a(n) = Sum_{d|n} (2^d - (-1)^d)*phi(3*n/d).

Terms

a(0) =6a(1) =12a(2) =36a(3) =48a(4) =90a(5) =180a(6) =294a(7) =576a(8) =1134a(9) =2160a(10) =4158a(11) =8496a(12) =16458a(13) =33096a(14) =65880a(15) =131712a(16) =262242a(17) =525852a(18) =1048686a(19) =2099520a(20) =4195296a(21) =8392824a(22) =16777350a(23) =33564672a(24) =67109250a(25) =134234256a(26) =268438860a(27) =536904480a(28) =1073741994a(29) =2147556240