A number like 11, 111, or 1111 i.e. a number containing only the digit 1
is called
repunit.
List all base8 prime repunits.
Let 8(n) denote the nth repunit, mod 8.
The even cases are trivial; each is divisible by 3, which has period 2 mod the sequence.
Other primes having very short periods mod the sequence are 31 (period 5) 127 (period 7) etc. 3,31,127 etc are Mersenne primes. It would be tempting to suggest a rule that 8(n) is divisible by Mersenne(n) with period p. Certainly, 8191 divides 8(13) and 131071 divides 8(17) while 524287 divides 8(19); but this is not true of 7 (period 7), nor does it account for 73, which is not a Mersenne Prime, yet has a period length of 3.
The rule 8(n) is of the form (an+1)(bn+1)(cn+1)..., for as many prime factors as needed applies to all the prime entries I have checked, 7 again excepted  299593 has factors 7, (18*7+1), (48*7+1).
Edited on November 25, 2016, 11:16 pm

Posted by broll
on 20161125 22:20:47 