a(n) = 3^n + n.

Terms

a(0) =1a(1) =4a(2) =11a(3) =30a(4) =85a(5) =248a(6) =735a(7) =2194a(8) =6569a(9) =19692a(10) =59059a(11) =177158a(12) =531453a(13) =1594336a(14) =4782983a(15) =14348922a(16) =43046737a(17) =129140180a(18) =387420507a(19) =1162261486a(20) =3486784421a(21) =10460353224a(22) =31381059631a(23) =94143178850a(24) =282429536505a(25) =847288609468