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 A117576 #9 Jun 13 2015 00:52:05 %S A117576 1,0,-1,-1,3,1,-5,-3,11,5,-21,-11,43,21,-85,-43,171,85,-341,-171,683, %T A117576 341,-1365,-683,2731,1365,-5461,-2731,10923,5461,-21845,-10923,43691, %U A117576 21845,-87381,-43691,174763,87381,-349525,-174763,699051 %N A117576 Expansion of (1-x^3)/((1-x^2)(1+2x^2)). %C A117576 A signed pair-reversal of the Jacobsthal numbers A001045. Diagonal sums of A116949. %H A117576 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (-1,-2,-2). %F A117576 G.f.: (1+x+x^2)/(1+x+2x^2+2x^3); a(n)=-a(n-1)-2a(n-2)-2a(n-3); a(n)=2^(n/2)(2*cos(pi*n/2)/3+sqrt(2)*sin(pi*n/2)/6)+(-1)^n/3; %F A117576 a(n) = floor(((-1)^(floor(n/2))*2^(2*floor(n/2)+1-floor((n+1)/2))+1)/3). - _Tani Akinari_, Nov 09 2012 %Y A117576 Cf. A112447. %K A117576 easy,sign %O A117576 0,5 %A A117576 _Paul Barry_, Mar 29 2006