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

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =4a(5) =11a(6) =33a(7) =114a(8) =438a(9) =1845a(10) =8458a(11) =41823a(12) =221539a(13) =1250269a(14) =7481758a(15) =47278652a(16) =314374316a(17) =2192798077a(18) =16000160519a(19) =121831654450a(20) =965946444587