O.g.f.: A(x) = 1 + 5*x + 109*x^2 + 22265*x^3 + 45300601*x^4 +...
where the g.f. may be expressed by the series identity:
A(x) = 1/(1-x) + 4*x/(1-x)^2 + 10^2*x^2/(1-x)^3 + 28^3*x^3/(1-x)^4 + 82^4*x^4/(1-x)^5 + 244^5*x^5/(1-x)^6 + 730^6*x^6/(1-x)^7 +...
A(x) = 1/(1-2*x) + 3*x/(1-4*x)^2 + 3^4*x^2/(1-10*x)^3 + 3^9*x^3/(1-28*x)^4 + 3^16*x^4/(1-82*x)^5 + 3^25*x^5/(1-244*x)^6 + 3^36*x^6/(1-730*x)^7 +...
Illustration of initial terms:
a(0) = 1;
a(1) = 1 + (1+3);
a(2) = 1 + 2*(1+3) + (1+3^2)^2;
a(3) = 1 + 3*(1+3) + 3*(1+3^2)^2 + (1+3^3)^3;
a(4) = 1 + 4*(1+3) + 6*(1+3^2)^2 + 4*(1+3^3)^3 + (1+3^4)^4;
a(5) = 1 + 5*(1+3) + 10*(1+3^2)^2 + 10*(1+3^3)^3 + 5*(1+3^4)^4 + (1+3^5)^5; ...
Also, by a binomial identity we have
a(0) = 1;
a(1) = 2 + 3;
a(2) = 2^2 + 2*(1+3)*3 + 3^4;
a(3) = 2^3 + 3*(1+3)^2*3 + 3*(1+3^2)*3^4 + 3^9;
a(4) = 2^4 + 4*(1+3)^3*3 + 6*(1+3^2)^2*3^4 + 4*(1+3^3)*3^9 + 3^16;
a(5) = 2^5 + 5*(1+3)^4*3 + 10*(1+3^2)^3*3^4 + 10*(1+3^3)^2*3^9 + 5*(1+3^4)*3^16 + 3^25; ...
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