Expansions of algebraically closed fields in o-minimal structures

We develop a notion of differentiability over an algebraically closed field K of characteristic zero with respect to a maximal real closed subfield R. We work in the context of an o-minimal expansion ? \cal {R} of the field R and obtain many of the standard results in complex analysis in this setting. In doing so we use the topological approach to complex analysis developed by Whyburn and others. We then prove a model theoretic theorem that states that the field R is definable in every proper expansion of the field K all of whose atomic relations are definable in ? \cal {R} . One corollary of this result is the classical theorem of Chow on projective analytic sets.     

A characterisation of PSp(4,K)

This paper gives a characterization of the group PS_p(4K) over some algebraically closed field K of characteristic not 2 inside the class of simple K ^*-groups of finite Morley rank not interpreting a bad field using the structure of centralizers of involutions.     

On some properties of coordinates in polynomial rings

In this paper, we study some properties of coordinates in the polynomial ring by introducing some concepts which are weaker than coordinates. In particular, in the polynomial ring in two variables over an algebraically closed field of characteristic zero, we show some relations between polynomials and their fibers on 𝔸².     

Irreducible characters for algebraic groups in characteristic two (iii)

In this note, we determine the irreducible characters for the special linear group SL(6,K) and S L(7,K) over an algebraically closed field K of characteristic 2, by using the theorem of Xi Nanhua [7] and the MATLAB software.     

Finite-dimensional pointed Hopf algebras over $\mathbb{S}_4$

Let K be an algebraically closed field of characteristic 0. We conclude the classification of finite-dimensional pointed Hopf algebras whose group of group-likes is \mathbbS₄\mathbb{S}_4. We also describe all pointed Hopf algebras over \mathbbS₅\mathbb{S}_5 whose infinitesimal braiding is associated to the rack of transpositions.     

Symmetric Elements,Hermitian Forms,and Cyclic Involutions

Let k be a field, K/k be a quadratic separable field extension, and 𝒜 a finite dimensional central simple algebra over K. If k is global or the field of fractions of a two-dimensional excellent henselian local domain with an algebraically closed residue field of characteristic zero and the degree of 𝒜 is odd, we prove that all K/k-involutions on 𝒜 are cyclic.     

Simple singularities of parametrized plane curves in positive characteristic

Let K be an algebraically closed field of characteristic p>0. The aim of the article is to give a classification of simple parametrized plane curve singularities over K. The idea is to give explicitly a class of families of singularities which are not simple such that almost all singularities deform to one of those and show that remaining singularities are simple.     

On Kummer extensions of the power series field

In this paper we study the Kummer extensions K ′ of a power series field K = k ((X₁, …, X_r)), where k is an algebraically closed field of arbitrary characteristic, with special emphasis in the case where K ′ is generated by a Puiseux power series. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A user friendly proof of Nagata's characterization of linearly reductive groups in positive characteristics

Let K be an algebraically closed field of positive characteristic p, and G be a linear algebraic group over K. We give a user friendly proof of Nagata's theorem that every finite-dimensional rational representation of G is completely reducible if and only if the connected component G ⁰ is a torus and p does not divide the index (G?:?G ⁰).     

Büchi’s problem for ultrametric meromorphic functions inside an open disk

Let K be a complete ultrametric algebraically closed field of characteristic zero such as ℂ _p . Büchi’s problem was solved for p-adic meromorphic functions in the whole field K. Here we show similar conclusions for meromorphic functions in an open disk that are not quotients of bounded analytic functions. The main method is the secondMain Theorem for p-adic meromorphic functions inside a disk, a specific p-adic theorem.     

Local flat duality of abelian varieties

 Let K be a field complete for a discrete valuation and with algebraically closed residue field of positive characteristic p. We prove the existence of a non-degenerate pairing between the first (flat) cohomology group of an abelian variety A _K over K and the fundamental group of the Néron model of the dual abelian variety. This pairing extends to the p-primary components a pairing introduced by Shafarevich in [16]. We relate this pairing with Grothendieck's pairing. Received: 7 January 2002 / Revised version: 6 December 2002 Published online: 24 April 2003 Mathematics Subject Classification (2000): 14k05, 14F20, 14G22     

On Certain Isomorphisms between Absolute Galois Groups

Let k be an algebraically closed field of characteristic zero, F be an algebraically closed extension of k of transcendence degree one, and G be the group of automorphisms over k of the field F. The purpose of this note is to calculate the group of continuous automorphisms of G.     

