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	<title>Comments on: Representations of GL(n)</title>
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	<pubDate>Fri, 21 Nov 2008 17:02:28 +0000</pubDate>
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		<title>By: Jacob Freeze</title>
		<link>http://www.arsmathematica.net/archives/2008/07/24/representations-of-gln/#comment-61247</link>
		<dc:creator>Jacob Freeze</dc:creator>
		<pubDate>Mon, 04 Aug 2008 06:45:27 +0000</pubDate>
		<guid isPermaLink="false">http://www.arsmathematica.net/?p=697#comment-61247</guid>
		<description>I didn't really expect my oblique reference to Grothendieck's &lt;em&gt;dessins d’enfant&lt;/em&gt; to attract Dr. Armstrong's attention, but as long as we're talking about the existence or non-existence of connections between Grothendieck's little graphs and Galois groups...

Belyi's theorem implies that the absolute Galois group acts "faithfully" on &lt;em&gt;dessins d’enfant&lt;/em&gt;, and in &lt;a href="http://www.ams.org/notices/200307/what-is.pdf" rel="nofollow"&gt;his brief introduction&lt;/a&gt; to &lt;em&gt;dessins d’enfant&lt;/em&gt;, Leonardo Zapponi sketches some deeper interactions:

&lt;blockquote&gt;Dessins correspond to covers of P1 defined over
Q. The covers are permuted by the Galois group
Gal(Q/Q), so this group also acts on the set of
dessins, and one consequence of Belyı ˘’s theorem
is that the action is faithful. The deepest open
question in the theory of dessins is this: Can the
Galois orbits of dessins be distinguished by combinatorial
or topological invariants? That is, is there
an effective way to tell whether two dessins belong
to the same Galois orbit?&lt;/blockquote&gt;

At the beginning of his textbook on algebraic geometry, Serge Lang warns the reader that it's possible to write &lt;em&gt;endlessly&lt;/em&gt; on the subject, and read likewise, but every now and then a glimmer of actual &lt;em&gt;insight&lt;/em&gt; would also be useful to orient the brain among millions of "interesting" results, so I'll repeat my original question, now that the punning reference to Grothendieck has been taken care of:

Why is it that we can only "see" two elements of the absolute Galois group over the rational numbers?</description>
		<content:encoded><![CDATA[<p>I didn&#8217;t really expect my oblique reference to Grothendieck&#8217;s <em>dessins d’enfant</em> to attract Dr. Armstrong&#8217;s attention, but as long as we&#8217;re talking about the existence or non-existence of connections between Grothendieck&#8217;s little graphs and Galois groups&#8230;</p>
<p>Belyi&#8217;s theorem implies that the absolute Galois group acts &#8220;faithfully&#8221; on <em>dessins d’enfant</em>, and in <a href="http://www.ams.org/notices/200307/what-is.pdf" rel="nofollow">his brief introduction</a> to <em>dessins d’enfant</em>, Leonardo Zapponi sketches some deeper interactions:</p>
<blockquote><p>Dessins correspond to covers of P1 defined over<br />
Q. The covers are permuted by the Galois group<br />
Gal(Q/Q), so this group also acts on the set of<br />
dessins, and one consequence of Belyı ˘’s theorem<br />
is that the action is faithful. The deepest open<br />
question in the theory of dessins is this: Can the<br />
Galois orbits of dessins be distinguished by combinatorial<br />
or topological invariants? That is, is there<br />
an effective way to tell whether two dessins belong<br />
to the same Galois orbit?</p></blockquote>
<p>At the beginning of his textbook on algebraic geometry, Serge Lang warns the reader that it&#8217;s possible to write <em>endlessly</em> on the subject, and read likewise, but every now and then a glimmer of actual <em>insight</em> would also be useful to orient the brain among millions of &#8220;interesting&#8221; results, so I&#8217;ll repeat my original question, now that the punning reference to Grothendieck has been taken care of:</p>
<p>Why is it that we can only &#8220;see&#8221; two elements of the absolute Galois group over the rational numbers?</p>
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		<title>By: John Armstrong</title>
		<link>http://www.arsmathematica.net/archives/2008/07/24/representations-of-gln/#comment-61246</link>
		<dc:creator>John Armstrong</dc:creator>
		<pubDate>Mon, 04 Aug 2008 05:45:37 +0000</pubDate>
		<guid isPermaLink="false">http://www.arsmathematica.net/?p=697#comment-61246</guid>
		<description>I'm pretty sure &lt;i&gt;dessins d'enfant&lt;/i&gt; are another subject entirely from Galois theory...</description>
		<content:encoded><![CDATA[<p>I&#8217;m pretty sure <i>dessins d&#8217;enfant</i> are another subject entirely from Galois theory&#8230;</p>
]]></content:encoded>
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		<title>By: Jacob Freeze</title>
		<link>http://www.arsmathematica.net/archives/2008/07/24/representations-of-gln/#comment-61243</link>
		<dc:creator>Jacob Freeze</dc:creator>
		<pubDate>Mon, 04 Aug 2008 02:48:15 +0000</pubDate>
		<guid isPermaLink="false">http://www.arsmathematica.net/?p=697#comment-61243</guid>
		<description>This is only tangentially related to GL(n), but I've been trying to find an appropriate place to send up an S.O.S. for a while, so...

Are there really only two elements of the absolute Galois group of the rational numbers that are given &lt;em&gt;explicitly&lt;/em&gt;, with a complete description: the identity and complex conjugation?

It's just a bunch of automorphisms! Shouldn't we be able to "see" a few more of them?

Why not?

I'm not looking for a link to some encyclopedia of Galois theory... just a little insight, like a child's drawing of an elephant for someone who has never seen an elephant.</description>
		<content:encoded><![CDATA[<p>This is only tangentially related to GL(n), but I&#8217;ve been trying to find an appropriate place to send up an S.O.S. for a while, so&#8230;</p>
<p>Are there really only two elements of the absolute Galois group of the rational numbers that are given <em>explicitly</em>, with a complete description: the identity and complex conjugation?</p>
<p>It&#8217;s just a bunch of automorphisms! Shouldn&#8217;t we be able to &#8220;see&#8221; a few more of them?</p>
<p>Why not?</p>
<p>I&#8217;m not looking for a link to some encyclopedia of Galois theory&#8230; just a little insight, like a child&#8217;s drawing of an elephant for someone who has never seen an elephant.</p>
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