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    <p>If you think of a graph with club members as vertices and 'like' as edges (i.e. if two members do <strong>not</strong> hate each other there is an edge between the corresponding vertices), one can reformulate the problem as follows:</p> <blockquote> <p>find the maximum subset of vertices for which there is an edge between <strong>all</strong> vertices in the subset. </p> </blockquote> <p>In fact, a subset where all vertices have mutual edges is called a <a href="http://en.wikipedia.org/wiki/Clique_%28graph_theory%29" rel="nofollow">Clique</a> or complete subgraph.</p> <p>To find the maximum clique takes exponential time if one can not exploit further features of the graph (see <a href="http://en.wikipedia.org/wiki/Clique_problem#Finding_maximum_cliques_in_arbitrary_graphs" rel="nofollow">this link</a>). This article suggests the <a href="http://en.wikipedia.org/wiki/Bron%E2%80%93Kerbosch_algorithm" rel="nofollow">Bron–Kerbosch algorithm</a>.</p> <p>Drawing the members in the <code>(S,B)</code> plane, one can see that a 'like' edge corresponds to lines going out of a vertex in the direction between 12 and 3 o'clock and between 6 and 9 o'clock. It is easy to construct an example where such edges intersect, so it's unfortunately not a planar graph.</p> <p>And unfortunately, the 'like' relationship is not <a href="http://en.wikipedia.org/wiki/Transitive_relation" rel="nofollow">transitive</a>, i.e. if <code>A</code> likes <code>B</code> and <code>B</code> likes <code>C</code> this does not imply that <code>A</code> also likes <code>C</code> (again, this is easy to see in the <code>(S,B)</code> plane).</p>
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