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

Terms

a(0) =1a(1) =-1a(2) =2a(3) =-8a(4) =32a(5) =-126a(6) =528a(7) =-2400a(8) =11216a(9) =-52844a(10) =252776a(11) =-1227850a(12) =6026860a(13) =-29820208a(14) =148667712a(15) =-746512524a(16) =3772351152a(17) =-19168404144a(18) =97881696096a(19) =-502052928468