a(n) = Sum_{d|n} mu(n/d)*4^(d-1).

Terms

a(0) =1a(1) =3a(2) =15a(3) =60a(4) =255a(5) =1005a(6) =4095a(7) =16320a(8) =65520a(9) =261885a(10) =1048575a(11) =4193220a(12) =16777215a(13) =67104765a(14) =268435185a(15) =1073725440a(16) =4294967295a(17) =17179802640a(18) =68719476735a(19) =274877644740