二阶曲线切线型方程的意义

本文论证了二阶曲线切线型直线方程的几何意义，其主要结论丰富了二阶曲线的一般理论．     

Rays of light trapped by conics

In a previous note [2], some geometric constructions were shown which can be interpreted as a ray of light trapped in a polygon by successive reflections on the sides. The same question is addressed here, replacing polygons by conics or pairs of conics. In some cases, the light is concentrated towards a specific segment. The tools used for the computations are elementary geometry, calculus and, in some cases, Commutative Algebra Software.     

Sketching the general quadratic equation using dynamic geometry software

This paper explores a geometrical way to sketch graphs of the general quadratic in two variables with Geometer's Sketchpad. To do this, a geometric procedure as described by De Temple is used, bearing in mind that this general quadratic equation (1) represents all the possible conics (conics sections), and the fact that five points (no three of which are collinear) uniquely determine a conic.     

Die zweifachen Blutelschen Kegelschnittflächen

A surface in projective space generated by a one parameter family of conics is called a conic surface of Blutel if the tangent planes of taken along a generating conic, envelop a quadratic cone. If the conjugate curves (with respect to the generating conics) are conics, too, we call a two-fold Blutel's conic surface. In an earlier paper [4] it was shown that the planes of both conic families, the generating and the conjugate one, belong to a pencil, each. The present paper completes these investigations by integrating the derivative equations (3), (8), (9), (10). As a final result, a complete classification of all these surfaces is given. They are all algebraic of at most fourth order and furthermore—besides the quadrics and certain degenerate cases—they are complex projectively equivalent to the cyclides of Dupin.     

Foliations on ℂℙ2 of degree 2 with degenerate singularities

In this paper we construct non-singular, locally-closed, algebraic varieties which are sets of foliations on ??² of degree 2 with a certain degenerate singularity. We obtain the dimension and closure of these varieties. To do that we construct a stratification (based on GIT, see [7]) of the space of foliations with respect to the action by change of coordinates. We prove that the set of unstable foliations has two irreducible components. We have the following corollary: a foliation of degree 2 defined by a pencil of conics is unstable if and only if the pencil is unstable. Finallywe give another proof of the fact that there are only 4 foliations of degree 2 with a unique singular point (see [5]).     

Real rational curves in Grassmannians

Fulton asked how many solutions to a problem of enumerative geometry can be real, when that problem is one of counting geometric figures of some kind having specified position with respect to some given general figures. For the problem of plane conics tangent to five general (real) conics, the surprising answer is that all 3264 may be real. Similarly, given any problem of enumerating -planes incident on some given general subspaces, there are general real subspaces such that each of the (finitely many) incident -planes is real. We show that the problem of enumerating parameterized rational curves in a Grassmannian satisfying simple (codimension 1) conditions may have all of its solutions real.

     

Edge symbolic structures of second generation

Operators on a manifold with (geometric) singularities are degenerate in a natural way. They have a principal symbolic structure with contributions from the different strata of the configuration. We study the calculus of such operators on the level of edge symbols of second generation, based on specific quantisations of the corner‐degenerate interior symbols, and show that this structure is preserved under composition (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Necessary and sufficient conditions for rational quartic representation of conic sections

Conic section is one of the geometric elements most commonly used for shape expression and mechanical accessory cartography. A rational quadratic Bézier curve is just a conic section. It cannot represent an elliptic segment whose center angle is not less than π$\pi$. However, conics represented in rational quartic format when compared to rational quadratic format, enjoy better properties such as being able to represent conics up to 2π$2\pi$ (but not including 2π$2\pi$) without resorting to negative weights and possessing better parameterization. Therefore, it is actually worth studying the necessary and sufficient conditions for the rational quartic Bézier representation of conics. This paper attributes the rational quartic conic sections to two special kinds, that is, degree-reducible and improperly parameterized; on this basis, the necessary and sufficient conditions for the rational quartic Bézier representation of conics are derived. They are divided into two parts: Bézier control points and weights. These conditions can be used to judge whether a rational quartic Bézier curve is a conic section; or for a given conic section, present positions of the control points and values of the weights of the conic section in form of a rational quartic Bézier curve. Many examples are given to show the use of our results.     

