most recent 30 from http://mathoverflow.net 2013-05-19T14:45:58Z http://mathoverflow.net/feeds/user/8018 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/16829/what-are-your-favorite-instructional-counterexamples/33786#33786 David 2010-07-29T13:04:52Z 2010-07-29T13:04:52Z

<p>The alternating group on 4 letters is nice because it provides a counterexample to the converse of Lagrange's theorem: It has order 12, but it does not have a subgroup of order 6.</p>