a(n) = 3^n - (n+1)*(n+2)/2.

Terms

a(0) =0a(1) =0a(2) =3a(3) =17a(4) =66a(5) =222a(6) =701a(7) =2151a(8) =6516a(9) =19628a(10) =58983a(11) =177069a(12) =531350a(13) =1594218a(14) =4782849a(15) =14348771a(16) =43046568a(17) =129139992a(18) =387420299a(19) =1162261257a(20) =3486784170a(21) =10460352950a(22) =31381059333a(23) =94143178527a(24) =282429536156a(25) =847288609092