a(n) = 4^n - 3^n - n*3^(n-1) - binomial(n,2)*3^(n-2).

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =13a(5) =106a(6) =694a(7) =3991a(8) =21067a(9) =104680a(10) =497452a(11) =2285053a(12) =10222777a(13) =44788342a(14) =192970834a(15) =820244467a(16) =3448381783a(17) =14367483412a(18) =59421385000a(19) =244271688313a(20) =999169721125