David Speyer gives a nice introduction to the representations of GL(n) at the Secret Blogging Seminar.
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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 explicitly, 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.
I didn’t really expect my oblique reference to Grothendieck’s dessins d’enfant 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 dessins d’enfant, and in his brief introduction to dessins d’enfant, Leonardo Zapponi sketches some deeper interactions:
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?
At the beginning of his textbook on algebraic geometry, Serge Lang warns the reader that it’s possible to write endlessly on the subject, and read likewise, but every now and then a glimmer of actual insight 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?
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 explicitly, 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.
I’m pretty sure dessins d’enfant are another subject entirely from Galois theory…
I didn’t really expect my oblique reference to Grothendieck’s dessins d’enfant 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 dessins d’enfant, and in his brief introduction to dessins d’enfant, Leonardo Zapponi sketches some deeper interactions:
At the beginning of his textbook on algebraic geometry, Serge Lang warns the reader that it’s possible to write endlessly on the subject, and read likewise, but every now and then a glimmer of actual insight 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?