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 A242203 #31 Sep 08 2022 08:46:08 %S A242203 1,3,10,16,20,22,24,34,39,56,63,108,128,194,202,212,214,218,314,364, %T A242203 662,722 %N A242203 Numbers n such that n*3^n + 1 is semiprime. %C A242203 The semiprimes of this form are 4, 82, 590491, 688747537, 69735688021, 690383311399, 6778308875545, 567024177788663347, 158049650967740074414, 29307467449532190083956645177, ... %C A242203 a(23) >= 894. - _Hugo Pfoertner_, Aug 03 2019 %H A242203 <a href="http://factordb.com/index.php?query=894*3%5E894%2B1">Status of 894*3^894+1 in factordb.com</a>. %t A242203 Select[Range[130], PrimeOmega[# 3^# + 1] == 2 &] %o A242203 (Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [1..130] | IsSemiprime(s) where s is n*3^n+1]; %o A242203 (PARI) isok(n) = bigomega(n*3^n + 1)==2; \\ _Michel Marcus_, Mar 30 2019 %Y A242203 Cf. numbers n such that n*k^n + 1 is semiprime: A242175 (k=2), this sequence (k=3), A242204 (k=4), A242205 (k=5), A242269 (k=6), A242270 (k=7), A242271 (k=8), A242272 (k=9), A216378 (k=10). %Y A242203 Cf. A006552, A050914. %K A242203 nonn,more %O A242203 1,2 %A A242203 _Vincenzo Librandi_, May 10 2014 %E A242203 a(14)-a(20) from _Luke March_, Jul 30 2015 %E A242203 a(21)-a(22) from _Daniel Suteu_, Mar 30 2019