Über Böschungslinien im dreidimensionalen elliptischen Raum

In accordance withH. R. Müller [3] we understand under a curve of constant slope in the elliptic 3-space an isogonal trajectory of the generators of an arbitrary Clifford cylinder. Using linegeometric methods in a special projective model, we study in particular those curves of constant slope, whose tangents form also a constant angle with a fixed plane. Thereby we meet with well-known classes of curves in the Euclidean space, such as spherical involutoids and tractrices of circles and loxodromes on a torus.     

Isogonal Prismatoids

The study of the polyhedra (in Euclidean 3-space) in which faces may be self-intersecting polygons, and distinct faces may intersect in various ways, was quite fashionable about a century ago. The Kepler—Poinsot regular polyhedra, and several of their generalizations, were investigated about that time by Cayley, Wiener, Badoureau, Fedorov, Hess, Pitsch, and others; the accumulated wisdom was presented in Max Brückner's well-known book Vielecke und Vielflache in 1900. Despite the intrinsic interest of the topic, and its relations to various other disciplines, there have been very few additional investigations during the intervening century, except for discussions of uniform polyhedra. In particular, there has been no mention or clarification of the many errors and other shortcomings of Brückner's book. One of our aims is to point out the extent of these inadequacies; they are illustrated by a discussion of isogonal prismatoids, the investigation of which is our main objective. A prismatoid is a polyhedron having all its vertices in two parallel planes. Familiar examples are prisms and antiprisms. A polyhedron P is isogonal if all its vertices form one transitivity class under isometric symmetries of P. Although these restrictions appear very severe, there exist many different kinds of isogonal prismatoids. Some general concepts concerning polyhedra with possible self-intersections are presented, and several classes of isogonal prismatoids are discussed in some detail. Received April 5, 1995.     

All cubics are self-isogonal

We develop techniques to prove that a cubic curve is invariant under the isogonal transformation of the projective plane determined by some triangle.     

Two circles and their common tangents

Given two circles C ₁ and C ₂ in a plane such that neither one of the two circles is contained in the other, there are either four common tangents when the circles do not intersect at all or the circles have three common tangents when they touch each other externally or only two common tangents when the circles intersect exactly at two points. The article oulines analytical procedures for computing the equations of these common tangents. Using the built-in Maple of the software Scientific Work Place 3.0, the diagrams of the circles with their common tangents are incorporated in this article.     

欧氏空间中等角基的性质

等角基是正交基的推广,等角基具有和正交基相似的性质,因此研究等角基的性质能够为研究欧氏空间提供一种工具,加深对欧氏空间的了解.本文主要把n维欧氏空间中正交基的一些性质推广到等角基上,得到了五个关于等角基性质的定理.     

Systeme halbkonzentrischer Kreise der euklidischen Ebene im komplexen Gebiet

All circles having the same tangent in one of the two absolute pointsJ, \(\bar J\) are said to be semi-concentric. They form a special net of conics Σ. By means of a complex conformal transformationT, Σ corresponds to the system Σ′ of the straight lines. Congruent circles in Σ correspond to parallel lines in Σ′. Moebius geometry in Σ is shown to be a model of the plane euclidean geometry. Furthermore, the plane is projected stereographically onto a Riemannian sphere; thenT corresponds to a biaxial involutionT ₁ which transforms the sphere into itself. The circles of Σ correspond to the intersections of the sphere with the planes passing throughJ. Finally, for a curvec having an ordinary point inJ, it follows thatc possesses inJ generally a well-defined radius of curvature but an infinite number of circles of curvature; one of them hyperosculatesc inJ. ByT, these circles correspond to a pencil of parallel lines. There are also considered examples of several special curves passing throughJ and \(\bar J\) .     

欧氏空间中等角基的性质

等角基是正交基的推广,等角基具有和正交基相似的性质,因此研究等角基的性质能够为研究欧氏空间提供一种工具,加深对欧氏空间的了解．本文主要把n维欧氏空间中正交基的一些性质推广到等角基上,得到了五个关于等角基性质的定理．     

A note on Pythagorean hodograph quartic spiral

By using the geometric constraints on the control polygon of a Pythagorean hodograph (PH) quartic curve, we propose a sufficient condition for this curve to have monotone curvature and provide the detailed proof. Based on the results, we discuss the construction of spiral PH quartic curves between two given points and formulate the transition curve of a G² contact between two circles with one circle inside another circle. In particular, we deduce an attainable range of the distance between the centers of the two circles and summarize the algorithm for implementation. Compared with the construction of a PH quintic curve, the complexity of the solution of the equation for obtaining the transition curves is reduced.     

über die Striktionslinien von einparametrigen Scharen linearer Räume

A generalisation of a ruled surface in n-dimensional euclidean space may be generated by euclidean motion of a s-plane A_s. For this one-parametric family {A_s} the curve of striction is defined and the following theorems are proved:

1. The generators A_s are parallel along the curve of striction, i.e. the multivectors representing A_s form a parallel vector field along the curve of striction.
2. If the curve of striction is geodesic on {A_s}, it is also an isogonal trajectory of the family of generators {A_s}.

     

Some cubic curves associated with a triangle

There are many interesting cubic curves which arise from the geometry of the triangle. In particular, those which are invariant under the operation of isogonal conjugacy have attracted much attention, and a class of these is here investigated by a variety of methods.     

Cyclic and ruled Lagrangian surfaces in Euclidean four space

We study those Lagrangian surfaces in complex Euclidean space which are foliated by circles or by straight lines. The former, which we call cyclic, come in three types, each one being described by means of, respectively, a planar curve, a Legendrian curve in the 3-sphere or a Legendrian curve in the anti-de Sitter 3-space. We describe ruled Lagrangian surfaces and characterize the cyclic and ruled Lagrangian surfaces which are solutions to the self-similar equation of the Mean Curvature Flow. Finally, we give a partial result in the case of Hamiltonian stationary cyclic surfaces.     

