a(n) = 3^n - n.

Terms

a(0) =1a(1) =2a(2) =7a(3) =24a(4) =77a(5) =238a(6) =723a(7) =2180a(8) =6553a(9) =19674a(10) =59039a(11) =177136a(12) =531429a(13) =1594310a(14) =4782955a(15) =14348892a(16) =43046705a(17) =129140146a(18) =387420471a(19) =1162261448a(20) =3486784381a(21) =10460353182a(22) =31381059587a(23) =94143178804a(24) =282429536457a(25) =847288609418