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
Resident registration number websites require users to submit a valid resident registration number to create an account. This practice ties each registered account to a unique online identity, rather than allowing anonymous registration. Since only a few large websites allow alternate means of identification (such as an alien registration number or passport number), foreigners are unable to use most South Korean websites. The principal means of validating a resident registration number is to use an algorithm to check the last digit against what it should be based upon the rest of the digits entered (i.e. checksum verification) however this only ensures it is