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 A216271 #14 Feb 01 2020 19:04:21
%S A216271 1,5,21,83,319,1209,4549,17051,63783,238337,890077,3322995,12403951,
%T A216271 46296905,172791861,644886923,2406788599,8982333009,33522674509,
%U A216271 125108627171,466912358463,1742541855257,6503257159717,24270490977915,90578715140551,338044386361505,1261598863859901
%N A216271 Expansion of (1-x)/((1-2x)(1-4x+x^2)).
%C A216271 Partial sums are in A216263.
%C A216271 Diagonal of square array A214846.
%H A216271 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (6,-9,2).
%F A216271 a(n) = A001353(n+2) - A087946(n+1).
%F A216271 G.f.: (1-x)/(1-6x+9x^2-2x^3).
%F A216271 a(n) = 6*a(n-1) - 9*a(n-2) + 2*a(n-3), a(0) = 1, a(1) = 5, a(2) = 21.
%F A216271 Sum_{k=0..n} a(k) = A216263(n).
%t A216271 CoefficientList[Series[(1-x)/((1-2x)(1-4x+x^2)),{x,0,30}],x] (* _Harvey P. Dale_, Oct 05 2019 *)
%Y A216271 Cf. A001353, A087946, A214846, A216263.
%K A216271 nonn,easy
%O A216271 0,2
%A A216271 _Philippe Deléham_, Mar 16 2013