凸域内两点间的平均距离

本文研究了凸域内两点间的平均距离公式,利用广义支持函数的方法分别求出了圆、矩形、椭圆域内两点间的平均距离,并给出了具体的求解过程.     

凸域内弦的平均长度

本文研究了凸域内弦的平均长度.通过广义支持函数与凸域的弦幂积分,建立了凸域内弦的平均长度的一般公式,并用此公式得出了圆域和矩形域内弦的平均长度.     

运动星体内部径向方向上两点间的平均距离

本文研究了两个相交非空凸体的交集及其线性径向组合体内部两点间的平均距离问题.利用对偶均质积分和对偶混合体积两个工具, 获得了平均距离的计算公式, 推广了已有的对偶运动公式的相关结果.     

运动星体内部径向方向上两点间的平均距离

本文研究了两个相交非空凸体的交集及其线性径向组合体内部两点间的平均距离问题.利用对偶均质积分和对偶混合体积两个工具,获得了平均距离的计算公式,推广了已有的对偶运动公式的相关结果.     

一类常宽"等腰梯形"

本文由对角线等于底边长的等腰梯形构造了一类新的常宽“等腰梯形”, 而著名的常宽凸集圆盘与Reuleaux 三角形为退化的特例. 我们还证明了关于这类常宽“等腰梯形” 面积的Blaschke-Lebesgue定理.     

凸域内矩形的运动测度

本文研究了凸域内矩形的运动测度,通过对凸域内定长线段运动测度的推广,建立了包含在凸域内且长、宽都确定的矩形运动测度的一般公式,利用此公式得到了圆域和矩形域内此类矩形的运动测度,并以此为基础得到了推广后的Buffon投针问题的一些结果.     

中考中的另类梯形

梯形是一类特殊的四边形,它具有不同于一般四边形的性质.在中考中,梯形一直是重点考查内容,尤其是对等腰梯形的考查,因为等腰梯形的两腰相等,这又使得等腰梯形具有不同于一般梯形的独特性质.近几年中考中,又出现了很多另类的梯形,下面介绍一下这方面的知识点. 一、“黄金梯形” 我们通常把顶角为36°的等腰三角形叫做“黄金三角形”,那么我们可以由“黄金三角形”得到“黄金梯形”.如图1,△AABC是等腰三角形,其中A B=AC,∠A=36°.BE、CD分别是△ABC的两底角平分线,BE、CD相交于点O,连接DE.     

数形结合求解一类复杂代数式的最值问题

平面内两点间的距离公式是平面解析几何中最基本的公式之一,最近的模考题以及自主招生考题中出现了一类以平面内两点间的距离公式为背景的复杂代数式求最值问题.本文举例说明如何借助两点间的距离公式利用数形结合的数学思想来快速求解这类问题.     

用“两平行线间的距离处处相等”解题

<正>"两平行线间的距离处处相等"是平行线的一条重要性质,在有关几何问题中,若能构造出平行线间的两条垂线段,应用上述性质往往可化难为易,思路清晰简洁.下面举例说明.例1如图1,等腰梯形ABCD中,AD∥BC,AB=CD,对角线AC⊥BD,垂足为O,过点A作AP∥BD,连接DP.若DP=DB,且AD=2^(1/2),     

等腰梯形判定定理另证

等腰梯形的判定定理:若一个梯形的对角线相等,则这个梯形是等腰梯形.如图,在梯形ABCD中,AD∥BC,AC=BD,求证:梯形ABCD为等腰梯形.证明∵在梯形ABCD中,AD∥BC,     

On the Average Taxicab Distance Function and Its Applications

Generalized conics are subsets in the space all of whose points have the same average distance from a given set of points (focal set). The function measuring the average distance is called the average distance function (or the generalized conic function). In general it is a convex function satisfying a kind of growth condition as the preliminary results of Sect. 2 show. Therefore any sublevel set is convex and compact. We can also conclude that such a function has a global minimizer.

The paper is devoted to the special case of the average taxicab distance function given by integration of the taxicab distance on a compact subset of positive Lebesgue measure in the Euclidean coordinate space.

The first application of the average taxicab distance function is related to its minimizer. It is uniquely determined under some natural conditions such as, for example, the connectedness of the integration domain. Geometrically, the minimizer bisects the measure of the integration domain in the sense that each coordinate hyperplane passing through the minimizer cuts the domain into two parts of equal measure. The convexity and the Lipschitzian gradient property allow us to use the gradient descent algorithm that is formulated in terms of a stochastic algorithm (Sect. 3) to find the bisecting point of a set in \(\mathbb{R}^{n}\).

