a(n) = 3*a(n-1) + 2^n - 6, with a(1) = 3.

Terms

a(0) =3a(1) =7a(2) =23a(3) =79a(4) =263a(5) =847a(6) =2663a(7) =8239a(8) =25223a(9) =76687a(10) =232103a(11) =700399a(12) =2109383a(13) =6344527a(14) =19066343a(15) =57264559a(16) =171924743a(17) =516036367a(18) =1548633383a(19) =4646948719a(20) =13942943303a(21) =41833024207a(22) =125507461223a(23) =376539160879