Another good one: 
<a href="http://www.ams.org/featurecolumn/archive/svd.html" rel="nofollow">http://www.ams.org/featurecolumn/archive/svd.html</a><p>and its HN thread:
<a href="http://news.ycombinator.com/item?id=736618" rel="nofollow">http://news.ycombinator.com/item?id=736618</a>