Example 1 in Sect. 4 shows the special form of the average taxicab distance function of a convex polygon. The level curves (generalized conics) admit semidefinite representations as algebraic curves in the plane because the average taxicab distance function is piecewise polynomial of degree at most three.

Some applications in geometric tomography are summarized as our main motivation to investigate the concept of the average taxicab distance function. Its second order partial derivatives give the coordinate \(X\)-rays of the integration domain almost everywhere and vice versa: the average taxicab distance function can be expressed in terms of the coordinate \(X\)-rays. Therefore the reconstruction of the sets given by their coordinate \(X\)-rays can be based on the average taxicab distance function instead of the direct comparison of the \(X\)-rays. In general (especially, in some classes of non-convex sets), the convergence property of the average taxicab distance function with respect to the Hausdorff convergence of the integration domain is better than the convergence of the \(X\)-rays (see regular and \(X\)-regular convergences in Sect. 4.1) and we can apply a standard approximation paradigm (Footnote 1) to solve the problem.

In the last section we prove that any compact convex body is uniquely determined by the diagonalization of its covariogram function measuring the average taxicab distance of the points from the intersection of the body with the translates of its axis parallel bounding box.

     

经典Hadamard不等式的高维推广

在n维Euclid空间利用多重积分的一般Stokes公式,将一元凸函数的经典H adam ard不等式在高维空间一般凸区域上进行了推广,得到了相应的高维Hadamard型不等式.这个结果蕴涵了经典的H adam ard不等式以及几个特殊凸体上的H adam ard型不等式.     

Equiaffine inner parallel curves of a plane convex body and the convex hulls of randomly chosen points

Summary. A general formula is proved, which relates the equiaffine inner parallel curves of a plane convex body and the probability that the convex hull of j independent random points is disjoint from the convex hull of k further independent random points. This formula is applied to improve some well-known results in geometric probability. For example, an estimate, which was established for a special case by L. C. G. Rogers, is obtained with the best possible bound, and an asymptotic formula due to A. Rényi and R.␣Sulanke is extended to an asymptotic expansion. Received: 21 May 1996     

Zwei integralgeometrische Formeln für Paare konvexer Körper

Zusammenfassung Betrachtet man Paare zufälliger Punkte, die in einem konvexen Körper gleichverteilt sind, sowie zufällige Geraden, die den konvexen Körper schneiden, so findet man eine einfache Beziehung der Momente der Verteilungen des Abstandes beider Punkte und der von der Geraden bestimmten Sehnenlänge. Diese Beziehung wird auf zwei Arten verallgemeinert. Die beiden Punkte sind in zwei konvexen Körpern gleichverteilt und zum anderen sind beliebige Funktionen des Abstandes zugelassen. Sodann werden einige Spezialfälle diskutiert.Drückt man die Geradendichte in Termen von Oberflächenelementen beider konvexer Körper aus, so gelangt man zu einer zweiten Klasse integralgeometrischer Formeln.

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Summary If we consider pairs of random points which are uniformly distributed in a convex body and random straight lines which hit the convex body, we find a simple relation between the moments of the distributions of the distance of the points and of the chord-length determined by the line. This relation can be generalized in two ways. On the one hand, the two points are taken from different convex bodies. On the other hand, we consider arbitrary functions of the distance. By expressing the density of lines in terms of surface-elements of the two bodies we obtain another integral-geometrical formula.

     

On ?-optimal points of convex, closed functions

The paper gives an estimate for the Hilbert space distance from a ?-optimal point to the minimum point of a convex, closed function, the subdifferential of which is a strongly monotone operator in its definition domain. Also, the Hausdorff distance between the ?-optimal points of the Tikhonov functions in the non-correct problems of mathematical programming is estimated.     

The Tilt Formula for Generalized Simplices in Hyperbolic Space

   Abstract. For a simplex in Lorentzian space whose vertices are in the positive light cone, Weeks defined the ``tilt' relative to each face. It gave us an efficient tool for deciding whether or not the dihedral angle between two simplices holding a face in common is convex. He also provided an efficient formula, called the ``tilt formula,' to obtain tilts from the intrinsic hyperbolic structure of the simplex when its dimension is two or three. Sakuma and Weeks generalized it to general dimensions. In this paper we generalize the concept of the tilt and the tilt formula to the case where not all vertices are in the positive light cone. A key to our generalization is to give a correspondence between points and hyperplanes (or half-spaces) in Lorentzian space.