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

Terms

a(0) =1a(1) =3a(2) =13a(3) =27a(4) =81a(5) =183a(6) =449a(7) =987a(8) =2317a(9) =5043a(10) =11265a(11) =24399a(12) =53249a(13) =114243a(14) =245853a(15) =523227a(16) =1114113a(17) =2357175a(18) =4980737a(19) =10480587a(20) =22020557a(21) =46126083a(22) =96468993a(23) =201302799a(24) =419430481a(25) =872361987a(26) =1811941645a(27) =3757981275a(28) =7784628225a(29) =16105886823