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 A216888 #33 Jul 18 2025 05:22:03 %S A216888 0,1,3,4,5,8,15,16,29,53,56,59,64,131,179,319,695,781,821,896,1251, %T A216888 1453,4216,5479,6224,7841,12095,13781,17719,43955,64821,82779,105105, %U A216888 152528,165895,191813,529679,1074725,1086111,1175231,1277861,1346541,3123035,3648968,5570080,6236771,10852676 %N A216888 Numbers k such that 6*3^k + 1 is prime. %F A216888 a(n) = A003306(n+1)-1. - _Bruno Berselli_, Sep 27 2012 %e A216888 3 is a term because 6*3^3 + 1 = 163 is prime. %e A216888 7 is not a term because 6*3^7 + 1 = 13123 = 11*1193 is composite. %t A216888 Select[Range[5000], PrimeQ[6 3^# + 1] &] %o A216888 (Magma) /* Gives only the terms up to 1453: */ [n: n in [0..1500] | IsPrime(6*3^n + 1)]; %o A216888 (PARI) is(n)=ispseudoprime(6*3^n+1) \\ _Charles R Greathouse IV_, Jun 13 2017 %Y A216888 Cf. A003306, A005537, A005538, A005539; A056800, A056805, A143279. %Y A216888 Associated primes are in A111974. %K A216888 nonn,hard %O A216888 1,3 %A A216888 _Vincenzo Librandi_, Sep 26 2012 %E A216888 More terms from _Vincenzo Librandi_, Oct 01 2012 %E A216888 a(41)-a(47) from the data at A003306 added by _Amiram Eldar_, Jul 18 2025