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

Terms

a(0) =1a(1) =1a(2) =4a(3) =13a(4) =43a(5) =124a(6) =388a(7) =1117a(8) =3370a(9) =9967a(10) =29860a(11) =88909a(12) =266896a(13) =798337a(14) =2394553a(15) =7178233a(16) =21533029a(17) =64579750a(18) =193738672a(19) =581159161a(20) =1743472426a(21) =5230263235a(22) =15690765067a(23) =47071824676a(24) =141215472685a(25) =423645012649