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 A133140 #18 Jul 03 2023 08:52:18
%S A133140 2,4,6,10,18,34,66,130,258,514,1026,2050,4098,8194,16386,32770,65538,
%T A133140 131074,262146,524290,1048578,2097154,4194306,8388610,16777218,
%U A133140 33554434,67108866,134217730,268435458,536870914,1073741826,2147483650,4294967298
%N A133140 a(0) = 2, a(n) = 2^n + 2 for n>=1.
%C A133140 Sum of the coefficients of the polynomial Q(n,x)=(1+x)[(1+x)^(n-1)+x^(n-1)], Q(0,x)=2.
%C A133140 Also row sums of A133138. - _R. J. Mathar_, Jun 12 2008
%H A133140 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (3, -2).
%F A133140 a(n)=2*A094373(n). a(n)=A052548(n), n>0. - _R. J. Mathar_, Jun 12 2008
%t A133140 Table[If[n < 2, 2, 1] + Total[Table[Coefficient[Expand[(1 + x) ((1 + x)^(n - 1) + x^(n - 1))], x^m], {m, 1, n}]], {n, 0, 22}] (* _Mats Granvik_, May 19 2012 *)
%t A133140 Join[{2},2^Range[40]+2] (* _Harvey P. Dale_, Aug 16 2014 *)
%K A133140 nonn,easy
%O A133140 0,1
%A A133140 _Paul Curtz_, Sep 21 2007
%E A133140 Edited by _R. J. Mathar_, Jun 12 2008