Searcharxiv⌕ Search

arXiv subjects

Hans Havlicek

Publications and source records attributed to Hans Havlicek.

At least 19 recordsLinked to original sources
arXiv2025

Quadratic forms and their duals

There are many specific results, spread over the literature, regarding the dualisation of quadrics in projective spaces and quadratic forms on vector spaces. In the present work we aim at generalising and unifying some of these. We start with a quadratic form $Q$ that is defined on a subspace $S$ of a finite-dimensional vector space $V$ over a field $F$. Whenever $Q$ satisfies a certain condition, which comes into effect only when $F$ is of characteristic two, $Q$ gives rise to a dual quadratic form $\hat{Q}$. The domain of the latter is a particular subspace $\hat{S}$ of the dual vector space of $V$. The connection between $Q$ and $\hat{Q}$ is given by a binary relation between vectors of $S$ and linear forms belonging to $\hat{S}$.

math.AG↗
arXiv2024

On Isomorphisms of Grassmann Spaces

We exhibit isomorphisms of Grassmann spaces and their relationship with collineations and embeddings of the underlying projective spaces.

math.AG↗
arXiv2023

Affine Metric Geometry and Weak Orthogonal Groups

By following the ideas underpinning the well-established ``homogeneous model'' of an $n$-dimensional Euclidean space, we investigate whether the motion group or the weak motion group of an $n$-dimensional affine metric space on a vector space $V$ over an arbitrary field admits a specific faithful linear representation as weak orthogonal group of an $(n+1)$-dimensional metric vector space. Apart from a few exceptions, such a representation exists precisely when the metric structure on $V$ is given by a quadratic form with a non-degenerate polar form.

math.MG↗
arXiv2022

Helmut Karzel (1928-2021)

Obituary for Professor Dr. Dr. h.c. Helmut Karzel, who passed away on June 22, 2021, at the age of 93.

math.HO↗
arXiv2022

Completing bases in four dimensions

Criteria and constructive methods for the completion of an incomplete basis of, or context in, a four-dimensional Hilbert space by (in)decomposable vectors are given.

quant-ph↗
arXiv2021

Projective metric geometry and Clifford algebras

Each vector space that is endowed with a quadratic form determines its Clifford algebra. This algebra, in turn, contains a distinguished group, known as the Lipschitz group. We show that only a quotient of this group remains meaningful in the context of projective metric geometry. This quotient of the Lipschitz group can be viewed as a point set in the projective space on the Clifford algebra and, under certain restrictions, leads to an algebraic description of so-called kinematic mappings.

math.MG↗
arXiv2020

Characterising Clifford parallelisms among Clifford-like parallelisms

We recall the notions of Clifford and Clifford-like parallelisms in a $3$-dimensional projective double space. In a previous paper the authors proved that the linear part of the full automorphism group of a Clifford parallelism is the same for all Clifford-like parallelisms which can be associated to it. In this paper, instead, we study the action of such group on parallel classes thus achieving our main results on characterisation of the Clifford parallelisms among Clifford-like ones.

math.AG↗
arXiv2019

Automorphisms of a Clifford-like parallelism

In this paper we focus on the description of the automorphism group $Γ_{\parallel}$ of a Clifford-like parallelism $\parallel$ on a $3$-dimensional projective double space $\bigl(\mathbb{P}(H_F),{\mathrel{\parallel_{\ell}}},{\mathrel{\parallel_{r}}}\bigr)$ over a quaternion skew field $H$ (with centre a field $F$ of any characteristic). We compare $Γ_{\parallel}$ with the automorphism group $Γ_{\ell}$ of the left parallelism $\mathrel{\parallel_{\ell}}$, which is strictly related to $\mathrm{Aut}(H)$. We build up and discuss several examples showing that over certain quaternion skew fields it is possible to choose $\parallel$ in such a way that $Γ_{\parallel}$ is either properly contained in $Γ_{\ell}$ or coincides with $Γ_{\ell}$ even though ${\parallel} \neq{\mathrel{\parallel_{\ell}}}$.

math.AG↗
arXiv2018

Clifford-like parallelisms

Given two parallelisms of a projective space we describe a construction, called blending, that yields a (possibly new) parallelism of this space. For a projective double space $(\mathbb{P},\parallel_\ell,\parallel_r)$ over a quaternion skew field we characterise the "Clifford-like" parallelisms, i.e. the blends of the Clifford parallelisms $\parallel_\ell$ and $\parallel_r$, in a geometric and an algebraic way. Finally, we establish necessary and sufficient conditions for the existence of Clifford-like parallelisms that are not Clifford.

math.AG↗
arXiv2017

Pencilled regular parallelisms

Over any field $\mathbb K$, there is a bijection between regular spreads of the projective space ${\rm PG}(3,{\mathbb K})$ and $0$-secant lines of the Klein quadric in ${\rm PG}(5,{\mathbb K})$. Under this bijection, regular parallelisms of ${\rm PG}(3,{\mathbb K})$ correspond to hyperflock determining line sets (hfd line sets) with respect to the Klein quadric. An hfd line set is defined to be \emph{pencilled} if it is composed of pencils of lines. We present a construction of pencilled hfd line sets, which is then shown to determine all such sets. Based on these results, we describe the corresponding regular parallelisms. These are also termed as being \emph{pencilled}. Any Clifford parallelism is regular and pencilled. From this, we derive necessary and sufficient algebraic conditions for the existence of pencilled hfd line sets.

