cp's OEIS Frontend

A090600 Number of n-element groupoids with an identity, considered to be equivalent when they are isomorphic or anti-isomorphic (by reversal of the operator).

%I A090600 #10 Feb 16 2025 08:32:52
%S A090600 1,2,30,22456,3179460875,118459556737869096,
%T A090600 1841479754519556293280702095,
%U A090600 17699004125822025124951660289473617122688,146207646053305863964379580591072585873215024249550168360
%N A090600 Number of n-element groupoids with an identity, considered to be equivalent when they are isomorphic or anti-isomorphic (by reversal of the operator).
%C A090600 Also partial groupoids with n-1 elements or groupoids with an absorbant (zero) element with n elements in each case the same equivalence.
%H A090600 Eric Postpischil <a href="http://groups.google.com/groups?&amp;hl=en&amp;lr=&amp;ie=UTF-8&amp;selm=11802%40shlump.nac.dec.com&amp;rnum=2">Posting to sci.math newsgroup, May 21 1990</a>
%H A090600 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Groupoid.html">Groupoid.</a>
%H A090600 <a href="/index/Gre#groupoids">Index entries for sequences related to groupoids</a>
%F A090600 a(n) = (A090601(n) + A090604(n))/2
%K A090600 nonn
%O A090600 1,2
%A A090600 _Christian G. Bower_, Dec 05 2003