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
Personal identity number (Sweden) of deceased people are reassigned to organisation numbers, in order to free up Personal identity numbers for future children. To calculate the checksum, multiply the individual digits in the 10-digit identity number and "212121-212". The resulting products (a two digit product, such as "16", would be converted to "1 + 6") are added together. The checksum is the last digit of "10" minus the last digit in this sum (note that if the last digit of the sum is zero, the checksum is "0"). For further details, see Luhn algorithm. For 12-digit numbers, the two first digits are omitted from