Number of 4-ary Lyndon words of length n with exactly two 1s.

Terms

a(0) =3a(1) =12a(2) =54a(3) =198a(4) =729a(5) =2538a(6) =8748a(7) =29484a(8) =98415a(9) =324648a(10) =1062882a(11) =3454002a(12) =11160261a(13) =35871174a(14) =114791256a(15) =365893848a(16) =1162261467a(17) =3680484804a(18) =11622614670a(19) =36611206686a(20) =115063885233a(21) =360882096930