a(n) = Sum_{d|n} Moebius(n/d)*d^(b-1)/phi(n) for b = 5.

Terms

a(0) =1a(1) =15a(2) =40a(3) =120a(4) =156a(5) =600a(6) =400a(7) =960a(8) =1080a(9) =2340a(10) =1464a(11) =4800a(12) =2380a(13) =6000a(14) =6240a(15) =7680a(16) =5220a(17) =16200a(18) =7240a(19) =18720a(20) =16000a(21) =21960a(22) =12720a(23) =38400a(24) =19500a(25) =35700a(26) =29160a(27) =48000a(28) =25260a(29) =93600