Number of reversible strings with n beads using exactly two different colors.

Terms

a(0) =0a(1) =1a(2) =4a(3) =8a(4) =18a(5) =34a(6) =70a(7) =134a(8) =270a(9) =526a(10) =1054a(11) =2078a(12) =4158a(13) =8254a(14) =16510a(15) =32894a(16) =65790a(17) =131326a(18) =262654a(19) =524798a(20) =1049598a(21) =2098174a(22) =4196350a(23) =8390654a(24) =16781310a(25) =33558526a(26) =67117054a(27) =134225918a(28) =268451838a(29) =536887294