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

Terms

a(0) =1a(1) =2a(2) =2a(3) =6a(4) =2a(5) =27a(6) =2a(7) =82a(8) =83a(9) =283a(10) =2a(11) =2047a(12) =2a(13) =4147a(14) =7188a(15) =20546a(16) =2a(17) =125964a(18) =2a(19) =343407a(20) =533844a(21) =1048699a(22) =2a(23) =10076747a(24) =390627a(25) =16777387a(26) =43053284a(27) =84003927a(28) =2a(29) =667311413