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

Terms

a(0) =1a(1) =2a(2) =0a(3) =4a(4) =-8a(5) =24a(6) =-72a(7) =224a(8) =-720a(9) =2368a(10) =-7936a(11) =27008a(12) =-93088a(13) =324288a(14) =-1140032a(15) =4039296a(16) =-14409728a(17) =51713792a(18) =-186577152a(19) =676334592a(20) =-2462090752a(21) =8997154816a(22) =-32992079872a(23) =121362092032a(24) =-447721572864