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<channel>
	<title>Ars Mathematica</title>
	<atom:link href="http://www.arsmathematica.net/feed/" rel="self" type="application/rss+xml" />
	<link>http://www.arsmathematica.net</link>
	<description>Dedicated to the mathematical arts.</description>
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		<item>
		<title>Hilbert&#8217;s Foundations of Geometry</title>
		<link>http://www.arsmathematica.net/archives/2012/02/03/hilberts-foundations-of-geometry/</link>
		<comments>http://www.arsmathematica.net/archives/2012/02/03/hilberts-foundations-of-geometry/#comments</comments>
		<pubDate>Fri, 03 Feb 2012 16:06:44 +0000</pubDate>
		<dc:creator>Walt</dc:creator>
				<category><![CDATA[Uncategorized]]></category>

		<guid isPermaLink="false">http://www.arsmathematica.net/?p=1524</guid>
		<description><![CDATA[Project Gutenberg has David Hilbert&#8217;s Foundations of Geometry available. It is a translation of Hilbert&#8217;s Grundlagen der Geometrie, which is famous as the first modern axiomization of Euclidean geometry. The difference between Hilbert&#8217;s approach and that of Euclid is that &#8230; <a href="http://www.arsmathematica.net/archives/2012/02/03/hilberts-foundations-of-geometry/">Continue reading <span class="meta-nav">&#8594;</span></a>]]></description>
			<content:encoded><![CDATA[<p>Project Gutenberg has David Hilbert&#8217;s <a href="http://www.gutenberg.org/ebooks/17384">Foundations of Geometry</a> available.  It is a translation of Hilbert&#8217;s <i>Grundlagen der Geometrie</i>, which is famous as the first modern <a href="http://en.wikipedia.org/wiki/Hilbert%27s_axioms">axiomization</a> of Euclidean geometry.  The difference between Hilbert&#8217;s approach and that of Euclid is that Hilbert fills in all of the fiddly little details required to meet modern standards of rigor.  </p>
<p>The book is elementary, and (as translated by Townsend) is a pleasant read.  Much of the book centers around constructing the field of real numbers in terms of the axiomized geometrical constructions.  This in turn allows Hilbert to show that the set of axioms is complete.  The topic leads naturally to one of the main themes of research in plane geometry in the early part of the last century, which is to consider different algebraic objects and how they can serve as coordinates for different notions of affine or projective planes.  The reals can be replaced with an arbitrary division ring, for example.  For a <a href="http://en.wikipedia.org/wiki/Projective_plane">projective plane</a>, the most general object is a <a href="http://en.wikipedia.org/wiki/Planar_ternary_ring">planar ternary ring</a>, with has a ternary operation that serves as a hybrid of addition and multiplication.  Determining the projective planes with a finite number of points is still an open question.</p>
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		<slash:comments>0</slash:comments>
		</item>
		<item>
		<title>A Cheerier Link</title>
		<link>http://www.arsmathematica.net/archives/2012/01/30/a-cheerier-link/</link>
		<comments>http://www.arsmathematica.net/archives/2012/01/30/a-cheerier-link/#comments</comments>
		<pubDate>Mon, 30 Jan 2012 20:54:29 +0000</pubDate>
		<dc:creator>Walt</dc:creator>
				<category><![CDATA[Physics]]></category>

		<guid isPermaLink="false">http://www.arsmathematica.net/?p=1522</guid>
		<description><![CDATA[I was told that the last two posts &#8212; on Hitler and serial killers &#8212; were too depressing, and that I need to post something more cheery So here is a careful analysis of the physics of My Little Pony.]]></description>
			<content:encoded><![CDATA[<p>I was told that the last two posts &mdash; on Hitler and serial killers &mdash; were too depressing, and that I need to post something more cheery  So here is a careful analysis of <a href="http://www.youtube.com/watch?v=muVfidujxRg">the physics of My Little Pony.</a></p>
]]></content:encoded>
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		<slash:comments>1</slash:comments>
		</item>
		<item>
		<title>Mathematics of a Serial Killer</title>
		<link>http://www.arsmathematica.net/archives/2012/01/26/mathematics-of-a-serial-killer/</link>
		<comments>http://www.arsmathematica.net/archives/2012/01/26/mathematics-of-a-serial-killer/#comments</comments>
		<pubDate>Thu, 26 Jan 2012 14:35:17 +0000</pubDate>
		<dc:creator>Walt</dc:creator>
				<category><![CDATA[Statistics]]></category>

