Number of length n word structures with all distinct runs using exactly 3 symbols.

Terms

a(0) =0a(1) =0a(2) =1a(3) =3a(4) =12a(5) =28a(6) =81a(7) =177a(8) =410a(9) =906a(10) =1869a(11) =4001a(12) =8094a(13) =16032a(14) =32355a(15) =62499a(16) =120078a(17) =227880a(18) =436743a(19) =805797a(20) =1487920a(21) =2751618a(22) =5017143a(23) =9063625a(24) =16153560a(25) =29066676a(26) =51334289a(27) =90784671a(28) =157941132a(29) =275244344