a(n) = ((2^b-1)/phi(n))*Sum_{d|n} Moebius(n/d)*d^(b-1) for b = 4.

Terms

a(0) =15a(1) =105a(2) =195a(3) =420a(4) =465a(5) =1365a(6) =855a(7) =1680a(8) =1755a(9) =3255a(10) =1995a(11) =5460a(12) =2745a(13) =5985a(14) =6045a(15) =6720a(16) =4605a(17) =12285a(18) =5715a(19) =13020a(20) =11115a(21) =13965a(22) =8295a(23) =21840a(24) =11625a(25) =19215a(26) =15795a(27) =23940a(28) =13065a(29) =42315