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 A124342 #7 Jan 25 2016 12:36:23
%S A124342 1,-1,2,-2,2,0,-4,12,-24,40,-56,64,-48,-16,160,-416,800,-1280,1728,
%T A124342 -1856,1152,1152,-6016,14336,-26368,40704,-52736,52736,-24064,-57344,
%U A124342 220160,-488448,862208,-1284096,1591296,-1458176,348160
%N A124342 Expansion of (1+x)/(1+2x-2x^3).
%C A124342 Diagonal sums of A124341. Binomial transform has g.f. (1-x)/(1-x-x^2-x^3).
%H A124342 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (-2,0,2).
%F A124342 a(n)=sum{k=0..floor(n/2), sum{j=0..n-k, (-1)^(n-k-j)*C(n-k,j)*C(k,j-k)}}
%F A124342 a(n) = (-1)^(n+1)*A073358(n+1). - _R. J. Mathar_, Feb 04 2014
%F A124342 a(n) = A077988(n-1)+A077988(n). - _R. J. Mathar_, Jan 25 2016
%K A124342 easy,sign
%O A124342 0,3
%A A124342 _Paul Barry_, Oct 26 2006