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

Terms

a(0) =1a(1) =0a(2) =1a(3) =1a(4) =4a(5) =9a(6) =27a(7) =76a(8) =226a(9) =680a(10) =2078a(11) =6441a(12) =20153a(13) =63684a(14) =202732a(15) =649930a(16) =2095854a(17) =6794684a(18) =22131765a(19) =72393439a(20) =237703654a(21) =783198068a(22) =2588645047a(23) =8580674778a(24) =28517805357a(25) =95009277576a(26) =317242351135