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 A027261 #21 Oct 26 2019 16:10:45
%S A027261 1,4,18,72,270,972,3402,11664,39366,131220,433026,1417176,4605822,
%T A027261 14880348,47829690,153055008,487862838,1549681956,4907326194,
%U A027261 15496819560,48814981614,153418513644,481176247338,1506290861232,4707158941350,14686335897012,45753584909922
%N A027261 a(n) = Sum_{k=0..2n} (k+1) * A025177(n, k).
%F A027261 a(n) = 2(n+1)*3^(n-1), for n>1 (conjectured). - _Ralf Stephan_, Feb 02 2004
%F A027261 From _Colin Barker_, Jul 28 2012: (Start)
%F A027261 Conjecture: a(n) = 6*a(n-1)-9*a(n-2), for n>3.
%F A027261 G.f.: (1-9*x^2+18*x^3)/(1-3*x)^2. (End)
%Y A027261 Cf. A014915, A064017, A079272.
%Y A027261 Cf. A006234.
%K A027261 nonn
%O A027261 0,2
%A A027261 _Clark Kimberling_
%E A027261 a(1) corrected and more terms from _Sean A. Irvine_, Oct 26 2019