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

Terms

a(0) =0a(1) =4a(2) =24a(3) =96a(4) =280a(5) =660a(6) =1344a(7) =2464a(8) =4176a(9) =6660a(10) =10120a(11) =14784a(12) =20904a(13) =28756a(14) =38640a(15) =50880a(16) =65824a(17) =83844a(18) =105336a(19) =130720a(20) =160440a(21) =194964a(22) =234784a(23) =280416a(24) =332400a(25) =391300a(26) =457704a(27) =532224a(28) =615496a(29) =708180