A repunit Rn is any integer written in decimal form as a string of 1’s. Thus
repunits have the form Rn = 10n9−1 . The term repunit was coined by Beiler [1]
in 1966. The great effort was devoted to testing of primality and finding all
their prime factors. It easily can be seen that R2 is prime. Hoppe [4] proved R19
to be prime in 1916 and Lehmer [7] and Kraitchik [6] independently found R23
to be prime in 1929. Williams proved that R317 is prime in 1978 and Williams
and Dubner [10] proved that R1031 is prime in 1986. No other repunit primes
are not known, but in recent time four probably prime repunits have known. In
1999 Dubner [3] found R49081, Baxter discovered R86453 in 2000, Dubner found
R109297 in 2007 and Voznyy and Budnyy found R270343 in 2007.