G.f. A(x) satisfies A(x) = 1 / (1 - x - x * A(x^3)).

Terms

a(0) =1a(1) =2a(2) =4a(3) =8a(4) =18a(5) =40a(6) =88a(7) =196a(8) =436a(9) =968a(10) =2152a(11) =4784a(12) =10632a(13) =23634a(14) =52536a(15) =116776a(16) =259576a(17) =577000a(18) =1282576a(19) =2850968a(20) =6337264a(21) =14086744a(22) =31312644a(23) =69603152a(24) =154716976a(25) =343911796a(26) =764462500a(27) =1699281320a(28) =3777238312a(29) =8396213840