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	<title>Comments on: Secrets of a scalar triple product identity</title>
	<link>http://realtimecollisiondetection.net/blog/?p=69</link>
	<description>Coding wisdom and rants of Christer Ericson</description>
	<pubDate>Fri, 10 Sep 2010 22:30:50 +0000</pubDate>
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		<title>By: adruab</title>
		<link>http://realtimecollisiondetection.net/blog/?p=69#comment-2204</link>
		<author>adruab</author>
		<pubDate>Mon, 28 Jul 2008 16:57:49 +0000</pubDate>
		<guid>http://realtimecollisiondetection.net/blog/?p=69#comment-2204</guid>
		<description>Strictly speaking, true enough.  I find restricting to two arguments to be less intuitive.  I'm sure it buys you something though.</description>
		<content:encoded><![CDATA[<p>Strictly speaking, true enough.  I find restricting to two arguments to be less intuitive.  I&#8217;m sure it buys you something though.</p>
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		<title>By: christer</title>
		<link>http://realtimecollisiondetection.net/blog/?p=69#comment-2200</link>
		<author>christer</author>
		<pubDate>Sat, 26 Jul 2008 20:28:01 +0000</pubDate>
		<guid>http://realtimecollisiondetection.net/blog/?p=69#comment-2200</guid>
		<description>adruab, there are certainly &lt;b&gt;cross product-&lt;u&gt;like&lt;/u&gt;&lt;/b&gt; operations that can be defined in arbitrary dimensions, as you describe. What I referred to with my parenthetical remark, however, is that when we use a stricter definition of cross product we find that the only two dimensions that have that "strict" cross product are dimensions 3 and 7. The late &lt;a href="http://users.tkk.fi/~ppuska/mirror/Lounesto/" rel="nofollow"&gt;Pertti Lounesto&lt;/a&gt; describes this in a lot of detail in &lt;a href="http://www.math.niu.edu/~rusin/known-math/95/prods" rel="nofollow"&gt;this usenet post&lt;/a&gt; of his (archived at Dave Rusin's excellent &lt;a href="http://www.math.niu.edu/~rusin/known-math/" rel="nofollow"&gt;The Mathematical Atlas&lt;a&gt; site).</description>
		<content:encoded><![CDATA[<p>adruab, there are certainly <b>cross product-<u>like</u></b> operations that can be defined in arbitrary dimensions, as you describe. What I referred to with my parenthetical remark, however, is that when we use a stricter definition of cross product we find that the only two dimensions that have that &#8220;strict&#8221; cross product are dimensions 3 and 7. The late <a href="http://users.tkk.fi/~ppuska/mirror/Lounesto/" rel="nofollow">Pertti Lounesto</a> describes this in a lot of detail in <a href="http://www.math.niu.edu/~rusin/known-math/95/prods" rel="nofollow">this usenet post</a> of his (archived at Dave Rusin&#8217;s excellent <a href="http://www.math.niu.edu/~rusin/known-math/" rel="nofollow">The Mathematical Atlas</a><a> site).</a></p>
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		<title>By: adruab</title>
		<link>http://realtimecollisiondetection.net/blog/?p=69#comment-2199</link>
		<author>adruab</author>
		<pubDate>Sat, 26 Jul 2008 19:37:41 +0000</pubDate>
		<guid>http://realtimecollisiondetection.net/blog/?p=69#comment-2199</guid>
		<description>I'm not sure if your don't post comments about generalizing was aimed at specifically 7D or not, so I'm going to anyways ;).

The cross product and scalar triple product generalize to any number of dimensions (probably with different names).  CrossN takes N-1 arguments and permuting the arguments has the same effect as the triple scalar product (even is equal, odd is negative).  The ScalarProductN would take N parameters and relates to permutations and determinants in the same way.

Example "useful" feature, a 4d plane (N,-d) equation is the "cross" of the homogeneous versions of 3 input points.  Obviously there's a cheaper way to compute it than a generic cross in this specific circumstance.  Inverses can also be computed in a similar way because of the relationship between cross and the null space of given set of vectors.  Specifically, your dual basis a'b'c' put in matrix form is the matrix inverse of abc.

An interesting possible extension was http://www.geometrictools.com/Documentation/LaplaceExpansionTheorem.pdf .  It kind of looks similar to cross, but I don't know if it's just coincidence yet.  That specific formulation leads to a decent, intuitively vectorized 4x4 matrix inverse at least.</description>
		<content:encoded><![CDATA[<p>I&#8217;m not sure if your don&#8217;t post comments about generalizing was aimed at specifically 7D or not, so I&#8217;m going to anyways ;).</p>
<p>The cross product and scalar triple product generalize to any number of dimensions (probably with different names).  CrossN takes N-1 arguments and permuting the arguments has the same effect as the triple scalar product (even is equal, odd is negative).  The ScalarProductN would take N parameters and relates to permutations and determinants in the same way.</p>
<p>Example &#8220;useful&#8221; feature, a 4d plane (N,-d) equation is the &#8220;cross&#8221; of the homogeneous versions of 3 input points.  Obviously there&#8217;s a cheaper way to compute it than a generic cross in this specific circumstance.  Inverses can also be computed in a similar way because of the relationship between cross and the null space of given set of vectors.  Specifically, your dual basis a&#8217;b'c&#8217; put in matrix form is the matrix inverse of abc.</p>
<p>An interesting possible extension was <a href="http://www.geometrictools.com/Documentation/LaplaceExpansionTheorem.pdf" rel="nofollow">http://www.geometrictools.com/Documentation/LaplaceExpansionTheorem.pdf</a> .  It kind of looks similar to cross, but I don&#8217;t know if it&#8217;s just coincidence yet.  That specific formulation leads to a decent, intuitively vectorized 4&#215;4 matrix inverse at least.</p>
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