Algebraically flat or projective algebras

We define and study algebraically flat algebras in order to have a better understanding of algebraically projective algebras of finite type (the projective algebras of literature). A close examination of the differential properties of these algebras leads to our main structure theorem. As a corollary, we get that an algebraically projective algebra of finite type over a field is either a polynomial ring or the affine algebra of a complete intersection.     

Konstruktion topologischer affiner Ebenen mit nichtstetigem Parallelismus durch Knicken von Geraden

We prove the existence of a class of topological affine planes having non-continuous parallelism by using [2, Satz 5.2]. For this, we introduce a new method of constructing affine Salzmann-planes with a monotonically increasing slope (see 2.1) by bending lines on two special curves, which are not necessary lines. Furthermore, the limit inferior of a sequence of topological planes with fixed point space is defined. As application of our new method, we construct a sequence of affine Salzmann-planes such that the limit inferior of this sequence is again an affine Salzmann-plane and fulfils the assumptions of [2, Satz 5.2]. Applying this theorem repeatedly, we get a sequence of non-isomorphic topological affine subplanes with non-continuous parallelism.     

THE DENSITY OF INTEGRAL POINTS ON COMPLETE INTERSECTIONS

In this paper, an upper bound for the number of integral pointsof bounded height on an affine complete intersection definedover is proven. The proof uses an extension to complete intersectionsof the method used for hypersurfaces by Heath-Brown (The densityof rational points on non-singular hypersurfaces, Proc. IndianAcad. Sci. Math. Sci. 104 (1994) 13–29), the so called‘q-analogue’ of van der Corput's AB process.     

Multiplicities of Pfaffian intersections,and the Łojasiewicz inequality

An effective estimate for the local multiplicity of a complete intersection of complex algebraic and Pfaffian varieties is given, based on a local complex analog of the Rolle-Khovanskii theorem. The estimate is valid also for the properly defined multiplicity of a non-isolated intersection. It implies, in particular, effective estimates for the exponents of the polar curves, and the exponents in the ojasiewicz inequalities for Pfaffian functions. For the intersections defined by sparse polynomials, the multiplicities outside the coordinate hyperplanes can be estimated in terms of the number of non-zero monomials, independent of degrees of the monomials.     

Complete intersections in Stein manifolds

The purpose of this note is to prove some theorems on set theoretic complete intersections in Stein manifolds (or Stein spaces) which are analogous to results in affine algebraic geometry. Due to the Oka principle in Stein theory one gets stronger results. For example any locally complete intersection Y of dimension 3 in a Stein space X with dim X>2 dim Y is a set theoretic₆ complete intersection. A 4-dimensional submanifold of⁶ is a set theoretic complete intersection if sc ₁ ² (Y)=0 for some integer s>0.Der erstgenannte Autor dankt der Alexander-von-Humboldt-Stiftung für ein Stipendium zu einem Gastaufenthalt an der Universität Münster     

Integral automorphisms of affine spaces over finite fields

A permutation of the point set of the affine space \({{\mathrm{AG}}}(n,q)\) is called an integral automorphism if it preserves the integral distance defined among the points. In this paper, we complete the classification of the integral automorphisms of \({{\mathrm{AG}}}(n,q)\) for \(n\ge 3\).     

Projective Generation of Curves in Polynomial Extensions of an Affine Domain (II)

Let Abe an affine domain of dimension nover an algebraically closed field kof characteristic 0. Let I A[T]be a local complete intersection ideal of height nsuch that I/I2 is generated by n elements. It is proved that there exists a projective A[T]module Pof rank nsuch that Iis a surjective image of P.     

On a Parameterized System of Nonlinear Equations with Economic Applications

We study a parameterized system of nonlinear equations. Given a nonempty, compact, and convex set, an affine function, and a point-to-set mapping from the set to the Euclidean space containing the set, we constructively prove that, under certain (boundary) conditions on the mapping, there exists a connected set of zero points of the mapping, i.e., the origin is an element of the image for every point in the connected set, such that the connected set has a nonempty intersection with both the face at which the affine function is minimized and the face at which that function is maximized. This result generalizes and unifies several well-known existence theorems including Browder??s fixed point theorem and Ky Fan??s coincidence theorem. An economic application with constrained equilibria is also discussed.     

Vector fields and framings on isolated complete intersection singularities

A theorem is proved for deciding as to when the complex orthogonal complement of a vector field on an isolated, complete intersection germ, is a trivial vector bundle.     

