G.f.: A(x) = x + 2*x^2 + 4*x^3 + 13*x^4 + 41*x^5 + 144*x^6 + 533*x^7 + 2072*x^8 + 8463*x^9 + 36142*x^10 + 160852*x^11 + 744491*x^12 + ...
where
A(x) = 1 * ((1+x) - 1) +
x * ((1+x)^2 - 1)*((1+x) - x) +
x^2 * ((1+x)^3 - 1)*((1+x)^2 - x)*((1+x) - x^2) +
x^3 * ((1+x)^4 - 1)*((1+x)^3 - x)*((1+x)^2 - x^2)*((1+x) - x^3) +
x^4 * ((1+x)^5 - 1)*((1+x)^4 - x)*((1+x)^3 - x^2)*((1+x)^2 - x^3)*((1+x) - x^4) +
x^5 * ((1+x)^6 - 1)*((1+x)^5 - x)*((1+x)^4 - x^2)*((1+x)^3 - x^3)*((1+x)^2 - x^4)*((1+x) - x^5) +
x^6 * ((1+x)^7 - 1)*((1+x)^6 - x)*((1+x)^5 - x^2)*((1+x)^4 - x^3)*((1+x)^3 - x^4)*((1+x)^2 - x^5)*((1+x) - x^6) + ...
equivalently,
A(x) = x +
(2*x^2 + x^3) +
(3*x^3 + 9*x^4 + 10*x^5 + 5*x^6 - 2*x^7 - 3*x^8 - x^9) +
(4*x^4 + 26*x^5 + 78*x^6 + 139*x^7 + 147*x^8 + 73*x^9 - 25*x^10 - 65*x^11 - 45*x^12 - 15*x^13 - 2*x^14) +
(5*x^5 + 55*x^6 + 290*x^7 + 965*x^8 + 2226*x^9 + 3689*x^10 + 4378*x^11 + 3463*x^12 + 1184*x^13 - 1161*x^14 - 2296*x^15 - 2002*x^16 - 1034*x^17 - 239*x^18 + 85*x^19 + 102*x^20 + 44*x^21 + 10*x^22 + x^23) +
(6*x^6 + 99*x^7 + 794*x^8 + 4099*x^9 + 15185*x^10 + 42667*x^11 + 93837*x^12 + 164301*x^13 + 229972*x^14 + 253682*x^15 + 208380*x^16 + 100483*x^17 - 28293*x^18 - 125093*x^19 - 157729*x^20 - 130285*x^21 - 73656*x^22 - 21858*x^23 + 7068*x^24 + 14241*x^25 + 10381*x^26 + 4903*x^27 + 1605*x^28 + 355*x^29 + 48*x^30 + 3*x^31) + ...
