An extension of Bruen chains

One way to obtain a new non-Desarguesian translation plane is by constructing a new spread that is not subregular. Chains of reguli in a regular spread of PG(3,q) were first introduced by Bruen as a method of obtaining a non-subregular spread. In this paper, we shall extend Bruen's notion of a chain of reguli. Let Ω be a regular spead of PG(3,q). A collection of reguli in Ω such that every line of Ω is contained in exactly none or two of these reguli will be called anest of reguli. Let γ be the spread obtained by replacing in Ω the lines of the nest with the lines of some other partial spread of PG(3,q) covering the same points. We shall show that in the case where the number of reguli in the nest is no more thanq, γ is not subregular and its full collineation group is the inherited group.     

Affine Spaces within Projective Spaces

We endow the set of complements of a fixed subspace of a projective space with the structure of an affine space, and show that certain lines of such an affine space are affine reguli or cones over affine reguli. Moreover, we apply our concepts to the problem of describing dual spreads. We do not assume that the projective space is finite-dimensional or pappian.     

Projective representations II. generalized chain geometries

In this paper, projective representations of generalized chain geometries are investigated, using the concepts and results of [5]. In particular, we study under which conditions such a projective representation maps the chains of a generalized chain geometry Σ (F, R) to reguli; this mainly depends on how the field F is embedded in the ring R. Moreover, we determine all bijective morphisms of a certain class of generalized chain geometries with the help of projective representations. Dedicated to Walter Benz on the occasion of his 70th birthday. The first author was supported by a Lise Meitner Research Fellowship of the Austrian Science Fund (FWF), projects M529-MAT, M574-MAT..     

Translation Ovoids of Generalized Quadrangles and Hexagnos

We define the notion of a translation ovoid in the classical generalized quadrangles and hexagons of order q, and we enumerate all known examples; translation spreads are defined dually. A modification of the known ovoids in the generalized hexagon H(q), q=3^2h+1, yields new ovoids of that hexagon. Dualizing and projecting along reguli, we obtain an alternative construction of the Roman ovoids due to Thas and Payne. Also, we construct a new translation spread in H(q) for any 1 mod 3, q odd, with the property that any projection along reguli yields the classical ovoid in the generalized quadrangle Q(4,q). Finally, we prove that for q odd, the new example is the only non-Hermitian translation spread in H(q) with the property that any projection along reguli yields the classical ovoid in Q(4,q).     

Note on exterior lines to two disjoint reguli

LetR ₁ andR ₂ be two disjoint reguli in the projective 3-space over the field GF(q) whereq=p ^e ,p an odd prime. Ifp is not a point in neither of the two doubly-ruled quadrics associated to the given reguli, then there is at least one line through P which does not meet neither of the two reguli. Supported in part by FONDECYT Project N. 0343 and DIB E-2586/8712.     

Infinite Nests of Reguli

In this article, infinite versions of t-nests for t=q,(q-1), (q+1),2(q-1) and mixed nests of reguli are constructed. Furthermore, a classification of all group replaceable spreads is given.     

Almost Desarguesian Maximal Partial Spreads

Constructions are given of various classesof maximal partial spreads in PG(3,2 ^r ) whose partialspreads consist of q/2 reguli sharing a line. Further,characterization results are given for the main classes of constructedmaximal partial spreads.     

Chain spaces over Jordan systems

Strong Jordan systems are certain subspaces of associative algebras closed under inversion and with many units. Every strong Jordan system gives rise to a chain space. We show that every homotopism of Jordan systems yields a morphism between the associated chain spaces and vice versa. By this, we obtain an isomorphy of categories.     

Further translation planes of order 19<Superscript>2</Superscript> admitting SL(2,5), obtained by nest replacement

We construct three translation planes of order 19² admitting SL(2,5), obtained by replacement of 24-nests of reguli in PG(3,19).     

Weak convergence of Markov-modulated random sequences

This work is concerned with weak convergence of non-Markov random processes modulated by a Markov chain. The motivation of our study stems from a wide variety of applications in actuarial science, communication networks, production planning, manufacturing and financial engineering. Owing to various modelling considerations, the modulating Markov chain often has a large state space. Aiming at reduction of computational complexity, a two-time-scale formulation is used. Under this setup, the Markov chain belongs to the class of nearly completely decomposable class, where the state space is split into several subspaces. Within each subspace, the transitions of the Markov chain varies rapidly, and among different subspaces, the Markov chain moves relatively infrequently. Aggregating all the states of the Markov chain in each subspace to a single super state leads to a new process. It is shown that under such aggregation schemes, a suitably scaled random sequence converges to a switching diffusion process.     

On the Grassmanian of lines in PG(4,q) and R(1,2) reguli

An R(1,2) regulus is a collection of q+1 mutually skew planes in PG(5,q) with the property that a line meeting three of the planes must meet all the planes. An (l,π)-configuration is the collection of lines in PG(4,q) meeting a line l and a plane π skew to l. A correspondence between (l,π)-configurations in PG(4-,q) and R(1,2) reguli in the associated Grassmanian space G(1,4) is examined. Bose has shown that R(1,2) reguli represent Baer subplanes of a Desarguesian projective plane in a linear representation of the plane. With the purpose of examining the relations between two Baer subplanes of PG(2,q²), the author examines the possible intersections of a 3-flat with an R(1,2) regulus.     

Square-central elements and standard generators for biquaternion algebras

Analyzing square-central elements in central simple algebras of degree 4, we show that every two elementary abelian Galois maximal subfields are connected by a chain of nontrivially-intersecting pairs. Similar results are proved for non-central quaternion subalgebras, and for central quaternion subalgebras when they exist. Along these lines we classify the maximal square-central subspaces. We also show that every two standard quadruples of generators of a biquaternion algebra are connected by a chain of basic steps, in each of which at most two generators are being changed.     

