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 A275669 #23 Sep 08 2022 08:46:17 %S A275669 3,5,7,9,11,12,13,15,17,19,21,22,23,25,26,27,29,31,32,33,35,37,39,40, %T A275669 41,42,43,45,47,48,49,51,52,53,54,55,57,59,61,62,63,65,67,68,69,70,71, %U A275669 72,73,74,75,77,79 %N A275669 Numbers k such that 3*k-1 is composite. %C A275669 Numbers n that can be expressed as 3xy+x-y for x>0, y>0. %H A275669 Vincenzo Librandi, <a href="/A275669/b275669.txt">Table of n, a(n) for n = 1..4100</a> %e A275669 a(1) = 3 because 3*3-1=8 is composite. %e A275669 a(2) = 5 because 3*5-1=14 is composite. %e A275669 1, 2, 4 give 2, 5, 11 which are primes, so 1, 2 and 4 are not terms. %t A275669 Select[Range@ 120, CompositeQ[3 # - 1] &] (* _Michael De Vlieger_, Aug 06 2016 *) %t A275669 Select[Range[200], !PrimeQ[3 # - 1] &] (* _Vincenzo Librandi_, Jun 09 2017 *) %o A275669 (PARI) isok(n) = !isprime(3*n-1); \\ _Michel Marcus_, Aug 04 2016 %o A275669 (Magma) [n: n in [2..90]| not IsPrime(3*n-1)]; // _Vincenzo Librandi_, Jun 09 2017 %Y A275669 Cf. A003627. %K A275669 nonn,easy %O A275669 1,1 %A A275669 _Ron R Spencer_, Aug 04 2016