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 A121205 #9 Apr 15 2018 07:38:21
%S A121205 342,438,546,666,798,942,1098,1266,1446,1638,1842,2058,2286,2526,2778,
%T A121205 3042,3318,3606,3906,4218,4542,4878,5226,5586,5958,6342,6738,7146,
%U A121205 7566,7998,8442,8898,9366,9846,10338,10842,11358,11886,12426
%N A121205 "666" in bases 7 and higher rewritten in base 10.
%H A121205 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (3,-3,1).
%F A121205 a(n) = 6*(n^2 + n + 1) = a(n-1) + 12*n.
%e A121205 In octal notation "666" means 6*8*8 + 6*8 + 6 = 438.
%t A121205 Table[FromDigits[{6,6,6},n],{n,7,50}] (* or *) LinearRecurrence[{3,-3,1},{342,438,546},40] (* _Harvey P. Dale_, Apr 15 2018 *)
%o A121205 (PARI) a(n) = 6*(n^2 + n + 1); \\ _Michel Marcus_, Aug 20 2013
%K A121205 easy,nonn
%O A121205 7,1
%A A121205 _Axel Harvey_, Aug 19 2006
%E A121205 More terms from _Michel Marcus_, Aug 20 2013