Dirichlet g.f.: Product_{k>=2} 1 / (1 - k^(-s))^(3^(k-1)).

Terms

a(0) =1a(1) =3a(2) =9a(3) =33a(4) =81a(5) =270a(6) =729a(7) =2278a(8) =6606a(9) =19926a(10) =59049a(11) =178173a(12) =531441a(13) =1596510a(14) =4783698a(15) =14356023a(16) =43046721a(17) =129162168a(18) =387420489a(19) =1162323189a(20) =3486790962a(21) =10460530350a(22) =31381059609a(23) =94143738789a(24) =282429539802a(25) =847290203766