Geometric algebra illustrated by Cinderella

Conventional illustrations of the rich elementary relations and physical applications of geometric algebra are helpful, but restricted in communicating full generality and time dependence. The main restrictions are one special perspective in each graph and the static character of such illustrations. Several attempts have been made to overcome such restrictions. But up till now very little animated and fully interactive, free, instant access, online material is available. This report presents therefore a set of over 90 newly developed (freely online accessible [1]) JAVA applets. These applets range from the elementary concepts of vector, bivector, outer product and rotations to triangle relationships, oscillations and polarized waves. A special group of 21 applets illustrates three geometrically different approaches to the representation of conics; and even more ways to describe ellipses. Next Clifford’s famous circle chain theorem is illustrated. Finally geometric applications important for crystallography and structural mechanics give a glimpse of the vast potential for applied mathematics. The interactive geometry software Cinderella [2] was used for creating these applets. The interactive features of many of the applets invite the user to freely explore by a few mouse clicks as many different special cases and perspectives as he likes. This is of great help in “visualizing” geometry encoded by geometric algebra.     

Combinatorial invariants for nets of conics in $$\mathrm {PG}(2,q)$$ PG ( 2 , q )

Designs, Codes and Cryptography - The problem of classifying linear systems of conics in projective planes dates back at least to Jordan, who classified pencils (one-dimensional systems) of conics...     

Triads, Flocks of Conics and Q -(5,q)

We show that if an ovoid of Q (4,q),q even, admits a flock of conics then that flock must be linear. It follows that an ovoid of PG (3,q),q even, which admits a flock of conics must be an elliptic quadric. This latter result is used to give a characterisation of the classical example Q ^-(5,q) among the generalized quadrangles T ₃( ), where is an ovoid of PG (3q) and q is even, in terms of the geometric configuration of the centres of certain triads.     

Duality for Curves in Affine Plane Geometry

The dual of a plane curve in equiaffine geometry is defined as a curve in the space of conics. The image and inverse of this duality are described. It is shown that the duality transforms equiaffine vertices into cusps. As an application, an analogue of Kneser's lemma for osculating conics is proved.     

A remarkable configuration: A generalization of computergenerated results by S.-C. Chou and its connection with a plane figure by A. Emch

The paper's starting point are four theorems on conics which can be found in a collection of computer proved results by C.-S. Chou from 1987. It not only contains a generalization of two of Chou's results but also a plane figure consisting of points, lines and conics. A suitable notation will reveal a striking symmetry of this figure. Moreover, it turns out that a plane figure from 1940 found by A. Emch using algebraic methods is very similar to ours, which we obtained synthetically. As an application in finite geometry we have gone some way towards regarding our figure as a real projective model of the finite projective plane of order 4.Dedicated to Dr. J. F. Rigby on the occasion of his 65th birthday     

On the Singularities of Surfaces Ruled by Conics

We classify the singularities of a surface ruled by conics: they are rational double points of type A _n or D _n . This is proved by showing that they arise from a precise series of blow-ups of a suitable surface geometrically ruled by conics. We determine also the family of such surfaces which are birational models of a given surface ruled by conics and obtained in a “minimal way” from it.     

Ivory's Theorem in Hyperbolic Spaces

According to the planar version of Ivory's theorem, the family of confocal conics has the property that in each curvilinear quadrangle formed by two pairs of conics the diagonals are of equal length. It turned out that this theorem is closely related to selfadjoint affine transformations. This point of view opens up a possibility of generalizing the Ivory theorem to the hyperbolic and other spaces.     

On Conics that are Ovals in a Hall Plane

The conics of a finite Desarguesian plane of square even order satisfying the following properties are classified. (1) Their two infinite points (one being the nucleus) do not intersect a certain derivation set. (2) They are also ovals in the Hall plane constructed from the derivation set. This leads to a construction of ovals from conics in certain subregular planes of even order (which are translation planes of dimension 2 over their kernel).     

Conics in finite projective planes

There does not exist a general theory of conics in finite projective planes, because the many definitions of conics which are equivalent in desarguesian projective planes yield different types of conics in more general situations. Thus even the use of the word conic can lead to confusion, particularly in the finite case. This note is an attempt to clarify these various definitions and give as an example in a finite projective plane a von Staudt conic which is not an Ostrom conic. We conjecture that any finite projective plane admitting an Ostrom conic must be desarguesian.