a(n) = 3^n - n^2.

Terms

a(0) =1a(1) =2a(2) =5a(3) =18a(4) =65a(5) =218a(6) =693a(7) =2138a(8) =6497a(9) =19602a(10) =58949a(11) =177026a(12) =531297a(13) =1594154a(14) =4782773a(15) =14348682a(16) =43046465a(17) =129139874a(18) =387420165a(19) =1162261106a(20) =3486784001a(21) =10460352762a(22) =31381059125a(23) =94143178298a(24) =282429535905a(25) =847288608818