a(n) = 3^n + 3*n + 1.

Terms

a(0) =2a(1) =7a(2) =16a(3) =37a(4) =94a(5) =259a(6) =748a(7) =2209a(8) =6586a(9) =19711a(10) =59080a(11) =177181a(12) =531478a(13) =1594363a(14) =4783012a(15) =14348953a(16) =43046770a(17) =129140215a(18) =387420544a(19) =1162261525a(20) =3486784462a(21) =10460353267a(22) =31381059676a(23) =94143178897a(24) =282429536554a(25) =847288609519