A simple grammar: cycles of pairs of sequences.

Terms

a(0) =0a(1) =0a(2) =1a(3) =2a(4) =4a(5) =6a(6) =12a(7) =18a(8) =34a(9) =58a(10) =106a(11) =186a(12) =350a(13) =630a(14) =1180a(15) =2190a(16) =4114a(17) =7710a(18) =14600a(19) =27594a(20) =52486a(21) =99878a(22) =190744a(23) =364722a(24) =699250a(25) =1342182a(26) =2581426a(27) =4971066a(28) =9587578a(29) =18512790