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 required even for local calls. All cellphones in North America ignore this and only require the user to dial 10 digits. The added dialing requirement, coupled with the need to remember which of the area's coincident area codes applied to a seven-digit local number, damaged the popularity of overlay plans, which themselves were introduced as a means to reduce the inconveniences associated with the traditional split plans. As overlay plans have spread to more areas, 10-digit dialing in the U.S. and Canada is becoming increasingly common. However, most areas not within an overlay plan can still use 7-digit dialing for