a(n) = Sum_{d|n} phi(d)*4^(n/d).

Terms

a(0) =0a(1) =4a(2) =20a(3) =72a(4) =280a(5) =1040a(6) =4200a(7) =16408a(8) =65840a(9) =262296a(10) =1049680a(11) =4194344a(12) =16782000a(13) =67108912a(14) =268451960a(15) =1073744160a(16) =4295033440a(17) =17179869248a(18) =68719747320a(19) =274877907016