The subregular variety to the variety of special lattices

We recall the basic geometric properties of the full lattice variety, the projective variety parametrizing special lattices over Witt vectors which was introduced in Haboush (2005) [6]. It is an analog in unequal characteristic, of a certain Schubert variety in the affine Grassmannian for , and it is normal and a locally complete intersection (Haboush and Sano, submitted for publication [7], Sano (2004) [15]). In this paper, I prove that the complement of its smooth locus, the subregular variety in it, is also normal and a locally complete intersection. The result is analogous to the geometry of the subregular subvariety of the nilpotent cone.     

Napoleon's Theorem with Weights in n-Space

The famous theorem of Napoleon was recently extended to higher dimensions. With the help of weighted vertices of an n-simplex T in , n 2, we present a weighted version of this generalized theorem, leading to a natural configuration of (n–1)-spheres corresponding with T by an almost arbitrarily chosen point. Besides, the Euclidean point of view, also affine aspects of the theorem become clear, and in addition a critical discussion on the role of the Fermat–Torricelli point in this framework is given.     

Lower complete intersection dimension over local homomorphisms

We introduce and study a theory of lower complete intersection dimension over local homomorphisms which encompasses the theory of lower complete intersection dimension for finite modules over local rings introduced by Gerko. In particular, we show that the lower complete intersection dimension over local homomorphisms reflects the complete intersection property of base rings as expected. As an application, we prove that the converse of a theorem of Sather-Wagstaff is also true.     

A new proof of Delsarte,Goethals and Mac Williams theorem on minimal weight codewords of generalized Reed–Muller codes

We give a new proof of Delsarte, Goethals and Mac Williams theorem on minimal weight codewords of generalized Reed–Muller codes published in 1970. To prove this theorem, we consider the intersection of the support of minimal weight codewords with affine hyperplanes and we proceed by recursion.     

REPRESENTATIONS OF AFFINE HECKEALGE BRAS OF TYPE G2

Let k be a field and q a nonzero element in k such that the square roots of q are in k. We use Hq to denote an affne Hecke algebra over k of type G2 with parameter q. The purpose of this paper is to study representations of Hq by using based rings of two-sided cells of an affne Weyl group W of type G2. We shall give the classification of irreducible representations of Hq. We also remark that a calculation in [11] actually shows that Theorem 2 in [1] needs a modification, a fact is known to Grojnowski and Tanisaki long time ago. In this paper we also show an interesting relation between Hq and an Hecke algebra corresponding to a certain Coxeter group. Apparently the idea in this paper works for all affne Weyl groups, but that is the theme of another paper.     

Biaffine planes and divisible semiplanes

We consider sets of g points of a biaffine plane of order q (an affine plane with one parallel class removed), satisfying the property that no three points of a set are collinear. We construct divisible semiplanes with parameters (2q²,2q) using collections of these sets and their duals, satisfying certain intersection properties.     

A cohomological characterization of Alexander schemes

Vistoli defined Alexander schemes in [19], which behave like smooth varieties from the viewpoint of intersection theory with Q-coefficients. In this paper, we will affirmatively answer Vistoli’s conjecture that Alexander property is Zariski local. The main tool is the abelian category of bivariant sheaves, and we will spend most of our time for proving basic properties of this category. We show that a scheme is Alexander if and only if all the first cohomology groups of bivariant sheaves vanish, which is an analogy of Serre’s theorem, which says that a scheme is affine if and only if all the first cohomology groups of quasi-coherent sheaves vanish. Serre’s theorem implies that the union of affine closed subschemes is again affine. Mimicking the proof line by line, we will prove that the union of Alexander open subschemes is again Alexander. Oblatum 1-XII-1997 & 14-XII-1998 / Published online: 10 May 1999     

Affine Versions of Singer's Theorem on Locally Homogeneous Spaces

A theorem of Singer says that an infinitesimaly homogeneous Riemannian manifold is locally homogeneous. We propose two result on affine connections similar to the theorem of Singer. As an application we prove a theorem giving a sufficient condition for local homogeneity in case of affine connections on 2-dimensional manifolds.     

On affine translation divisible designs

We prove that a translation divisible design (TDD) with an abelian translation group can be embedded in PG (n,q) for some n2. Moreover we study affine TDD's showing that they have an (elementary) abelian translation group. A construction for TDD's with an abelian translation group which are not affine is given too.     

Asymptotes and centers of affine algebraic curves

In the and century many beautiful theorems about the intersection of plane algebraic curves have been discovered. The book of Coolidge [C] contains a large collection of such results. One may ask which of these theorems can be generalized to curves in higher dimensional spaces. In this note we wish to discuss a generalization of a theorem of Waring and apply it to the question which affine algebraic curves have a unique "center". Received: 22 February 1995 / Accepted: 11 July 1995