New integer pairs for Hjelmslev planes

Associated with every finite projective Hjelmslev plane is an invariant pair(t, r); t is the order of the Hjelmslev plane andr is the order of the underlying projective plane. The aim of this paper is to give some new constructions of Hjelmslev planes with an invariant pair (t, 2). First we construct a PH-plane with the invariant pair (20, 2). Using this, 16 more invariant pairs (t, 2) witht 1000 are obtained. In all, we thus obtain 17 new PH-planes with invariant pairs (t, 2),t 1000.     

Some new invariant pairs (t,3) for projective hjelmslev planes

Associated with every finite projective Hjelmslev plane is an invariant pair (t,r): t is the number of neighbours of a given point on a given line passing through it and r is the order of the underlying projective plane. The Drake-Lenz method [2],[3] of using auxiliary matrices for the constructions of projective Hjelmslev planes has become standard by now. This paper is intended to give some new constructions of projective Hjelmslev planes with invariant pairs (t,3) by making use of the generalization and improvement of the Drake-Lenz theorem [3] obtained by the author in [6] and [7]. The results of this paper add 8 new values to the list ([5], example 3.7(ii)) of invariant pairs (t,3) with t 1,000 for projective Hjelmslev planes.     

Polarities in n-uniform projective Hjelmslev planes

In [9] the author has studied polarities in finite 2-uniform projective Hjelmslev planes. The present paper deals with polarities in finite n-uniform projective Hjelmslev planes (n 2).The author's research was supported by IWONL grant no. 840037.     

Order and topology in projective Hjelmslev planes

The concept of an ordered projective Hjelmslev plane was intuitively introduced by Hjelmslev in Einleitung in die allgemeine Kongruenglehre ([9], [10]).This paper is concerned with formalizing and examing preorderings and orderings for projective Hjelmslev planes. In addition we show that orderings generated topologies of the point and line sets which render the plane a topological Hjelmslev plane ([19], [13]). These planes — unlike the ordinary ordered planes ([18]) — are, due to the existence of infinitesimals, non-archimedian, non-compact and disconnected with the neighbour classes as certain quasi-components.The authors gratefully acknowledge the support of the Natural Sciences and Engineering Research Council of Canada.     

Polarities in finite 2-uniform projective Hjelmslev planes

In this paper we deal with polarities in finite projective Hjelmslev planes. A polarity is an involutory one-to-one mapping of the points onto the lines and of the lines onto the points such that the incidence relation and the neighbor relation are preserved. Most of the results are obtained in the case of 2-uniform PH-planes. In a forthcoming paper [5], we shall deal with the case of n-uniform (n > 2) PH-planes.The author's research was supported by I.W.O.N.L. grant No. 840037.     

Regular Hjelmslev planes

A projective Hjelmslev plane is called regular iff it admits an Abelian collineation group that is regular on both the points and lines of the plane and that splits into a summand regular on the elements of any given neighborhood and another summand permuting the points and lines of the projective image plane regularly. Regular Hjelmslev planes are shown to correspond to so-called special difference sets. We construct regular Hjelmslev planes with parameters (qⁿ, q) for any prime power q and any natural number n as well as for infinitely many series of parameters (t, q), where t is not a power of q. Our construction also yields series of parameters for which the existence of a Hjelmslev plane was not known up to now as well as the first information on the existence of nontrivial collineations in the case of parameters (t, q) with t not a power of q.     

Preordered uniform Hjelmslev planes

In Order and topology in projective Hjelmslev planes, the authors introduced the notions of preorder and order on projective Hjelmslev planes (PH-planes); cf. [3: 3.7, 5.3]. They also showed that in both ordered and preordered uniform PH-planes singular segments are uniquely determined by their endpoints. In this note, we show that the notions of order and preorder actually coincide on uniform PH-planes.The author gratefully acknowledges the support of the Natural Sciences and Engineering Research Council of Canada.     

Constructions of Hjelmslev planes

We present a new construction method for strongly n-uniform Hjelmslev planes which gives greater control in selecting the point and line neighborhood structures of the constructed planes. As a consequence, we are able to complete the proof of the Drake-Törner Theorem which asserts: the spectrum of invariant pairs for the class of finite, regular, minimally uniform projective Hjelmslev planes is the set of all Lenz-pairs.     

Affine planes over finite rings,a summary

In Keppens (Innov. Incidence Geom. 15: 119–139, 2017) we gave a state of the art concerning “projective planes” over finite rings. The current paper gives a complementary overview for “affine planes” over rings (including the important subclass of desarguesian affine Klingenberg and Hjelmslev planes). No essentially new material is presented here but we give a summary of known results with special attention to the finite case, filling a gap in the literature.     

Hjelmslevgruppen mit Nachbarhomomorphismus

Hjelmslev groups have been introduced by F. Bachmann ([1], [2]) in order to study plane metric geometries in a general sense: For example two points may have none or two lines joining them. Let (G,S) and (-G,¯ S) be Hjelmslev groups and let be a Hjelmslev homomorphism from (G,S) onto (¯G, ¯S). It is shown that — under certain assumptions — the group plane of (G, S) can be embedded into the projective Hjelmslev plane over a local ringR and thatG is isomorphic to a subgroup of an orthogonal group O ₃ ⁺ (V,f). The result may be considered as a generalization of the main theorem in Bachmann [1].     

On polarities in the k-uniform n-dimensional projective Hjelmslev space PH(n,GF(q)[t]/tk), q odd

The study of polarities in finite projective Hjelmslev spaces started with [5] where the plane 2-uniform case was treated. The results obtained there have been extended in [6] to the plane k-uniform case (k≧2). The present paper deals with polarities in the k-uniform n-dimensional PH-space PH(n,GF(q) [t] _/tk)(k≧2, n≧2) with q odd.     