math.AG↗
arXiv2017

Divisible Designs, Laguerre Geometry, and Beyond

In these notes we aim at bringing together design theory and projective geometry over a ring. Both disciplines are well established, but the results on the interaction between them seem to be rare and scattered over the literature. Thus our main goal is to present the basics from either side, to develop, or at least sketch, the principal connections between them, and to make recommendations for further reading. There is no attempt to provide encyclopedic coverage with expansive notes and references.

math.CO↗
arXiv2016

Linear sets in the projective line over the endomorphism ring of a finite field

Let $\mathrm{PG}(1,E)$ be the projective line over the endomorphism ring $E=End_q({\mathbb F}_{q^t})$ of the $\mathbb F_q$-vector space ${\mathbb F}_{q^t}$. As is well known there is a bijection $Ψ:\mathrm{PG}(1,E)\rightarrow{\cal G}_{2t,t,q}$ with the Grassmannian of the $(t-1)$-subspaces in $\mathrm{PG}(2t-1,q)$. In this paper along with any $\mathbb F_q$-linear set $L$ of rank $t$ in $\mathrm{PG}(1,q^t)$, determined by a $(t-1)$-dimensional subspace $T^Ψ$ of $\mathrm{PG}(2t-1,q)$, a subset $L_T$ of $\mathrm{PG}(1,E)$ is investigated. Some properties of linear sets are expressed in terms of the projective line over the ring $E$. In particular the attention is focused on the relationship between $L_T$ and the set $L'_T$, corresponding via $Ψ$ to a collection of pairwise skew $(t-1)$-dimensional subspaces, with $T\in L'_T$, each of which determine $L$. This leads among other things to a characterization of the linear sets of pseudoregulus type. It is proved that a scattered linear set $L$ related to $T\in\mathrm{PG}(1,E)$ is of pseudoregulus type if and only if there exists a projectivity $φ$ of $\mathrm{PG}(1,E)$ such that $L_T^φ=L'_T$.

math.CO↗
arXiv2015

Clifford Parallelisms and External Planes to the Klein quadric

For any three-dimensional projective space ${\mathbb P}(V)$, where $V$ is a vector space over a field $F$ of arbitrary characteristic, we establish a one-one correspondence between the Clifford parallelisms of ${\mathbb P}(V)$ and those planes of ${\mathbb P}(V\wedge V)$ that are external to the Klein quadric representing the lines of ${\mathbb P}(V)$. We also give two characterisations of a Clifford parallelism of ${\mathbb P}(V)$, both of which avoid the ambient space of the Klein quadric.

math.AG↗
arXiv2014

A note on Clifford parallelisms in characteristic two

It is well known that a purely inseparable field extension $L/F$ with some extra property and degree $[L:F]=4$ determines a Clifford parallelism on the set of lines of the three-dimensional projective space over $F$. By extending the ground field of this space from $F$ to $L$, we establish the following geometric description of such a parallelism in terms of a distinguished `absolute pencil of lines' of the extended space: Two lines are Clifford parallel if, and only if, there exists a line of the absolute pencil that meets both of them.

math.AG↗
arXiv2014

Veldkamp-Space Aspects of a Sequence of Nested Binary Segre Varieties

Let $S_{(N)} \equiv PG(1,\,2) \times PG(1,\,2) \times \cdots \times PG(1,\,2)$ be a Segre variety that is $N$-fold direct product of projective lines of size three. Given two geometric hyperplanes $H'$ and $H''$ of $S_{(N)}$, let us call the triple $\{H', H'', \overline{H' ΔH''}\}$ the Veldkamp line of $S_{(N)}$. We shall demonstrate, for the sequence $2 \leq N \leq 4$, that the properties of geometric hyperplanes of $S_{(N)}$ are fully encoded in the properties of Veldkamp {\it lines} of $S_{(N-1)}$. Using this property, a complete classification of all types of geometric hyperplanes of $S_{(4)}$ is provided. Employing the fact that, for $2 \leq N \leq 4$, the (ordinary part of) Veldkamp space of $S_{(N)}$ is $PG(2^N-1,2)$, we shall further describe which types of geometric hyperplanes of $S_{(N)}$ lie on a certain hyperbolic quadric $\mathcal{Q}_0^+(2^N-1,2) \subset PG(2^N-1,2)$ that contains the $S_{(N)}$ and is invariant under its stabilizer group; in the $N=4$ case we shall also single out those of them that correspond, via the Lagrangian Grassmannian of type $LG(4,8)$, to the set of 2295 maximal subspaces of the symplectic polar space $\mathcal{W}(7,2)$.

math.CO↗