And while I was exploring Artin's conjecture, I started looking at "full reptend primes" (primes p with decimal repeating block of maximal length p-1). In the MathWorld article on Full Reptend Primes, we find: A necessary (but not sufficient) condition that p be a full reptend prime is that the number 9R_(p-1) (where R_p is a repunit) is divisible by p, which is equivalent to 10^(p-1)-1 being divisible by p. For example, values of n such that 10^(n-1)-1 is divisible by n are given by 1, 3, 7, 9, 11, 13, 17, 19, 23, 29, 31, 33, 37, ... (Sloane's A104381). A104381 is titled "Numbers n such that 10^(n-1) == 1 (mod n)." By Fermat's Little Theorem, A104381 includes every prime except 2 and 5. Additionally, it contains the base-10 Fermat pseudoprimes, A005939. So the comment "Superset of full reptend primes" is not particularly noteworthy, it amounts to saying 2 and 5 are not full reptend primes.