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 A231968 #4 Nov 20 2013 13:39:36 %S A231968 2530,59455,4112125651,66487065301,1069364586601,4313828719351, %T A231968 146264252084551,252626415665851,3398860950259501,10045301361318901, %U A231968 110736954977078251 %N A231968 Squarefree numbers (A005117) of the form p*q*r*s with prime factors p, q, r, s with q = 2*p + 1, r = 2*q + 1 and s = 2*r + 1. %e A231968 4112125651 = 89*179*359*719; 179 = 2*89 + 1; 359 = 2*179 + 1, 719 = 2*359 + 1. %Y A231968 Cf. A005117, A000040, A231967, A231969, A231966. %K A231968 nonn %O A231968 1,1 %A A231968 _Jaroslav Krizek_, Nov 18 2013