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 A177103 #13 Sep 08 2022 08:45:53 %S A177103 0,4,5,6,9,10,11,13,14,15,16,17,18,19,21,22,24,25,26,28,29,30,31,32, %T A177103 33,34,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,57, %U A177103 58,59,60,61,63,64,65,66,67,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84 %N A177103 Numbers n >= 0 such that 2*3^n - 1 is not prime. %H A177103 Vincenzo Librandi, <a href="/A177103/b177103.txt">Table of n, a(n) for n = 1..1000</a> %t A177103 Select[Range[0, 100], !PrimeQ[2*3^# - 1] &] (* _Vincenzo Librandi_, Oct 15 2012 *) %o A177103 (Magma) [ n: n in [0..100] | not IsPrime(2*3^n-1) ]; %Y A177103 Cf. A003307 (Numbers n such that 2*3^n-1 is prime). %K A177103 nonn,easy %O A177103 1,2 %A A177103 _Vincenzo Librandi_