The decimal expansion of 1/13 has a period of six digits. Find all primes p for which 1/p has a period of exactly** four **digits.

Guest Apr 13, 2019

#2**0 **

The reciprocal of every prime p (other than two and five) has a period, that is the decimal expansion of 1/p repeats in blocks of some set length. This period is the period of p. ... **For example: 3, 11, 37 and 101 are the only primes with periods one, two, three and four respectively−so they are unique-primes.**

**http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.693.5325&rep=rep1&type=pdf**

Guest Apr 14, 2019