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

Terms

a(0) =0a(1) =1a(2) =2a(3) =5a(4) =9a(5) =19a(6) =35a(7) =71a(8) =135a(9) =271a(10) =527a(11) =1055a(12) =2079a(13) =4159a(14) =8255a(15) =16511a(16) =32895a(17) =65791a(18) =131327a(19) =262655a(20) =524799a(21) =1049599a(22) =2098175a(23) =4196351a(24) =8390655a(25) =16781311a(26) =33558527a(27) =67117055a(28) =134225919a(29) =268451839