a(n) = 3^n - (3*n^2 + n + 2)/2.

Terms

a(0) =0a(1) =0a(2) =1a(3) =11a(4) =54a(5) =202a(6) =671a(7) =2109a(8) =6460a(9) =19556a(10) =58893a(11) =176959a(12) =531218a(13) =1594062a(14) =4782667a(15) =14348561a(16) =43046328a(17) =129139720a(18) =387419993a(19) =1162260915a(20) =3486783790a(21) =10460352530a(22) =31381058871a(23) =94143178021a(24) =282429535604a(25) =847288608492