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
Ten-digit dialing Ten-digit dialing In the United States and Canada, ten-digit dialing is the practice of including the area code of a telephone number when dialing to initiate a telephone call. When necessary, a ten-digit number may be prefixed with the trunk code "1", which is often referred to as "11-digit dialing" or "national format". After the implementation of the North American Numbering Plan, placing a local call within the caller's area code involved only seven-digit dialing. Callers dialed only the central office code (three digits) and the four-digit station number of the telephone subscriber to reach. For example, a person at