a(n) = 4^n - 3^n - binomial(n,2)*3^(n-2).

Terms

a(0) =0a(1) =1a(2) =6a(3) =28a(4) =121a(5) =511a(6) =2152a(7) =9094a(8) =38563a(9) =163729a(10) =694282a(11) =2934592a(12) =12348541a(13) =51697075a(14) =215291356a(15) =891989002a(16) =3677964295a(17) =15099277669a(18) =61745907934a(19) =251632677604