Number of primitive (aperiodic) words of length n which contain exactly two different symbols.

Terms

a(0) =0a(1) =2a(2) =6a(3) =12a(4) =30a(5) =54a(6) =126a(7) =240a(8) =504a(9) =990a(10) =2046a(11) =4020a(12) =8190a(13) =16254a(14) =32730a(15) =65280a(16) =131070a(17) =261576a(18) =524286a(19) =1047540a(20) =2097018a(21) =4192254a(22) =8388606a(23) =16772880a(24) =33554400a(25) =67100670a(26) =134217216a(27) =268419060a(28) =536870910a(29) =1073708010