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A191469
Numbers n such that 7^n - 6 is prime.
11
2, 3, 6, 9, 21, 25, 33, 49, 54, 133, 245, 255, 318, 1023, 1486, 3334, 6821, 8555, 11605, 42502, 44409, 90291, 92511, 140303
OFFSET
1,1
COMMENTS
a(14)=1023 and a(15)=1486 correspond to BPSW strong probable primes (passing PARI's ispseudoprime()). - Joerg Arndt, Jun 06 2011
a(25) > 2*10^5. - Robert Price, Nov 14 2014
MAPLE
A191469:=n->`if`(isprime(7^n-6), n, NULL): seq(A191469(n), n=1..10^3); # Wesley Ivan Hurt, Nov 14 2014
MATHEMATICA
Select[Range[1, 5000], PrimeQ[7^#-6]&] (* Vincenzo Librandi, Aug 05 2012 *)
PROG
(Magma) [n: n in [1..1000]| IsPrime(7^n-6)]
(PARI) for(n=1, 10^6, if(isprime(7^n-6), print1(n, ", ")))
KEYWORD
nonn,hard,more
AUTHOR
Vincenzo Librandi, Jun 06 2011
EXTENSIONS
a(17)-a(23) from Robert Price, Jan 24 2014
a(24) from Robert Price, Nov 14 2014
STATUS
approved