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

Terms

a(0) =1a(1) =1a(2) =3a(3) =10a(4) =35a(5) =132a(6) =519a(7) =2105a(8) =8746a(9) =37033a(10) =159229a(11) =693343a(12) =3051290a(13) =13550083a(14) =60642857a(15) =273248824a(16) =1238567263a(17) =5643738611a(18) =25837579578a(19) =118785766683a(20) =548182891007