Projective deformation and biholomorphic equivalence of real hypersurfaces

The concepts of first order projective deformation, biholomorphic equivalence, and equivalence of induced Cauchy-Riemann structure are all equivalent for real analytic hypersurfaces in complex projective space. Studying the first concept leads to a realization of the Cauchy-Riemann structure bundle as a submanifold of the projective group. The Chern-Moser connection on this bundle can then be given in terms of the Maurer-Cartan form of the projective group, and equations analogous to the Gauss equations of Euclidean geometry give the Chern-Moser invariants. Research supported by the National Science Foundation under Grant Number MCS-8103370.     

Holomorphically projective transformations on complex Riemannian manifold

The aim of the paper is to prove that if a complex Riemannian manifold with holomorphic characteristic connection is holomorphically projective equivalent to a locally symmetric space then it is a complex Riemannian manifold of pointwise constant holomorphic characteristic sectional curvature.Dedicated to N.K. Stephanidis on the occasion of his 65 th birthday.     

Einstein metrics in projective geometry

It is well known that pseudo–Riemannian metrics in the projective class of a given torsion free affine connection can be obtained from (and are equivalent to) the solutions of a certain overdetermined projectively invariant differential equation. This equation is a special case of a so-called first Bernstein–Gelfand–Gelfand (BGG) equation. The general theory of such equations singles out a subclass of so-called normal solutions. We prove that non-degenerate normal solutions are equivalent to pseudo–Riemannian Einstein metrics in the projective class and observe that this connects to natural projective extensions of the Einstein condition.     

Reductions of the object and the affine torsion tensor of a centroprojective connection on a distribution of planes

The projective group is represented as a bundle of centroprojective frames. This bundle is endowed with a centroprojective connection and becomes the space of this centroprojective connection. Structure equations of this space are found, which include the affine torsion tensor and the centroprojective curvature tensor containing the affine curvature subtensor. A distribution of planes in projective space and its associated principal bundle (which has two simplest and two simple (in the sense of [1]) quotient principal bundles) are considered. On the associated bundle, a group connection is defined. The object of the centroprojective connection is reduced to the object of the group connection. The object of the group connection contains the objects of the flat and normal linear connections, the centroprojective subconnection, and the affine-group connection as subobjects. The torsion object of the affine-group connection is determined. It is proved that it forms a tensor, which contains the torsion tensor of the normal linear connection as a subtensor. It is shown that the affine torsion tensor of the centroprojective connection reduces to the torsion tensor of the affine-group connection.     

On the Non-Existence of Certain Cameron-Liebler Line Classes in PG(3, q)

Our main result is a non-existence theorem for certain families of lines in three dimensional projective space PG(3, q) over a finite field GF(q). Specifically, a Cameron-Liebler line class in PG(3, q) is a set of lines which intersects every spread of PG(3, q) in the same number x of lines (this number is called its parameter). These sets arose in connection with an attempt by Cameron and Liebler to determine the subgroups of PGL(n+1, q) which have the same number of orbits on points (of PG(n, q)) as on lines; they satisfy several equivalent properties. Here we prove that for 2 < x q, no Cameron-Liebler line class of parameter x exists in PG(3, q). A relevant general question on incidence matrices is described.     

Projective spread sets

In this paper the notion of a spread set for at-spread ofPG(2t+1,q) is generalised and it is shown that certaint-spreads ofPG(n, q) correspond to these generalised spread sets. Then a projective spread set is defined and it is shown that anyt-spread ofPG(n, q) corresponds to a projective spread set. Connections between the spread set and the projective spread set of at-spread are discussed, in particular in the case of at-spread ofPG(2t + 1,q) the spread set and the projective spread set are equivalent, giving a new and straightforward construction of a spread set. The methods developed are used to show, with the aid of a computer, that the 1-packing ofPG(7,2) constructed by Baker is regulus-free.Dedicated to Professor Giuseppe Tallini on the occasion of his 60th birthday     

Canonical decomposition of projective and affine killing vectors on the tangent bundle

For an affine connection on the tangent bundle T(M) obtained by lifting an affine connection on M, the structure of vector fields on T(M) which generate local one-parameter groups of projective and affine collineations is described. On the T(M) of a complete irreducible Riemann manifold, every projective collineation is affine. On the T(M) of a projectively Euclidean space, every affine collineation preserves the fibration of T(M), and on the T(M) of a projectively non-Duclidean space which is maximally homogeneous (in the sense of affine collineations) there exist affine collineations permuting the fibers of T(M).Translated from Matematicheskie Zametki, Vol. 19, No. 2, pp. 247–258, February, 1976.     

Hyperplane partitions and difference systems of sets

Difference Systems of Sets (DSS) are combinatorial configurations that arise in connection with code synchronization. This paper gives new constructions of DSS obtained from partitions of hyperplanes in a finite projective space, as well as DSS obtained from balanced generalized weighing matrices and partitions of the complement of a hyperplane in a finite projective space.     

Compact spreads and compact translation planes over locally compact fields

We prove that a spread S over a locally compact nondlscrete field F defines a topological translation plane if and only if the spread is compact. For F=R, this is implicit in Breuning's thesis [Bre], cf. [B 2]. For the proof, we describe the point set of the projective translation plane as a quotient space of some projective space, with identifications taking place in one hyperplane. This is new even for F=R.     

