a(n) = Sum_{d|n} phi(n/d)*2^(d+1), with a(0) = 0.

Terms

a(0) =0a(1) =4a(2) =12a(3) =24a(4) =48a(5) =80a(6) =168a(7) =280a(8) =576a(9) =1080a(10) =2160a(11) =4136a(12) =8448a(13) =16432a(14) =33096a(15) =65760a(16) =131712a(17) =262208a(18) =525672a(19) =1048648a(20) =2099520a(21) =4194960a(22) =8392824a(23) =16777304a(24) =33564096a(25) =67109200a(26) =134234256a(27) =268437672a(28) =536904480a(29) =1073741936