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 A242339 #28 Sep 08 2022 08:46:08 %S A242339 3,6,9,13,21,24,32,45,61,62,73,132,150,174,187,217,266,331,534 %N A242339 Numbers k such that k*8^k-1 is semiprime. %C A242339 The semiprimes of this form are: 1535, 1572863, 1207959551, 7146825580543, 193690812773950291967, 113336795588871485128703, 2535301200456458802993406410751, 1960026433464605549549037738806984897986559, ... %t A242339 Select[Range[80], PrimeOmega[# 8^# - 1]==2&] %o A242339 (Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [2..80] | IsSemiprime(s) where s is n*8^n-1]; %o A242339 (PARI) isok(n)=bigomega(n*8^n-1)==2 /* _Anders Hellström_, Aug 18 2015 */ %Y A242339 Cf. similar sequences listed in A242273. %Y A242339 Cf. A064754, A242201. %K A242339 nonn,more %O A242339 1,1 %A A242339 _Vincenzo Librandi_, May 12 2014 %E A242339 a(12)-a(19) from _Luke March_, Aug 14 2015