a(n) is the number of different linear hydrocarbon molecules with n carbon atoms.

Terms

a(0) =1a(1) =3a(2) =4a(3) =10a(4) =18a(5) =42a(6) =84a(7) =192a(8) =409a(9) =926a(10) =2030a(11) =4577a(12) =10171a(13) =22889a(14) =51176a(15) =115070a(16) =257987a(17) =579868a(18) =1301664a(19) =2925209a(20) =6569992a(21) =14763529a(22) =33166848a(23) =74527233a(24) =167446566a(25) =376253517a(26) =845401158a(27) =1899609267a(28) =4268309531a(29) =9590827171