This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A272057 #10 Apr 20 2016 01:20:10 %S A272057 1,2,3,9,17,19,32,38,47,103,108,153,162,229,235,637,1638,2102,2567, %T A272057 6338,7449,12845,20814,40165,61815,77965,117380,207420,351019,496350, %U A272057 600523,1156367,2117707,5742009,5865925,5947859 %N A272057 Numbers n such that 3*4^n - 1 is prime. %C A272057 These are Williams primes to base 3. %C A272057 Half of the even terms of A002235. %H A272057 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 A272057 Select[Range[0, 100000], PrimeQ[3*4^# - 1] &] %o A272057 (PARI) for(n=1,10000, if(isprime(3*4^n-1), print1(n,", "))) %Y A272057 Cf. A002235, A000043, A003307, A046865, A079906, A046866, A268061, A268356, A056725, A046867, A079907. %K A272057 nonn,hard,more %O A272057 1,2 %A A272057 _Tim Johannes Ohrtmann_, Apr 19 2016 %E A272057 a(25) corrected and a(33)-a(36) added by _Giovanni Resta_, Apr 19 2016, using data from A002235.