		<guid isPermaLink="false">http://www.arsmathematica.net/?p=1501</guid>
		<description><![CDATA[Someone sent me a link to this story about a mathematical model of a particular serial killer&#8217;s behavior. Two things struck me about it: How much it sounded like the kind of bizarre model you&#8217;d see on Charline on Numb3rs &#8230; <a href="http://www.arsmathematica.net/archives/2012/01/26/mathematics-of-a-serial-killer/">Continue reading <span class="meta-nav">&#8594;</span></a>]]></description>
			<content:encoded><![CDATA[<p>Someone sent me a link to <a href="http://science.slashdot.org/story/12/01/17/0420232/statisticians-uncover-the-mathematics-of-a-serial-killer">this story</a> about a mathematical model of a particular serial killer&#8217;s behavior.  Two things struck me about it:</p>
<ol>
<li>How much it sounded like the kind of bizarre model you&#8217;d see on Charline on Numb3rs come up with in order to crack the case.
</li>
<li>That Cosma Shalizi would hate the model, since it&#8217;s the kind of a casual use of power laws he regularly criticizes.  And <a href="http://cscs.umich.edu/~crshalizi/weblog/857.html">here&#8217;s his analysis of the paper</a>.  He points out that, as in many other cases, a lognormal distribution provides a better fit.
</li>
</ol>
]]></content:encoded>
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		<slash:comments>0</slash:comments>
		</item>
		<item>
		<title>Hitler on Topology</title>
		<link>http://www.arsmathematica.net/archives/2012/01/20/hitler-on-topology/</link>
		<comments>http://www.arsmathematica.net/archives/2012/01/20/hitler-on-topology/#comments</comments>
		<pubDate>Fri, 20 Jan 2012 16:56:18 +0000</pubDate>
		<dc:creator>Walt</dc:creator>
				<category><![CDATA[Mathematics]]></category>

		<guid isPermaLink="false">http://www.arsmathematica.net/?p=1502</guid>
		<description><![CDATA[At this point, I&#8217;m sure everyone has seen at least one of the YouTube videos of Hitler ranting (actually actor Bruno Ganz from the movie Downfall) with fake subtitles. Here&#8217;s one showing Hitler&#8217;s reaction to discovering that in topology a &#8230; <a href="http://www.arsmathematica.net/archives/2012/01/20/hitler-on-topology/">Continue reading <span class="meta-nav">&#8594;</span></a>]]></description>
			<content:encoded><![CDATA[<p>At this point, I&#8217;m sure everyone has seen at least one of the YouTube videos of Hitler ranting (actually actor Bruno Ganz from the movie <i>Downfall</i>) with fake subtitles.  Here&#8217;s <a href="http://www.youtube.com/watch?v=SyD4p8_y8Kw">one showing Hitler&#8217;s reaction</a> to discovering that in topology a set can be both closed and open.  I think we all know how he felt.  (This is the clip with <a href="http://www.youtube.com/watch?v=_YQR36fQ_Xc">accurate subtitles</a> &#8212; I&#8217;d never seen it before.)</p>
<p>Via <a href="http://twistedphysics.typepad.com/cocktail_party_physics/2011/07/big-announcement.html">Cocktail Party Physics</a>.</p>
]]></content:encoded>
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		<slash:comments>4</slash:comments>
		</item>
		<item>
		<title>Best Possible Rejection Letter</title>
		<link>http://www.arsmathematica.net/archives/2012/01/16/best-possible-rejection-letter/</link>
		<comments>http://www.arsmathematica.net/archives/2012/01/16/best-possible-rejection-letter/#comments</comments>
		<pubDate>Mon, 16 Jan 2012 12:32:49 +0000</pubDate>
		<dc:creator>Walt</dc:creator>
				<category><![CDATA[Uncategorized]]></category>