Geodesic Webs on a Two-Dimensional Manifold and Euler Equations

We prove that any planar 4-web defines a unique projective structure in the plane in such a way that the leaves of the web foliations are geodesics of this projective structure. We also find conditions for the projective structure mentioned above to contain an affine symmetric connection, and conditions for a planar 4-web to be equivalent to a geodesic 4-web on an affine symmetric surface. Similar results are obtained for planar d-webs, d>4, provided that additional d−4 second-order invariants vanish.     

Parallel submanifolds of complex projective space and their normal holonomy

The object of this article is to compute the holonomy group of the normal connection of complex parallel submanifolds of the complex projective space. We also give a new proof of the classification of complex parallel submanifolds by using a normal holonomy approach. Indeed, we explain how these submanifolds can be regarded as the unique complex orbits of the (projectivized) isotropy representation of an irreducible Hermitian symmetric space. Moreover, we show how these important submanifolds are related to other areas of mathematics and theoretical physics. Finally, we state a conjecture about the normal holonomy group of a complete and full complex submanifold of the complex projective space. Research partially supported by GNSAGA (INdAM) and MIUR of Italy.     

Strong connections and invertible weak entwining structures

In this paper we obtain a criterion under which the bijectivity of the canonical morphism of a weak Galois extension associated to a weak invertible entwining structure is equivalent to the existence of a strong connection form. Also we obtain an explicit formula for a strong connection under equivariant projective conditions or under coseparability conditions.     

Projective complex Finsler metrics

In this paper we obtain the conditions under which two complex Finsler metrics are projective, i.e. have the same geodesics as point sets. Two important classes of such metrics are considered: conformal projective and weakly projective complex Finsler spaces. For each of them we study the transformations of the canonical connection. We pay attention to local projectivity in a pure Hermitian or Kähler space.     

4-Webs on hypersurfaces of 4-axial space

V. B. Lazareva investigated 3-webs formed by shadow lines on a surface embedded in 3-dimensional projective space and assumed that the lighting sources are situated on 3 straight lines. The results were used, in particular, for the solution of the Blaschke problem of classification of regular 3-webs formed by pencils of circles in a plane. In the present paper, we consider a 4-web W formed by shadow surfaces on a hypersurface V embedded in 4-dimensional projective space assuming that the lighting sources are situated on 4 straight lines. We call the projective 4-space with 4 fixed straight lines a 4-axial space. Structure equations of 4-axial space and of the surface V , asymptotic tensor of V , torsions and curvatures of 4-web W, and connection form of invariant affine connection associated with 4-web W are found.     

Opers and theta functions

We construct natural maps (the Klein and Wirtinger maps) from moduli spaces of semistable vector bundles over an algebraic curve X to affine spaces, as quotients of the nonabelian theta linear series. We prove a finiteness result for these maps over generalized Kummer varieties (moduli space of torus bundles), leading us to conjecture that the maps are finite in general. The conjecture provides canonical explicit coordinates on the moduli space. The finiteness results give low-dimensional parametrizations of Jacobians (in for generic curves), described by 2Θ functions or second logarithmic derivatives of theta.We interpret the Klein and Wirtinger maps in terms of opers on X. Opers are generalizations of projective structures, and can be considered as differential operators, kernel functions or special bundles with connection. The matrix opers (analogues of opers for matrix differential operators) combine the structures of flat vector bundle and projective connection, and map to opers via generalized Hitchin maps. For vector bundles off the theta divisor, the Szegö kernel gives a natural construction of matrix oper. The Wirtinger map from bundles off the theta divisor to the affine space of opers is then defined as the determinant of the Szegö kernel. This generalizes the Wirtinger projective connections associated to theta characteristics, and the associated Klein bidifferentials.     

On the finsler space admitting a holonomy group

Summary The ideas of holonomy group fixing an m-dimensional plane in a Finsler space were given by one of the present authors[1]. In that paper the deformation properties of the space admitting such holonomy group were of main consideration and, indeed, the decomposition characteristics of the space were not touched upon. In the present paper we consider the decomposition of the space due to the existence of holonomy group. The geometry is constructed on the decomposed metric of the space. The decomposition properties of various entities such as the connection parameters, the covariant derivatives, the curvature tensors, and the projective curvature tensors have been studied. In all there are six articles in the paper. The first of these is introductory. The next three articles are dealt with the Cartan's approach to Finsler space whereas the fifth one is dealt with Berwald's approach. The last article is devoted to the theory of decomposition in the projective curvature tensors. Entrata in Redazione il 18 ottobre 1969.     

Some new two-weight codes and strongly regular graphs

It is well known (and due to Delsarte [3]) that the three concepts (i) two-weight projective code, (ii) strongly regular graph defined by a difference set in a vector space, and (iii) subset X of a projective space such that |X∩H| takes only two values when H runs over all hyperplanes, are equivalent. Here we construct some new examples (formulated as in (iii)) by taking a quadric defined over a small field and cutting out a quadratic defined over a larger field.     

Connections for weighted projective lines

We introduce a notion of a connection on a coherent sheaf on a weighted projective line (in the sense of Geigle and Lenzing). Using a theorem of Hübner and Lenzing we show, under a mild hypothesis, that if one considers coherent sheaves equipped with such a connection, and one passes to the perpendicular category to a nonzero vector bundle without self-extensions, then the resulting category is equivalent to the category of representations of a deformed preprojective algebra.     

Scattered Spaces with Respect to a Spread in PG(n,q)

A scattered subspace of PG(n-1,q) with respect to a (t-1)-spread S is a subspace intersecting every spread element in at most a point. Upper and lower bounds for the dimension of a maximum scattered space are given. In the case of a normal spread new classes of two intersection sets with respect to hyperplanes in a projective space are obtained using scattered spaces.