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

Terms

a(0) =1a(1) =4a(2) =9a(3) =20a(4) =35a(5) =78a(6) =133a(7) =280a(8) =531a(9) =1070a(10) =2057a(11) =4212a(12) =8203a(13) =16534a(14) =32865a(15) =65840a(16) =131087a(17) =262818a(18) =524305a(19) =1049740a(20) =2097459a(21) =4196390a(22) =8388629a(23) =16782024a(24) =33554575a(25) =67117102a(26) =134218809a(27) =268452212a(28) =536870939a(29) =1073777010