G.f.: (1-x)*(1-2*x-x^2-(1-x)*sqrt(1-2*x-3*x^2))/(2*x*(1-2*x-x^2)).

Terms

a(0) =0a(1) =0a(2) =1a(3) =3a(4) =10a(5) =30a(6) =88a(7) =252a(8) =712a(9) =1992a(10) =5537a(11) =15323a(12) =42286a(13) =116506a(14) =320773a(15) =883191a(16) =2433112a(17) =6709832a(18) =18529067a(19) =51251265a(20) =142021430a(21) =394336930a(22) =1097215414a(23) =3059549286a(24) =8550284176a(25) =23947824960a(26) =67221675367a(27) =189102714045a(28) =533104654538