199

199 is a centered triangular number. It is a prime number and the fourth part of a prime quadruplet: 191, 193, 197, 199. 199 is the smallest natural number that takes more than two iterations to compute its digital root as a repeated digit sum: 199 ↦ 1 + 9 + 9 = 19 ↦ 1 + 9 = 10 ↦ 1 + 0 = 1. {\displaystyle {\begin{aligned}199&\mapsto 1+9+9=19\&\mapsto 1+9=10\&\mapsto 1+0=1.\end{aligned}}} Thus, its additive persistence is three, and it is the smallest number of persistence three.