The number of length n ternary sequences in which no symbol appears exactly once.

Terms

a(0) =1a(1) =0a(2) =3a(3) =3a(4) =21a(5) =63a(6) =243a(7) =969a(8) =3657a(9) =12987a(10) =43959a(11) =143685a(12) =458109a(13) =1435047a(14) =4439451a(15) =13612257a(16) =41474577a(17) =125798643a(18) =380343519a(19) =1147320285a(20) =3455328261a(21) =10394294175a(22) =31242648963a(23) =93853773369a(24) =281825558361a(25) =846030320043