OFFSET
1,1
COMMENTS
a(7) > 350028.
Exponents of Mersenne prime pairs {2^n - (2k + 1), (2k + 1)*2^n - 1}:
for k = 11: 6, 8, 12, ...
for k = 12: 9, ...
for k = 13: 5, 8, 10, ...
for k = 14:
EXAMPLE
a(1) = 4 because 2^4 - 5 = 11 and 5*2^4 - 1 = 79 are both primes.
MATHEMATICA
fQ[n_] := PrimeQ[2^n - 5] && PrimeQ[5*2^n - 1]; k = 1; While[ k < 15001, If[fQ@ k, Print@ k]; k++] (* Robert G. Wilson v, Mar 05 2014 *)
Select[Range[1000], PrimeQ[2^# - 5] && PrimeQ[5 2^# - 1] &] (* Vincenzo Librandi, May 17 2015 *)
PROG
(PARI) isok(n) = isprime(2^n - 5) && isprime(5*2^n - 1); \\ Michel Marcus, Mar 04 2014
(Magma) [n: n in [0..100] | IsPrime(2^n-5) and IsPrime(5*2^n-1)]; // Vincenzo Librandi, May 17 2015
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Ilya Lopatin and Juri-Stepan Gerasimov, Mar 04 2014
STATUS
approved