The Brauer group of a curve over a strictly local discrete valuation ring

LetK be the field of fractions of a curve overR whereR is the henselization of a regular local ring on an algebraic curve over a field which is algebraically closed and has characteristic 0. ThenK has the exponent=degree property for division algebras. In fact every central finite dimensionalK-division algebra with exponentn is a cyclic algebra of degreen. In memory of Professor S. A. Amitsur     

Triangulation of <Emphasis Type="Italic">n</Emphasis>-tuple solvable Lie algebras

We prove an analog of the Lie theorem for finite-dimensional n-tuple solvable Lie algebras over an algebraically closed field of characteristic 0.     

Simple groups of finite morley rank and Tits buildings

Theorem A:If ℬ is an infinite Moufang polygon of finite Morley rank, then ℬ is either the projective plane, the symplectic quadrangle, or the split Cayley hexagon over some algebraically closed field. In particular, ℬ is an algebraic polygon. It follows that any infinite simple group of finite Morley rank with a spherical MoufangBN-pair of Tits rank 2 is eitherPSL ₃(K),PSp ₄(K) orG ₂(K) for some algebraically closed fieldK. Spherical irreducible buildings of Tits rank ≥ 3 are uniquely determined by their rank 2 residues (i.e. polygons). Using Theorem A we show Theorem B:If G is an infinite simple group of finite Morley rank with a spherical Moufang BN-pair of Tits rank ≥ 2, then G is (interpretably) isomorphic to a simple algebraic group over an algebraically closed field. Theorem C:Let K be an infinite field, and let G(K) denote the group of K-rational points of an isotropic adjoint absolutely simple K-algebraic group G of K-rank ≥ 2. Then G(K) has finite Morley rank if and only if the field K is algebraically closed. We also obtain a result aboutBN-pairs in splitK-algebraic groups: such aBN-pair always contains the root groups. Furthermore, we give a proof that the sets of points, lines and flags of any ℵ₁-categorical polygon have Morley degree 1. Partially sponsored by the Edmund Landau Center for Research in Mathematical Analysis, supported by the Minerva Foundation (Germany). Supported by the Minerva Foundation (Germany). Research Director at the Fund for Scientific Research-Flanders (Belgium).     

Hochschild cohomology of algebras of quaternionic type: The family Q(2ℬ)<Subscript>1</Subscript>(<Emphasis Type="Italic">k,s, a,c</Emphasis>) over a field of characteristic different from 2

In a previous joint paper of the author with A.I. Generalov and S.O. Ivanov, the Hochschild cohomology algebra of quaternionic-type algebras from the family Q(2ℬ)₁ over an algebraically closed field of characteristic 2 was calculated. In this paper, the Hochschild cohomology groups of algebras from this family over an algebraically closed field of characteristic different from 2 are calculated. As a corollary, the additive structure of the Hochschild cohomology of algebras of type Q(2 $ A $ A ) over a field of characteristic not 2 is described.     

Moving subvarieties by endomorphisms

Let K be an algebraically closed field of characteristic zero and let A be a semiabelian variety defined over K. Let End(A) be the ring of endomorphisms of A. Let X ⊂ A be a subvariety of smaller dimension. We show that does not equal A(K). In particular, we may take K to be countable.     

Toroidal varieties and the weak factorization theorem

We develop the theory of stratified toroidal varieties, which gives, together with the theory of birational cobordisms [73], a proof of the weak factorization conjecture for birational maps in characteristic zero: a birational map between complete nonsingular varieties over an algebraically closed field K of characteristic zero is a composite of blow-ups and blow-downs with smooth centers. Dedicated to Professor Andrzej Biaynicki-Birula