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
25 (number) 25 (number) 25 (twenty-five) is the natural number following 24 and preceding 26. It is a square number, being 5 = 5 × 5. It is one of two two-digit numbers whose square and higher powers of the number also ends in the same last two digits, e.g. 25 = 625, the other is 76. It is the smallest square that is also a sum of two (non-zero) squares: 25 = 3 + 4. Hence it often appears in illustrations of the Pythagorean theorem. 25 is the sum of the single-digit odd natural numbers 1, 3, 5, 7 and 9,