Number of primitive (period n) periodic palindromic structures using exactly two different symbols.

Terms

a(0) =0a(1) =1a(2) =1a(3) =2a(4) =3a(5) =4a(6) =7a(7) =10a(8) =14a(9) =21a(10) =31a(11) =42a(12) =63a(13) =91a(14) =123a(15) =184a(16) =255a(17) =371a(18) =511a(19) =750a(20) =1015a(21) =1519a(22) =2047a(23) =3030a(24) =4092a(25) =6111a(26) =8176a(27) =12222a(28) =16383a(29) =24486