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

Terms

a(0) =1a(1) =2a(2) =2a(3) =4a(4) =2a(5) =15a(6) =2a(7) =74a(8) =83a(9) =643a(10) =2a(11) =12635a(12) =2a(13) =117715a(14) =397188a(15) =2359426a(16) =2a(17) =103572204a(18) =2a(19) =1260918355a(20) =13841818644a(21) =25937425627a(22) =2a(24) =152587890627a(28) =2a(30) =2