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

Terms

a(0) =1a(1) =1a(2) =5a(3) =21a(4) =73a(5) =233a(6) =717a(7) =2173a(8) =6545a(9) =19665a(10) =59029a(11) =177125a(12) =531417a(13) =1594297a(14) =4782941a(15) =14348877a(16) =43046689a(17) =129140129a(18) =387420453a(19) =1162261429a(20) =3486784361a(21) =10460353161a(22) =31381059565a(23) =94143178781a(24) =282429536433a(25) =847288609393