A272057 - OEIS

%I #10 Apr 20 2016 01:20:10

%S 1,2,3,9,17,19,32,38,47,103,108,153,162,229,235,637,1638,2102,2567,

%T 6338,7449,12845,20814,40165,61815,77965,117380,207420,351019,496350,

%U 600523,1156367,2117707,5742009,5865925,5947859

%N Numbers n such that 3*4^n - 1 is prime.

%C These are Williams primes to base 3.

%C Half of the even terms of A002235.

%H H. C. Williams, <a href="http://matwbn.icm.edu.pl/ksiazki/aa/aa39/aa3912.pdf">The primality of certain integers of the form 2Ar^n - 1</a>, Acta Arith. 39 (1981), 7-17.

%t Select[Range[0, 100000], PrimeQ[3*4^# - 1] &]

%o (PARI) for(n=1,10000, if(isprime(3*4^n-1), print1(n,", ")))

%Y Cf. A002235, A000043, A003307, A046865, A079906, A046866, A268061, A268356, A056725, A046867, A079907.

%K nonn,hard,more

%O 1,2

%A _Tim Johannes Ohrtmann_, Apr 19 2016

%E a(25) corrected and a(33)-a(36) added by _Giovanni Resta_, Apr 19 2016, using data from A002235.