a(0) = 1; for n > 0, a(n) = number of rings with n elements.

Terms

a(0) =1a(1) =1a(2) =2a(3) =2a(4) =11a(5) =2a(6) =4a(7) =2a(8) =52a(9) =11a(10) =4a(11) =2a(12) =22a(13) =2a(14) =4a(15) =4a(16) =390a(17) =2a(18) =22a(19) =2a(20) =22a(21) =4a(22) =4a(23) =2a(24) =104a(25) =11a(26) =4a(27) =59a(28) =22a(29) =2