OFFSET
1,1
COMMENTS
Known divisors of Mersenne(p) (2^p-1 or Mp for short) are of the form 2*k*p+1. See crossrefs for other k's. If k == 2 (mod 4), there are no such divisors in general. Here k is 14/2 = 7.
LINKS
EXAMPLE
See LINKS for example of a(13).
MATHEMATICA
Select[Range[45000], PrimeQ[#] && PowerMod[2, #, 14*# + 1] == 1 &] (* Amiram Eldar, Jan 27 2022 *)
PROG
(PARI) forprime(p=1, 1e6, if (Mod(2, p*14+1)^p==1, print1(p, ", ")))
(Python) from sympy import sieve
print([p for p in sieve[1:1000000] if pow(2, p, 14*p+1) == 1])
CROSSREFS
KEYWORD
nonn
AUTHOR
Karl-Heinz Hofmann, Jan 27 2022
STATUS
approved