G.f. A(x) satisfies A(x) = 1 + x/(1-x^3)^2 * A(x)^2.

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =16a(5) =50a(6) =162a(7) =544a(8) =1870a(9) =6551a(10) =23310a(11) =84016a(12) =306104a(13) =1125550a(14) =4171522a(15) =15567213a(16) =58445096a(17) =220598990a(18) =836611770a(19) =3186364485a(20) =12182475716a(21) =46739872301a(22) =179893583304a(23) =694387330970