a(n) = 3^n - 3*n.

Terms

a(0) =1a(1) =0a(2) =3a(3) =18a(4) =69a(5) =228a(6) =711a(7) =2166a(8) =6537a(9) =19656a(10) =59019a(11) =177114a(12) =531405a(13) =1594284a(14) =4782927a(15) =14348862a(16) =43046673a(17) =129140112a(18) =387420435a(19) =1162261410a(20) =3486784341a(21) =10460353140a(22) =31381059543a(23) =94143178758a(24) =282429536409a(25) =847288609368