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 A087223 #12 Oct 11 2020 05:52:01
%S A087223 1,2,5,14,36,96,254,676,1792,4756,12621,33490,88868,235818,625764,
%T A087223 1660510,4406296,11692452,31026836,82332140,218474784,579739960,
%U A087223 1538385398,4082226194,10832507040,28744906148,76276860598,202406625820
%N A087223 G.f. satisfies A(x) = f(x) + x*A(x)*f(x)^3, where f(x) = Sum_{k>=0} x^((4^k-1)/3).
%F A087223 a(n) = A087221(3n+1).
%e A087223 Given f(x) = 1 + x + x^5 + x^21 + x^85 + x^341 + ...
%e A087223 so that f(x)^3 = 1 + 3x + 3x^2 + x^3 + 3x^5 + 6x^6 + 3x^7 + 3x^10 + ...
%e A087223 then A(x) = (1 + x + x^5 + ...) + x*A(x)*(1 + 3x + 3x^2 + x^3 + 3x^5 + 6x^6 + ...)
%e A087223 = 1 + 2x + 5x^2 + 14x^3 + 36x^4 + 96x^5 + 254x^6 + ...
%o A087223 (PARI) a(n)=local(A,m); if(n<1,n==0,m=1; A=1+O(x); while(m<=3*n+3,m*=4; A=1/(1/subst(A,x,x^4)-x)); polcoeff(A,3*n+1))
%Y A087223 Cf. A087221, A087222, A087224.
%K A087223 nonn
%O A087223 0,2
%A A087223 _Paul D. Hanna_, Aug 27 2003