G.f.: x*(1 - 2*x^2)/(1 - x - 3*x^2 - 3*x^3 - x^4).

Terms

a(0) =0a(1) =1a(2) =1a(3) =2a(4) =8a(5) =18a(6) =49a(7) =129a(8) =338a(9) =890a(10) =2340a(11) =6153a(12) =16181a(13) =42550a(14) =111892a(15) =294238a(16) =773745a(17) =2034685a(18) =5350526a(19) =14070054a(20) =36999432a(21) =97295857a(22) =255854841a(23) =672810762a(24) =1769262288a(25) =4652554954