A191469 Numbers n such that 7^n - 6 is prime.

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

Author

Vincenzo Librandi, Jun 06 2011

Keywords

• nonn
• hard
• more

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

Crossrefs

Cf. A014224, A059266, A059613, A059614, A095714, A177093.

Cf. A236371, A217131, A191469, A090669, A096305, A217130, A217132, A152213.

Programs

Magma

[n: n in [1..1000]| IsPrime(7^n-6)]

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 *)

PARI

for(n=1, 10^6, if(isprime(7^n-6), print1(n, ", ")))

Extensions

a(17)-a(23) from Robert Price, Jan 24 2014

a(24) from Robert Price, Nov 14 2014