Number of 2-divided words of length n over a 4-letter alphabet.

Terms

a(0) =0a(1) =0a(2) =6a(3) =40a(4) =186a(5) =816a(6) =3396a(7) =14040a(8) =57306a(9) =233000a(10) =943608a(11) =3813000a(12) =15378716a(13) =61946640a(14) =249260316a(15) =1002158880a(16) =4026527706a(17) =16169288640a(18) =64901712996a(19) =260410648680