Geometric construction of some families of two-class and three-class association schemes from nondegenerate quadrics in characteristic two

1.IntroductionThetheoryoflinearspacesintriteprojectivegeometryhasbeenusedbyseveralauthorsinconstructingBIBandPBIBdesigns.BoseI21firstusedthepropertiesofquadricsurfaCesinfiniteprojectivegeometryoftwoandthreedimensionsforconstr-netingexperimelltaldesigns.D.K.Ray-Chaudhurils]usedthegeometryofquadricstoconstructseveralseriesofPBIBdesignswithtwoassociateclasses.I.M.Chakravartila]usednondegenerateanddegenerateHebotianvarietiestoconstructsomefamiliesoftwo-classandthree-classassociationschemes…     

A new characterization of projections of quadrics in finite projective spaces of even characteristic

We will classify, up to linear representations, all geometries fully embedded in an affine space with the property that for every antiflag {p,L} of the geometry there are either 0, α, or q lines through p intersecting L. An example of such a geometry with α=2 is the following well known geometry . Let Q_n+1 be a nonsingular quadric in a finite projective space , n≥3, q even. We project Q_n+1 from a point r∉Q_n+1, distinct from its nucleus if n+1 is even, on a hyperplane not through r. This yields a partial linear space whose points are the points p of , such that the line 〈p,r〉 is a secant to Q_n+1, and whose lines are the lines of which contain q such points. This geometry is fully embedded in an affine subspace of and satisfies the antiflag property mentioned. As a result of our classification theorem we will give a new characterization theorem of this geometry.     

A finiteness theorem for low-codimensional nonsingular subvarieties of quadrics

We prove that there are only finitely many families of codimension two nonsingular subvarieties of quadrics which are not of general type, for and . We prove a similar statement also for the case of higher codimension.

     

New Families of Complete Caps, and the Asymptotic Size of the Largest Caps, of Quadrics over Prime Fields

A cap of a quadric is a set of its points whose pairwise joins are all chords. Such a cap is complete if it is not part of a larger one. Few examples of complete caps are known except for quadrics in low dimensions. In this paper, we consider the case when the coordinate field is GF(p), with p an odd prime, and construct, in each projective space GF(n,p) with n p – 1 and n – 2(mod p), a cap on one of its nonsingular quadrics. We use this in two ways. Firstly, we combine its size with the recent Blokhuis–Moorhouse upper bound for quadric caps to show that the size of the largest cap of any nonsingular quadric in PG(N,p) is asymptotic to Np – 1/(p – 1) ! as N tends to infinity. Secondly, by establishing situations when our cap is complete, we produce various infinite families of complete quadric caps over GF(p) for each p. Earlier work determined all complete caps of all nonsingular quadrics over GF(2).     

Classification of pairs of natural quadrics from the point of view of Laguerre geometry

The problem of classification of pairs of natural quadrics (spheres, circular cones and circular cylinders) arises in problems such as the blending of two or more natural quadrics using canal surfaces. In this paper, considering this problem from the point of view of Laguerre geometry, we reduce it to the classification of point/line and line/line pairs in the pseudo-Euclidean space with Lorenz metrics. We establish equations of the surfaces of centers for all canonical positions and their projective equivalence with toric surfaces.__________Translated from Lietuvos Matematikos Rinkinys, Vol. 45, No. 1, pp. 64–83, January–March, 2005.Translated by M. Kazakeviit     

Birational automorphisms of algebraic varieties with a pencil of double quadrics

The birational rigidity of multidimensional algebraic varieties with a pencil of double quadrics of index 1 sufficiently twisted with respect to the base is established. Translated fromMathematicheskie Zametki, Vol. 67, No. 2, pp. 241–249, February, 2000.     

Hyperelliptic Jacobians as billiard algebra of pencils of quadrics: Beyond Poncelet porisms

The thirty years old programme of Griffiths and Harris of understanding higher-dimensional analogues of Poncelet-type problems and synthetic approach to higher genera addition theorems has been settled and completed in this paper. Starting with the observation of the billiard nature of some classical constructions and configurations, we construct the billiard algebra, that is a group structure on the set T of lines simultaneously tangent to d−1 quadrics from a given confocal family in the d-dimensional Euclidean space. Using this tool, the related results of Reid, Donagi and Knörrer are further developed, realized and simplified. We derive a fundamental property of T: any two lines from this set can be obtained from each other by at most d−1 billiard reflections at some quadrics from the confocal family. We introduce two hierarchies of notions: s-skew lines in T and s-weak Poncelet trajectories, s=−1,0,…,d−2. The interrelations between billiard dynamics, linear subspaces of intersections of quadrics and hyperelliptic Jacobians developed in this paper enabled us to obtain higher-dimensional and higher-genera generalizations of several classical genus 1 results: Cayley's theorem, Weyr's theorem, Griffiths-Harris theorem and Darboux theorem.     

The maximum size of the intersection of two ovoids

In this article we show that the maximum size for the intersection of two distinct ovoids of PG(3,q), q even, is . This is less than the old bound of given by Segre.     

The base of the intersection of two free submonoids

Given two free submonoids of a free monoid, one wishes to find a specification for the base of the intersection. An algorithm to construct a graph-theoretic specification of the base is presented. From this specification it can easily be determined whether the base is finite. In addition, a a polynomial-time algorithm to determine if a regular set is a circular code is presented.     

Characterization of the Graded Betti Numbers for Almost Complete Intersections

Using standard mapping cone procedure of the inclusion map of an ideal generated by a suitable regular sequence inside a Gorenstein ideal and by a deep analysis of possible cancellations, we give a numerical characterization of the graded Betti sequences admissible for almost complete intersections of height three.     

The category of complete Boolean algebras is not an intersection of reflective subcategories of the category of frames

We answer a question of J. Rosický and W. Tholen by showing that the class of complete Boolean algebras (which is a prereflective subcategory of the category of frames) is not an intersection of reflective subcategories of the category of frames. In order to deduce this result, we first prove the following observation (using some basic facts about congruence frames): the three-element chain belongs to every reflective subcategory of the category of frames which contains the class of complete Boolean algebras.Supported by the Categorical Topology Research Group at the University of Cape Town, under funding from the Foundation for Research Development and the University of Cape Town.     

Young Tableaux and the Frobenius–Pullback of Tensor Differential Forms

In the case of prime characteristic char k = p > the pullback of the Frobenius provides an opportunity to define new discrete birational invariants of algebraic manifolds using the p^s–th powers (df) instead of the differentials df. Earlier results concerning T–symmetrical tensor forms on complete intersections will be generalized with regard to these differential forms.