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

Terms

a(0) =1a(1) =-1a(2) =2a(3) =-7a(4) =26a(5) =-98a(6) =388a(7) =-1615a(8) =6914a(9) =-30118a(10) =133244a(11) =-597558a(12) =2709700a(13) =-12400612a(14) =57200520a(15) =-265683199a(16) =1241557778a(17) =-5833098686a(18) =27536326476a(19) =-130548675698a(20) =621319892556