G.f. A(x) satisfies: A(x)^2 = A( x^2/(1 - 4*x - 2*x^2) ).

Terms

a(0) =1a(1) =2a(2) =8a(3) =32a(4) =138a(5) =612a(6) =2784a(7) =12896a(8) =60635a(9) =288614a(10) =1388104a(11) =6735808a(12) =32938438a(13) =162156828a(14) =803026176a(15) =3997462368a(16) =19991321445a(17) =100387500906a(18) =505950179016