		<guid isPermaLink="false">http://www.arsmathematica.net/?p=1498</guid>
		<description><![CDATA[Andrew Gelman quotes from the best possible rejection letter from a journal (sent to Charles Babbage): It is no inconsiderable degree of reluctance that I decline the offer of any Paper from you. I think, however, you will upon reconsideration &#8230; <a href="http://www.arsmathematica.net/archives/2012/01/16/best-possible-rejection-letter/">Continue reading <span class="meta-nav">&#8594;</span></a>]]></description>
			<content:encoded><![CDATA[<p>Andrew Gelman quotes from the <a href="http://andrewgelman.com/2012/01/a-model-rejection-letter/">best possible rejection letter</a> from a journal (sent to Charles Babbage):</p>
<blockquote><p>
It is no inconsiderable degree of reluctance that I decline the offer of any Paper from you. I think, however, you will upon reconsideration of the subject be of the opinion that I have no other alternative. The subjects you propose for a series of Mathematical and Metaphysical Essays are so profound, that there is perhaps not a single subscriber to our Journal who could follow them.
</p></blockquote>
<p>I encourage all journals to adopt this as the standard form letter for rejection.</p>
]]></content:encoded>
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		<slash:comments>1</slash:comments>
		</item>
		<item>
		<title>Back, From Outer Space!</title>
		<link>http://www.arsmathematica.net/archives/2012/01/09/back-from-outer-space/</link>
		<comments>http://www.arsmathematica.net/archives/2012/01/09/back-from-outer-space/#comments</comments>
		<pubDate>Mon, 09 Jan 2012 22:34:43 +0000</pubDate>
		<dc:creator>Walt</dc:creator>
				<category><![CDATA[site]]></category>

		<guid isPermaLink="false">http://www.arsmathematica.net/?p=1496</guid>
		<description><![CDATA[And you just walked in to find me here with that sad look upon my face. The computer that hosted Ars Math (which was at a dedicated web-site hosting company) died a horrible death a couple of months ago. For &#8230; <a href="http://www.arsmathematica.net/archives/2012/01/09/back-from-outer-space/">Continue reading <span class="meta-nav">&#8594;</span></a>]]></description>
			<content:encoded><![CDATA[<p>And you just walked in to find me here with that sad look upon my face.</p>
<p>The computer that hosted Ars Math (which was at a dedicated web-site hosting company) died a horrible death a couple of months ago.  For a while, it looked like several years of posts had been lost.  They could be recovered by cutting-and-pasting from the Internet Archive, but I found the idea so depressing that I didn&#8217;t do anything about recovering the site.  Fortunately, we were able to extract the posts from the database anyway.  Someone offered to help out with the WordPress hosting, so we&#8217;re back online!</p>
<p>At the moment the last couple of years of comments are missing, but we&#8217;re still investigating what happened to them.</p>
]]></content:encoded>
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		<slash:comments>1</slash:comments>
		</item>
		<item>
		<title>Shapley-Folkman-Starr Theorem</title>
		<link>http://www.arsmathematica.net/archives/2011/08/15/shapley-folkman-starr-theorem/</link>
		<comments>http://www.arsmathematica.net/archives/2011/08/15/shapley-folkman-starr-theorem/#comments</comments>
		<pubDate>Mon, 15 Aug 2011 11:57:40 +0000</pubDate>
		<dc:creator>Walt</dc:creator>
				<category><![CDATA[Economics]]></category>
		<category><![CDATA[Mathematics]]></category>

		<guid isPermaLink="false">http://www.arsmathematica.net/?p=1485</guid>
		<description><![CDATA[While economic theory sometimes uses advanced mathematics, such as Brouwer&#8217;s fixed point theorem, it&#8217;s less common for economic theory to lead to new mathematical developments. The Shapley-Folkman-Starr Theorem is an example of the latter. Roughly, the theorem states that the &#8230; <a href="http://www.arsmathematica.net/archives/2011/08/15/shapley-folkman-starr-theorem/">Continue reading <span class="meta-nav">&#8594;</span></a>]]></description>
			<content:encoded><![CDATA[<p>While economic theory sometimes uses advanced mathematics, such as Brouwer&#8217;s fixed point theorem, it&#8217;s less common for economic theory to lead to new mathematical developments.  The <a href="http://en.wikipedia.org/wiki/Shapley%E2%80%93Folkman_lemma">Shapley-Folkman-Starr Theorem</a> is an example of the latter.  Roughly, the theorem states that the (Minkowski) sum of a large number of arbitrary sets in a finite-dimensional vector space will be close to convex.  Starr was an economics undergraduate who was working on a term paper on approximating non-convex optimization problems with convex ones.  This led to collaboration with Shapley (a game theorist), and Folkman (a mathematician), and the eponymous theorem.</p>
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		<slash:comments>0</slash:comments>
		</item>
		<item>
		<title>TheoryMine</title>
		<link>http://www.arsmathematica.net/archives/2011/07/19/theorymine/</link>
		<comments>http://www.arsmathematica.net/archives/2011/07/19/theorymine/#comments</comments>
		<pubDate>Tue, 19 Jul 2011 14:58:54 +0000</pubDate>
		<dc:creator>Walt</dc:creator>
				<category><![CDATA[Uncategorized]]></category>

