Number of nonisomorphic connected functions with no fixed points, or proper rings with n edges.

Terms

a(0) =0a(1) =1a(2) =2a(3) =5a(4) =11a(5) =31a(6) =77a(7) =214a(8) =576a(9) =1592a(10) =4375a(11) =12183a(12) =33864a(13) =94741a(14) =265461a(15) =746372a(16) =2102692a(17) =5938630a(18) =16803610a(19) =47639902a(20) =135288198a(21) =384812502a(22) =1096141974a(23) =3126648842a(24) =8929715592a(25) =25533447030a(26) =73090099586a(27) =209438176485a(28) =600721031344