18,302 research outputs found
Reverse Mathematics and Algebraic Field Extensions
This paper analyzes theorems about algebraic field extensions using the
techniques of reverse mathematics. In section 2, we show that
is equivalent to the ability to extend -automorphisms of field extensions to
automorphisms of , the algebraic closure of . Section 3 explores
finitary conditions for embeddability. Normal and Galois extensions are
discussed in section 4, and the Galois correspondence theorems for infinite
field extensions are treated in section 5.Comment: 25 page
Complete reducibility and separable field extensions
Let G be a connected reductive linear algebraic group. The aim of this note
is to settle a question of J-P. Serre concerning the behaviour of his notion of
G-complete reducibility under separable field extensions. Part of our proof
relies on the recently established Tits Centre Conjecture for the spherical
building of the reductive group G.Comment: 5 pages; to appear in Comptes rendus Mathematiqu
Algebraic extensions in free groups
The aim of this paper is to unify the points of view of three recent and
independent papers (Ventura 1997, Margolis, Sapir and Weil 2001 and Kapovich
and Miasnikov 2002), where similar modern versions of a 1951 theorem of
Takahasi were given. We develop a theory of algebraic extensions for free
groups, highlighting the analogies and differences with respect to the
corresponding classical field-theoretic notions, and we discuss in detail the
notion of algebraic closure. We apply that theory to the study and the
computation of certain algebraic properties of subgroups (e.g. being malnormal,
pure, inert or compressed, being closed in certain profinite topologies) and
the corresponding closure operators. We also analyze the closure of a subgroup
under the addition of solutions of certain sets of equations.Comment: 35 page
Hilbert 90 for biquadratic extensions
Hilbert's Theorem 90 is a classical result in the theory of cyclic
extensions. The quadratic case of Hilbert 90, however, generalizes in noncyclic
directions as well. Informed by a poem of Richard Wilbur, the article explores
several generalizations, discerning connections among multiplicative groups of
fields, values of binary quadratic forms, a bit of module theory over group
rings, and even Galois cohomology.Comment: v2 (15 pages); followed Monthly style sheet and added additional
expositio
Extensions of differential representations of SL(2) and tori
Linear differential algebraic groups (LDAGs) measure differential algebraic
dependencies among solutions of linear differential and difference equations
with parameters, for which LDAGs are Galois groups. The differential
representation theory is a key to developing algorithms computing these groups.
In the rational representation theory of algebraic groups, one starts with
SL(2) and tori to develop the rest of the theory. In this paper, we give an
explicit description of differential representations of tori and differential
extensions of irreducible representation of SL(2). In these extensions, the two
irreducible representations can be non-isomorphic. This is in contrast to
differential representations of tori, which turn out to be direct sums of
isotypic representations.Comment: 21 pages; few misprints corrected; Lemma 4.6 adde
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