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

Terms

a(0) =1a(1) =1a(2) =1a(3) =4a(4) =7a(5) =16a(6) =34a(7) =73a(8) =157a(9) =337a(10) =724a(11) =1555a(12) =3340a(13) =7174a(14) =15409a(15) =33097a(16) =71089a(17) =152692a(18) =327967a(19) =704440a(20) =1513066a(21) =3249913a(22) =6980485a(23) =14993377a(24) =32204260a(25) =69171499a(26) =148573396a(27) =319120654a(28) =685438945a(29) =1472253649