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 A124280 #16 Aug 25 2025 15:17:56
%S A124280 1,1,2,2,4,5,9,12,20,28,45,65,102,150,232,345,529,792,1208,1816,2761,
%T A124280 4161,6314,9530,14444,21821,33049,49956,75628,114356,173077,261761,
%U A124280 396110,599150,906576,1371377,2074913,3138864,4748976,7184304,10869329,16443521
%N A124280 Expansion of 1/(1-x-x^2+x^3-x^4).
%C A124280 Diagonal sums of number triangle A124279.
%H A124280 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (1,1,-1,1).
%F A124280 a(n) = Sum_{k=0..floor(n/2)} Sum_{j=0..n-2k}, C(j,n-2k-j)*C(k,n-2k-j) for n > 0.
%F A124280 a(n) = a(n-1)+a(n-2)-a(n-3)+a(n-4) for n > 3. - _Carl Najafi_, May 06 2014
%t A124280 CoefficientList[Series[1/(1-x-x^2+x^3-x^4),{x,0,37}],x] (* _Stefano Spezia_, Aug 25 2025 *)
%Y A124280 Cf. A124279.
%K A124280 easy,nonn,changed
%O A124280 0,3
%A A124280 _Paul Barry_, Oct 24 2006