		<guid isPermaLink="false">http://www.arsmathematica.net/?p=1489</guid>
		<description><![CDATA[Here&#8217;s an odd web site. TheoryMine is a site that sells original mathematical theorems. You pay them money, and TheoryMine uses an automated theorem prover to find a theorem and its proof. The paper The Theory Behind TheoryMind explains more &#8230; <a href="http://www.arsmathematica.net/archives/2011/07/19/theorymine/">Continue reading <span class="meta-nav">&#8594;</span></a>]]></description>
			<content:encoded><![CDATA[<p>Here&#8217;s an odd web site.  <a href="http://theorymine.co.uk/">TheoryMine</a> is a site that sells original mathematical theorems.  You pay them money, and TheoryMine uses an automated theorem prover to find a theorem and its proof.  The paper <a href="http://dream.inf.ed.ac.uk/projects/isaplanner/papers/cacm-theorymine-draft.pdf">The Theory Behind TheoryMind</a> explains more about the how it works.</p>
<p>New Scientist has an <a href="http://www.newscientist.com/article/dn19809-mathematical-immortality-name-that-theorem.html">article</a> with some interviews.  The article&#8217;s author also bought <a href="http://www.newscientist.com/articleimages/dn19809/1-you-too-can-get-that-pythagoras-feeling.html">this theorem</a>.</p>
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		<item>
		<title>Machine Learning Book</title>
		<link>http://www.arsmathematica.net/archives/2011/06/30/machine-learning-book/</link>
		<comments>http://www.arsmathematica.net/archives/2011/06/30/machine-learning-book/#comments</comments>
		<pubDate>Thu, 30 Jun 2011 14:42:11 +0000</pubDate>
		<dc:creator>Walt</dc:creator>
				<category><![CDATA[Computer science]]></category>

		<guid isPermaLink="false">http://www.arsmathematica.net/?p=1486</guid>
		<description><![CDATA[Machine learning is an exciting new area of applied mathematics and computer science. It&#8217;s philosophically interesting because it requires to make a precise mathematical formulation of what it means to learn, and to be able to distinguish concepts from each &#8230; <a href="http://www.arsmathematica.net/archives/2011/06/30/machine-learning-book/">Continue reading <span class="meta-nav">&#8594;</span></a>]]></description>
			<content:encoded><![CDATA[<p><a href="http://en.wikipedia.org/wiki/Machine_learning">Machine learning</a> is an exciting new area of applied mathematics and computer science.  It&#8217;s philosophically interesting because it requires to make a precise mathematical formulation of what it means to learn, and to be able to distinguish concepts from each other.  Nils Nilsson has an early draft of a <a href="http://ai.stanford.edu/~nilsson/MLBOOK.pdf">textbook</a> on the subject.</p>
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		<slash:comments>5</slash:comments>
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		<item>
		<title>XKCD goes bad</title>
		<link>http://www.arsmathematica.net/archives/2011/05/19/xkcd-goes-bad/</link>
		<comments>http://www.arsmathematica.net/archives/2011/05/19/xkcd-goes-bad/#comments</comments>
		<pubDate>Thu, 19 May 2011 11:46:15 +0000</pubDate>
		<dc:creator>Walt</dc:creator>
				<category><![CDATA[Uncategorized]]></category>

		<guid isPermaLink="false">http://www.arsmathematica.net/?p=1482</guid>
		<description><![CDATA[Web comic XKCD has gone bad, and is now telling flagrant and terrible lies about the number line: Number Line. (Original here.)]]></description>
			<content:encoded><![CDATA[<p>Web comic XKCD has gone bad, and is now telling flagrant and terrible lies about the number line:</p>
<p><img src="http://imgs.xkcd.com/comics/number_line.png"/>Number Line.</p>
<p>(Original <a href="http://xkcd.com/899/">here</a>.)</p>
]]></content:encoded>
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