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 A217777 #17 Oct 02 2023 13:47:00 %S A217777 1,2,4,8,15,30,55,110,200,400,725,1450,2625,5250,9500,19000,34375, %T A217777 68750,124375,248750,450000,900000,1628125,3256250,5890625,11781250, %U A217777 21312500,42625000,77109375,154218750,278984375,557968750,1009375000,2018750000,3651953125 %N A217777 Expansion of (1+x)*(1+2*x)*(1-x)/(1-5*x^2+5*x^4). %H A217777 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (0, 5, 0, -5). %F A217777 a(n) = A216212(n+1)/2. %F A217777 a(2n) = A039717(n+1), a(2n+1) = 2*a(2n) = 2*A039717(n+1). %F A217777 a(n) = sum(A217770(n-k,k), 0<=k<=n). %F A217777 a(n) = 5*a(n-2) - 5*a(n-4) for n>=4, a(0) = 1, a(1) = 2, a(2) = 4, a(3) = 8. %F A217777 G.f.: (1+x)*(1+2*x)*(1-x)/(1-5*x^2+5*x^4). %o A217777 (PARI) Vec((1+x)*(1+2*x)*(1-x)/(1-5*x^2+5*x^4)+O(x^66)) /* _Joerg Arndt_, Mar 29 2013 */ %Y A217777 Cf. A217770 %K A217777 nonn,easy %O A217777 0,2 %A A217777 _Philippe Deléham_, Mar 24 2013 %E A217777 Corrected name (g.f.), _Joerg Arndt_, Mar 29 2013