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
National identification number Personal Numerical Code ("Cod Numeric Personal", CNP), which is created by using the citizen's gender and century of birth (1/3/5/7 for male, 2/4/6/8 for female and 9 for foreign citizen), date of birth (six digits, YYMMDD), the country zone (two digits, from 01 to 52, or 99), followed by a serial number (3 digits), and finally a checksum digit. The first digit encodes the gender of person as follows: The country zone is a code of Romanian county in alphabetical order. For Bucharest the code is 4 followed by the sector number. To calculate the checksum digit, every digit from