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

Terms

a(0) =0a(1) =0a(2) =1a(3) =8a(4) =42a(5) =172a(6) =611a(7) =2004a(8) =6292a(9) =19304a(10) =58533a(11) =176464a(12) =530558a(13) =1593204a(14) =4781575a(15) =14347196a(16) =43044648a(17) =129137680a(18) =387417545a(19) =1162258008a(20) =3486780370a(21) =10460348540a(22) =31381054251a(23) =94143172708a(24) =282429529532a(25) =847288601592