G cubic transition between two circles with shape control

This paper describes a method for joining two circles with an S-shaped or with a broken back C-shaped transition curve, composed of at most two spiral segments. In highway and railway route design or car-like robot path planning, it is often desirable to have such a transition. It is shown that a single cubic curve can be used for blending or for a transition curve preserving G²$G^2$ continuity with local shape control parameter and more flexible constraints. Provision of the shape parameter and flexibility provide freedom to modify the shape in a stable manner which is an advantage over previous work by Meek, Walton, Sakai and Habib.     

Fair cubic transition between two circles with one circle inside or tangent to the other

This paper describes a method for joining two circles with a C-shaped and an S-shaped transition curve, composed of a cubic Bézier segment. As an extension of our previous work; we show that a single cubic curve can be used for blending or for a transition curve preserving G ² continuity regardless of the distance of their centers and magnitudes of the radii which is an advantage. Our method with shape parameter provides freedom to modify the shape in a stable manner.     

Inversive distance

Summary Any two non-intersecting circles have a kind of distance that is invariant for inversion, namely, the natural logarithm of the ratio of the radii (the larger to the smaller) of two concentric circles into which the given circles can be inverted. When the inversive plane is used as a conformal model for hyperbolic space [3, p 266], the inversive distance between two non-intersecting circles is equal to the hyperbolic distance between the corresponding ultra-parallel planes. In memory of Guido Castelnuovo, in the recurrence of the first centenary of his birth.     

Bonnesen's inequality for the isoperimetric deficiency of closed curves in the plane

An inequality of T. Bonnesen for the isoperimetric deficiency of a convex closed curve in the plane is extended to arbitrary simple closed curves. As a primary tool it is shown that, for any such curve, there exist two concentric circles such that the curve is between these and passes at least four times between them. Next, it is shown that Bonnesen's inequality holds for any such pair of concentric circles.This work was supported by The Danish Natural Science Council and carried out during the author's stay 1989–90 as a Visiting Member at the Institute for Advanced Study, Princeton, N.J., U.S.A.     

Director circles of conic sections

Given a conic section, the locus of a moving point in the plane of the conic section such that the two tangent lines drawn to the conic section from the moving point are all mutually perpendicular is a curve. In the case of an ellipse and hyperbola this curve is a circle referred to as the director circle. In the case of the parabola this curve coincides with the directrix of the parabola. The last section is devoted to the graphical illustrations of director circles for circles, parabolas, ellipses and hyperbolas using the built-in Maple V software of Scientific Work Place 3.0.     

REMARKS ON THE ZIG-ZAG THEOREM

The classical Zig-zag Theorem [1] says that if an equilateral closed 2m-gon shuttles between two given circles of the Euclidean 3-space, then the vertices of the polygon can be moved smoothly along the circles without changing the lengths of the sides of the polygon. First we prove that the Zig-zag Theorem holds also in the hyperbolic, Euclidean and spherical n-spaces, and in fact the circles can be replaced by straight lines or any kind of cycles. In the second part of the paper we restrict our attention to planar zig-zag configurations. With the help of an alternative formulation of the Zig-zag Theorem, we establish two duality theorems for periodic zig-zags between two circles. This revised version was published online in August 2006 with corrections to the Cover Date.     

Bézier曲线降多阶逼近的一种方法

文献[1,2]讨论了Bezier曲线一次降多阶逼近问题,得到了很好的结果.文献[1]利用广义逆矩阵得到不保端点插值的降多阶逼近曲线的控制顶点的表达式.但却没有得到带端点任意阶插值条件的降多阶逼近曲线的控制顶点的表达式.文献[2]得到了带端点任意阶插值的降多阶逼近曲线的控制顶点的解析表达式.本文首先给出两Bezier曲线间距离的定义;然后根据降阶曲线与原曲线间的距离最小,分别得到了用矩阵表示的不保端点插值和保端点任意阶插值的降多阶逼近曲线的控制顶点的显示表达式.所给数值例子显示,用本文方法得到的降多阶逼近曲线对原曲线有很好的逼近效果.     

On the Complexity of Arrangements of Circles in the Plane

Continuing and extending the analysis in a previous paper [15], we establish several combinatorial results on the complexity of arrangements of circles in the plane. The main results are a collection of partial solutions to the conjecture that (a) any arrangement of unit circles with at least one intersecting pair has a vertex incident to at most three circles, and (b) any arrangement of circles of arbitrary radii with at least one intersecting pair has a vertex incident to at most three circles, under appropriate assumptions on the number of intersecting pairs of circles (see below for a more precise statement). Received June 26, 2000, and in revised form January 30, 2001. Online publication October 5, 2001.     

On the Parameterization of Rational Ringed Surfaces and Rational Canal Surfaces

Ringed surfaces and canal surfaces are surfaces that contain a one-parameter family of circles. Ringed surfaces can be described by a radius function, a directrix curve and vector field along the directrix curve, which specifies the normals of the planes that contain the circles. In particular, the class of ringed surfaces includes canal surfaces, which can be obtained as the envelopes of a one-parameter family of spheres. Consequently, canal surfaces can be described by a spine curve and a radius function. We present parameterization algorithms for rational ringed surfaces and rational canal surfaces. It is shown that these algorithms may generate any rational parameterization of a ringed (or canal) surface with the property that one family of parameter lines consists of circles. These algorithms are used to obtain rational parameterizations for Darboux cyclides and to construct blends between pairs of canal surfaces and pairs of ringed surfaces.