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 A180423 #2 Mar 30 2012 18:40:58 %S A180423 2,28,9906,43803136 %N A180423 Number of unbreakable loops of order n. %C A180423 From p. 3 of Beaudry, Figure 1: Unbreakable loops of size 5 to 9. We say that a finite loop is unbreakable whenever it doesn't have proper subloops, that is, other than itself and the trivial one-element loop. While it is easy to see that the finite associative unbreakable loops are exactly the cyclic groups of prime order, it turns out that finite, nonassociative unbreakable loops are numerous and diverse. While the cyclic groups of prime order are the only unbreakable finite groups, we show that nonassociative unbreakable loops exist for every order n >= 5. We describe two families of commutative unbreakable loops of odd order, n >= 7, one where the loop's multiplication group is isomorphic to the alternating group A_n and another where the multiplication group is isomorphic to the symmetric group S_n. We also prove for each even n >= 6 that there exist unbreakable loops of order n whose multiplication group is isomorphic to S_n. %H A180423 Martin Beaudry, Louis Marchand, <a href="http://arxiv.org/abs/1009.0487">Unbreakable Loops</a>, Sep 02, 2010. %e A180423 a(5) = 2 because there are 6 loops of order 5, of which 2 are unbreakable. %Y A180423 Cf. A057771 Number of loops (quasigroups with an identity element) of order n. %K A180423 nonn %O A180423 5,1 %A A180423 _Jonathan Vos Post_, Sep 03 2010