03.05.2014 Aufrufe

Eine Herausforderung für die Mathematik(didaktik)?

Eine Herausforderung für die Mathematik(didaktik)?

Eine Herausforderung für die Mathematik(didaktik)?

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D. M. Evans, D. Macpherson and A. A. Ivanov: Finite covers;<br />

Z. Chatzidakis: Definable subgroups of algebraic groups over pseudofinite<br />

fields;<br />

W. Hodges: Groups in pseudofinite fields;<br />

D. Lascar: The group of automorphisms of the field of complex numbers<br />

leaving fixed the algebraic numbers is simple;<br />

D. Evans, D. Lascar: The automorphism group of the field of complex numbers<br />

is complete;<br />

P. J. Cameron: The algebra of an age;<br />

M. Boffa: Elimination of inverses in groups;<br />

F. Oger: Model-theoretic properties of polycyclic-by-finite groups;<br />

I. M. Chiswell: Non-standard free groups;<br />

P. H. Pfander: Finitely generated subgroups of the free Z[t]-group on two<br />

generators;<br />

K. Burke, M. Prest: Rings of definable scalars and biendomorphism rings;<br />

B. Kim: Recent results on simple first-order theories.<br />

The main theme is the strong interplay between model theory and group<br />

theory. The first article for instance (72 pages!) starts by explaining how<br />

Zil’ber’s ladder theorem leads to the study of the so called cover problem<br />

which can be described entirely in group theoretic terms. Surprisingly, even<br />

certain cohomology groups can be computed by the careful analysis of this<br />

problem.<br />

From the next two articles let me quote two results after saying that a<br />

pseudo-finite field is one that is a non-principal ultraproduct of finite fields<br />

and that those of characteristic zero represent precisely the infinite models<br />

of the theory of finite fields of characteristic p (p a given prime).<br />

Theorem. Let G be a connected algebraic group defined over F and G 1 a<br />

proper subgroup of finite index of G(F ) which is definable in F . There exists<br />

a connected algebraic group H defined over F and a F -rational surjective<br />

homomorphism g : H → G with finite central non-trivial kernel such that<br />

imH(F ) has finite index in G 1.<br />

Theorem (Matthews, Vaserstein, Weisfeiler). If G is an almost simple, simply<br />

connected closed subgroup of Gl n(Q) and Γ a finitely generated subgroup<br />

of G(Q), which is Zariski dense in G, then but for a finite number of primes<br />

p, Γ/p = G p(F p).<br />

The proof of this theorem is noneffective but much simpler than the<br />

original one.<br />

Lascar’s article contains the proof of the fact that the subgroup of automorphisms<br />

of the complex numbers whose elements fix every algebraic<br />

number is a simple group.<br />

In the next article, using CH, its completeness is proved and generalizations<br />

are given.<br />

Cameron shows how to oligomorphic groups G (more specifically a permutation<br />

group G of an infinite set Ω such that for all n the induced action<br />

on the set of subsets of cardinality n has only finitely many orbits) can be<br />

attached the reduced incidence algebra a la Rota and that it is a polynomial<br />

algebra (in general it is not) for certain such groups.<br />

The next two articles have to do with finite extensions of nilpotent<br />

groups. Indeed, linear groups with elimiation of inverses in identities (i.e.,<br />

one can find a conjunction of monoidal identities) are of that sort.<br />

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