a(n) = Sum_{1<=k<=n, gcd(k,n)=1} 2^(k-1).

Terms

a(0) =1a(1) =1a(2) =3a(3) =5a(4) =15a(5) =17a(6) =63a(7) =85a(8) =219a(9) =325a(10) =1023a(11) =1105a(12) =4095a(13) =5397a(14) =13515a(15) =21845a(16) =65535a(17) =70737a(18) =262143a(19) =333125a(20) =890523a(21) =1397077a(22) =4194303a(23) =4527185a(24) =16236015a(25) =22365525a(26) =57521883a(27) =88429845a(28) =268435455a(29) =272962625