The numerals sum of irreducible proper fraction a/3n+2 is 9(t-1)/2+r,where t is the period of a/3n+2 and r is the least non-negative residue of a modulo 9.If the period of 1/p equal to p-1 or (p-1)/2,then p is a prime.

• 既约真分数a/3n+2的数码和为9(t-1)/2+r；这里t是a/3n+2的周期；r是a模9的最小非负剩余.；如果1/p的周期等于p-1或(p-1)/2；那么p是一个素数