a(n) = Sum_{d|n} d * 3^(n/d-1).

Terms

a(0) =1a(1) =5a(2) =12a(3) =37a(4) =86a(5) =276a(6) =736a(7) =2261a(8) =6597a(9) =19870a(10) =59060a(11) =177780a(12) =531454a(13) =1595816a(14) =4783272a(15) =14353429a(16) =43046738a(17) =129154113a(18) =387420508a(19) =1162301342a(20) =3486786672a(21) =10460471356a(22) =31381059632a(23) =94143540948a(24) =282429536911a(25) =847289672390