This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A118542 #4 Jun 16 2016 23:27:30 %S A118542 1,2,12,3342,178985294,2483527716080119,14325590005802419238355799, %T A118542 50976900301828909677297289506452525838, %U A118542 155682086691137998248942804080553139214788341933547854 %N A118542 Number of nonisomorphic groupoids with <= n elements. %C A118542 The number of isomorphism classes of closed binary operations on sets of order <= n. See formulas by Christian G. Bower in A001329 Number of nonisomorphic groupoids with n elements. %F A118542 a(n) = SUM[i=0..n] A001329(i). a(n) = SUM[i=0..n] (A079173(i)+A027851(i)). a(n) = SUM[i=0..n] (A079177(i)+A079180(i)). a(n) = SUM[i=0..n] (A079183(i)+A001425(i)). a(n) = SUM[i=0..n] (A079187(i)+A079190(i)). a(n) = SUM[i=0..n] (A079193(i)+A079196(i)+A079199(i)+A001426(i)). %e A118542 a(5) = 1 + 1 + 10 + 3330 + 178981952 + 2483527537094825 = 2483527716080119 is prime. %Y A118542 Cf. A001424, A001425, A002489, A006448, A029850, A030245-A030265, A030271, A038015-A038023. %K A118542 nonn %O A118542 0,2 %A A118542 _Jonathan Vos Post_, May 06 2006