a(n) = (1/n) * Sum_{d|n} phi(n/d)*2^(d+1).

Terms

a(0) =4a(1) =6a(2) =8a(3) =12a(4) =16a(5) =28a(6) =40a(7) =72a(8) =120a(9) =216a(10) =376a(11) =704a(12) =1264a(13) =2364a(14) =4384a(15) =8232a(16) =15424a(17) =29204a(18) =55192a(19) =104976a(20) =199760a(21) =381492a(22) =729448a(23) =1398504a(24) =2684368a(25) =5162856a(26) =9942136a(27) =19175160a(28) =37025584a(29) =71585136