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 A220978 #19 Aug 01 2015 10:40:44
%S A220978 1,19,217,2107,19441,176419,1592137,14342347,129120481,1162202419,
%T A220978 10460176057,94142647387,847287015121,7625592702019,68630363015977,
%U A220978 617673353237227,5559060437415361,50031544711579219,450283904728735897,4052555149532191867
%N A220978 a(n) = 3^(2*n+1) - 3^(n+1) + 1: The left Aurifeuillian factor of 3^(6*n+3) + 1.
%C A220978 The corresponding right Aurifeuillian factor is A198410(n+2): 3^(6*n+3) + 1 = (3^(2*n+1) + 1) * a(n) * A198410(n+2).
%H A220978 Wikipedia, <a href="http://en.wikipedia.org/wiki/Cunningham_project">Cunningham Project</a>
%H A220978 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (13, -39, 27).
%F A220978 a(n) = 13*a(n-1) - 39*a(n-2) + 27*a(n-3).
%F A220978 G.f.: (1 + 3*x)^2/((1 - x)*(1 - 3*x)*(1 - 9*x)).
%t A220978 Table[3^(2n+1) - 3^(n+1) + 1, {n, 0, 30}]
%t A220978 LinearRecurrence[{13,-39,27},{1,19,217},30] (* _Harvey P. Dale_, Mar 17 2013 *)
%o A220978 (PARI) Vec((1 + 3*x)^2/((1 - x)*(1 - 3*x)*(1 - 9*x)) + O(x^30)) \\ _Michel Marcus_, Feb 12 2015
%Y A220978 Cf. A092440, A085601, A198410, A220979-A220990.
%K A220978 nonn,easy
%O A220978 0,2
%A A220978 _Stuart Clary_, Dec 27 2012