From: "George Janelidze" <janelidze@mat.ua.pt>
To: <categories@mta.ca>
Subject: field and Galois theory
Date: Fri, 16 Feb 2001 18:27:24 -0000 [thread overview]
Message-ID: <000001c09846$d8a1f800$175188c1@mat.ua.pt> (raw)
Dear All,
The following recent message of Tobias Schroeder
>Hello,
>all introductions to field and Galois theory I've found are written in
a
>"classical" way, i.e. making not much use of categorical notions. A lot
of
>computation is done where someone who is "categorical minded" has the
>feeling that the results could be established in a more comprehensible
and
>clear way by category theory. -- Does somebody have a reference to a
short
>and good introduction to field and Galois theory from a categorical
>viewpoint?
>
>Thanks
>
>Tobias Schroeder
was already answered by Francis Borceux, who has beautifully written
most of the book "Galois theories". I would like to add:
I would say, "Field and Galois theory" sounds too general. For instance
any such introduction should include a lot of Group theory and
polynomials, which of course would look much nicer if various parts of
Category theory were involved - but this is a very long story!
So, let me replace "Field and Galois theory" by just "The fundamental
theorem of Galois theory" - and call it GFT for short.
The standard formulation of GFT includes the following assertions about
a finite Galois extension E/K with the Galois group G = Gal(E/K):
GFT1: The opposite lattice of subextensions of E/K is isomorphic to the
lattice of subgroups of G; under this isomorphism a subextension F/K
corresponds to the subgroup Gal(E/F) = {g in G: ga = a for all a in
F}, and therefore a subgroup H in G corresponds to {a in E: ga = a for
all g in H}.
GFT2: A subextension F/K of E/K is normal (equivalently, Galois) if and
only if its corresponding subgroup Gal(E/F) is normal, and if this is
the case, then Gal(F/K) is canonically isomorphic to the quotient group.
Moreover, every K-homomorphism of subextensions of E/K extends to a
K-automorphism of E.
Unfortunately even today all books in Algebra give only this kind of
formulation. I think actually the right name for it is not "standard"
but "prehistoric" - since more than 40 years ago Chevalley and
Grothendieck understood that it is a straightforward consequence of the
following simple and nice formulation:
Grothendieck's GFT restricted: The category of subextensions of E/K
(with morphisms all K-homomorphisms) is equivalent to the category of
transitive G-sets, where E/K and G are as above.
Moreover, one does not really want what I called "restricted", and then
the right formulation becomes:
Grothendieck's GFT: The category of K-algebras split over E/K is
equivalent to the category of finite G-sets. Here a K-algebra A is said
to be split over E/K if its tensor product over K with E is isomorphic
to the Cartesian product of a finite number of copies of E; note that A
is split over E/K if and only if it is itself isomorphic to the
Cartesian product of a finite number of subextensions of E/K.
There are many theorems similar or more general then this, proved by
Chevalley and Grothendieck themselves, by A. R. Magid, M. Barr and R.
Diaconescu, and others. In 1984 I realized that there is a purely
categorical formulation and a purely categorical proof - before that the
topos-theoretic level was considered as the most general, although there
was no topos-theoretic extension of Magid's theorem. And what I call now
Categorical Galois theory - let us say CGT for short - has important
examples very far from Grothendieck and topos theory. One of them,
studied in joint work with G. M. Kelly is of what we called generalized
central extensions in universal algebra. CGT actually uses very simple
category theory (pullbacks, adjoint functors, monadicity, internal
category actions), but after many attempts I found it very difficult to
explain it to "non-category-theorists" - I would say, simply because
most of them do not believe that General Category theory can have
non-trivial applications! A further generalization of CGT to so-called
variable categories was developed in joint work with D. Schumacher and
R. H. Street. In some sense it includes Street's theory of torsors,
Joyal - Tierney's theorem on geometric morphisms of toposes, and Tannaka
duality.
George Janelidze
next reply other threads:[~2001-02-16 18:27 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2001-02-16 18:27 George Janelidze [this message]
-- strict thread matches above, loose matches on Subject: below --
2001-02-09 11:02 BORCEUX Francis
2001-02-07 10:07 Tobias Schroeder
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