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

Terms

a(0) =1a(1) =7a(2) =28a(3) =115a(4) =406a(5) =1492a(6) =5104a(7) =17611a(8) =59077a(9) =197242a(10) =649540a(11) =2127364a(12) =6908734a(13) =22325632a(14) =71744968a(15) =229600123a(16) =731794258a(17) =2324583475a(18) =7360989292a(19) =23245426690a(20) =73222477552a(21) =230128420012a(22) =721764371008