"4") + (7×"3") + (2×"2") = 139. IMO number The International Maritime Organization (IMO) number is a unique reference for ships, registered ship owners and management companies. IMO numbers were introduced to improve maritime safety and security and to reduce maritime fraud. They consist of the three letters "IMO"