Curves of constant diameter and inscribed polygons
Mathieu Baillif
备注:
6 pages, 10 figures. Aimed at an undergraduate audiance. V2 includes the published version, with many suggestions from the referee, and a short addendum with some improvements of the results
发表信息:
Elemente der Mathematik, 2009, vol. 64, no. 3, p. 102-108
A simple closed curve in the Euclidean plane is said to have property C_n(R) if at each point we can inscribe a unique regular $n$-gon with edges length $R$. C_2(R) is equivalent to having constant diameter. We show that smooth curves satisfying C_n(R) other than the circle do exist for all n, and that the circle is the only $C^2$ regular curve satisfying C_2(R) and C_4(R') where $R'=R/\sqrt{2}$. In an addendum, we show that the last assertion holds for any R and R'. The proofs use only elementary differential calculus and geometry.
A linear-time algorithm for the maximum-area inscribed triangle in a convex polygon
给定一个凸多边形的n个循环阶顶点,能否用O(n)时间复杂度的算法求出内接于P的最大面积三角形?1979年Dobkin和Snyder提出的线性时间算法最近被Keikha、Löffler、Urhausen和van der Hoog证明是不正确的。这些作者给出了一个替代算法的O(n log n)时间复杂度。这里我们给出了一个具有线性时间复杂度的算法。
我们调查的“广义海伦多项式”,涉及的平方面积的n边形内接在一个圆的平方,其边长。对于一个(2 m +1)-边形或(2 m +2)-边形,我们将它表示为从具有m-1个二重根的二元(2 m-1)-形式簇导出的某个簇的定义多项式。从而得到了循环七边形和循环八边形面积的显式公式,并阐明了循环五边形和循环六边形面积的Robbins公式的某些神秘性质。我们还介绍了一个同伴家庭的多项式,涉及的平方面积的n边形内接在一个圆,其一侧是直径,平方长度的其他方面。通过类似的代数技巧,我们得到了这些多项式的显式公式,所有n <= 7。
On the maximum product of distances of diameter $2$ point sets
KU Leuven Campus Kulak-Kortrijk(荷语鲁汶大学库尔拉克-科特赖克校区);Ghent University(根特大学);Institute of Software, Chinese Academy of Sciences(中国科学院软件研究所);Xi’an Jiaotong University(西安交通大学)