Ovoids and translation planes revisited

Kantor has previously described the translation planes which may be obtained by projecting sections of ovoids in ⁺(8, q)-spaces to ovoids in corresponding ⁺(6, q)-spaces. Since the Klein correspondence associates spreads in 4-dimensional vector spaces with ovoids in ⁺(6, q)-spaces, there are corresponding translation planes of order q ² and kernel containing GF(q). In this article, we revisit some of these translation planes and give some presentations of the spreads. Motivated by various properties of the planes, we study, in general, translation planes which admit certain homology groups and/or elation groups. In particular, we develop new constructions of projective planes of Lenz-Barlotti class II-1.Finally, we show how certain projective planes of order q ² of Lenz-Barlotti class II-1 may be considered equivalent to flocks of quadratic cones in PG(3, q).This work was partially supported by NSF grant DMS-8800843.     

Twisted derivations and distinct sets of lines that cover the same pairs of points

A partial projective plane of ordern consists of lines andn ² +n + 1 points such that every line hasn+1 points and distinct lines meet in a unique point. Suppose that two essentially different partial projective planes and of ordern, n a perfect square, that are defined on the same set of points cover the same pairs of points. For sufficiently largen we show that this implies that and have at leastn(n+1) lines. This bound is sharp and there exist essentially two different types of examples meeting the bound.As an application, we can show that derived planes provide an example for a pair of projective planes of square order with as much structure as possible in common, that is, as many lines as possible in common. Furthermore, we present a new method (twisted derivations) to obtain planes from one another by replacing the same number of lines as in a derivation.     

On an assertion of Dembowski

In his Finite Geometries, Dembowski asserted that the class of uniform projective Hjelmslev planes of order q and a certain class of symmetric divisible partial designs coincide. We give a counterexample for q=2 and prove the validitiy of Dembowski's assertion for all q3.     

Linear Perfect Codes and a Characterization of the Classical Designs

A new definition for the dimension of a combinatorial t-(v,k,) design over a finite field is proposed. The complementary designs of the hyperplanes in a finite projective or affine geometry, and the finite Desarguesian planes in particular, are characterized as the unique (up to isomorphism) designs with the given parameters and minimum dimension. This generalizes a well-known characterization of the binary hyperplane designs in terms of their minimum 2-rank. The proof utilizes the q-ary analogue of the Hamming code, and a group-theoretic characterization of the classical designs.     

A structural characterization of non-Arguesian lattices

This is the first of a planned series of papers on the structure of non-Arguesian modular lattices. Apart from the (subspace lattices of) non-Arguesian projective planes, the best known examples of such lattices are obtained via the Hall-Dilworth construction by badly gluing together two projective planes of the same order. Our principal result shows that every non-Arguesian modular lattice L retains some of the flavor of these examples: There exist in the ideal lattice of L 20 intervals, not necessarily distinct, that form non-degenerate projective plains, and 10 points and 10 lines in these planes that constitute in a natural sense a classical non-Arguesian configuration.Research supported by NSERC Operating Grant A8190.Research supported by NSF Grant DMS-8300107.     

Non-free extensions of the simplex codes over a chain ring with four elements

Let R be a chain ring with four elements. In this paper, we present two new constructions of R-linear codes that contain a subcode associated with a simplex code over the ring R. The simplex codes are defined as the codes generated by a matrix having as columns the homogeneous coordinates of all points in some projective Hjelmslev geometry PHG(R ^k ). The first construction generalizes a recent result by Kiermaier and Zwanzger to codes of arbitrary dimension. We provide a geometric interpretation of their construction which is then extended to projective Hjelmslev spaces of arbitrary dimension. The second construction exploits the possibility of adding two non-free rows to the generator matrix of a linear code over R associated with a given point set. Though the construction works over both chain rings with four elements, the better codes are obtained for ${R=\mathbb{Z}_4}$ .     

Blocking sets of Rédei type in projective Hjelmslev planes

In this paper, we introduce Rédei type blocking sets in projective Hjelmslev planes over finite chain rings. We construct, in Hjelmslev planes over chain rings of nilpotency index 2 that contain the residue field as a proper subring, the Baer subplanes associated with this subring as Rédei type blocking sets. Two further examples of Rédei type blocking sets are given for planes over Galois rings generalizing familiar constructions in projective planes over finite fields.     

On balanced regular Hjelmslev planes

We consider regular Hjelmslev planes satisfying additional assumptions; in particular, we always assume the planes to be balanced. The main result of the paper is the determination of the spectra of invariants of regular n-uniform H-planes and of balanced minimally uniform regular H-planes. These are determined exactly (not only up to orders of projective planes), the only other spectrum known exactly up to now being the spectrum of desarguesian H-planes. We also prove that our construction method for balanced minimally uniform regular H-planes is canonical.The author acknowledges the hospitality of the University of Florida while doing this research.     

Planar Functions and Planes of Lenz-Barlotti Class II

Planar functions were introduced by Dembowski and Ostrom [4] to describe projective planes possessing a collineation group with particular properties. Several classes of planar functions over a finite field are described, including a class whose associated affine planes are not translation planes or dual translation planes. This resolves in the negative a question posed in [4]. These planar functions define at least one such affine plane of order 3^e for every e 4 and their projective closures are of Lenz-Barlotti type II. All previously known planes of type II are obtained by derivation or lifting. At least when e is odd, the planes described here cannot be obtained in this manner.