A Bruen chain forq=19

The one-to-one correspondence between the class of two-dimensional translation planes of orderq ² and the collection of spreads ofPG(3,q) has long provided a natural context for describing new planes. The method often used for constructing interesting spreads is to start with a regular spread, corresponding to a desarguesian plane, and then replace some nice subset of lines by another partial spread covering the same set of points. Indeed the first approach was replacing the lines of a regulus by the lines of its opposite regulus, or doing this process for a set of disjoint reguli. Nontrivial generalizations of this idea include thechains of Bruen and thenests of Baker and Ebert. In this paper we construct a replaceable subset of a regular spread ofPG (3, 19) which is the union of 11 reguli double covering the lines in their union, hence is a chain in the terminology of Bruen or a 11-nest in the Baker-Ebert terminology.     

Zur Darstellung der Unterräume von Kettengeometrien

A subspace of a chain geometry in the sense of W. Benz [1] is defined as a subset S of points such that the chain joining any three pairwise non-parallel points of S is contained in S. In this note we are concerned with the problem of determining those subspaces that can be represented by subalgebras.     

On a projective representation of chain geometries

We define a distance d on the set of r-spaces of an n-space. By the transfer of d to the GraßmannianG=G(n, r) we obtain a distinguished class of normal rational curves of order 1, the 1-distance lines, 1=1,..., r, which are in 1–1-correspondence to the so-called generalized reguli of type (r, 1).To every chain geometry there are subspaces T and Z of the surrounding space ofG, such that forV=GT andW = VZ we have a projective representation of on V\W as pointset, where the chains of are exactly the r-distance lines on V\W.Dedicated to Prof.A. Barlotti on occasion of his 60 birthday     

Localic subspaces and colimits of localic spaces

A spectral space is localic if it corresponds to a frame under Stone Duality. This class of spaces was introduced by the author (under the name ’locales’) as the topological version of the classical frame theoretic notion of locales, see Johnstone and also Picado and Pultr). The appropriate class of subspaces of a localic space are the localic subspaces. These are, in particular, spectral subspaces. The following main questions are studied (and answered): Given a spectral subspace of a localic space, how can one recognize whether the subspace is even localic? How can one construct all localic subspaces from particularly simple ones? The set of localic subspaces and the set of spectral subspaces are both inverse frames. The set of localic subspaces is known to be the image of an inverse nucleus on the inverse frame of spectral subspaces. How can the inverse nucleus be described explicitly? Are there any special properties distinguishing this particular inverse nucleus from all others? Colimits of spectral spaces and localic spaces are needed as a tool for the comparison of spectral subspaces and localic subspaces.     

Resolvent convergence and spectral approximations of sequences of self-adjoint subspaces

This paper studies resolvent convergence and spectral approximations of sequences of self-adjoint subspaces (relations) in complex Hilbert spaces. Concepts of strong resolvent convergence, norm resolvent convergence, spectral inclusion, and spectral exactness are introduced. Fundamental properties of resolvents of subspaces are studied. By applying these properties, several equivalent and sufficient conditions for convergence of sequences of self-adjoint subspaces in the strong and norm resolvent senses are given. It is shown that a sequence of self-adjoint subspaces is spectrally inclusive under the strong resolvent convergence and spectrally exact under the norm resolvent convergence. A sufficient condition is given for spectral exactness of a sequence of self-adjoint subspaces in an open interval lacking essential spectral points. In addition, criteria are established for spectral inclusion and spectral exactness of a sequence of self-adjoint subspaces that are defined on proper closed subspaces.     

Perturbation analysis of singular subspaces and deflating subspaces

Summary. Perturbation expansions for singular subspaces of a matrix and for deflating subspaces of a regular matrix pair are derived by using a technique previously described by the author. The perturbation expansions are then used to derive Fr\'echet derivatives, condition numbers, and th-order perturbation bounds for the subspaces. Vaccaro's result on second-order perturbation expansions for a special class of singular subspaces can be obtained from a general result of this paper. Besides, new perturbation bounds for singular subspaces and deflating subspaces are derived by applying a general theorem on solution of a system of nonlinear equations. The results of this paper reveal an important fact: Each singular subspace and each deflating subspace have individual perturbation bounds and individual condition numbers. Received July 26, 1994     

由给定的子空间构造新的子空间

本给出并证明了若干个子空间的并以及两个子空间的基构成子空间的充要条件，从而本质地揭示了除子空间的交与和是构造新的予空间的方法外，集合的其它运算不能构造新的子空间，最后分析了子空间直和的两种不同定义的优缺点，指出了张禾瑞教材中子空间直和定义推广时应注意的一个问题。     

The Lattice of Synchrony Subspaces of a Coupled Cell Network: Characterization and Computation Algorithm

Coupled cell systems are networks of dynamical systems (the cells), where the links between the cells are described through the network structure, the coupled cell network. Synchrony subspaces are spaces defined in terms of equalities of certain cell coordinates that are flow-invariant for all coupled cell systems associated with a given network structure. The intersection of synchrony subspaces of a network is also a synchrony subspace of the network. It follows, then, that, given a coupled cell network, its set of synchrony subspaces, taking the inclusion partial order relation, forms a lattice. In this paper we show how to obtain the lattice of synchrony subspaces for a general network and present an algorithm that generates that lattice. We prove that this problem is reduced to obtain the lattice of synchrony subspaces for regular networks. For a regular network we obtain the lattice of synchrony subspaces based on the eigenvalue structure of the network adjacency matrix.