A simple grammar: pairs of cycles of sequences.

Terms

a(0) =0a(1) =0a(2) =1a(3) =4a(4) =10a(5) =22a(6) =43a(7) =84a(8) =157a(9) =294a(10) =551a(11) =1032a(12) =1941a(13) =3666a(14) =6955a(15) =13224a(16) =25269a(17) =48362a(18) =92875a(19) =178640a(20) =344453a(21) =665130a(22) =1286699a(23) =2492184a(24) =4834061a(25) =9386650a(26) =18247971a(27) =35508348a(28) =69161705a(29) =134822934