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arXiv 1503.03111math.MG
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关于凸体的覆盖指数

On the covering index of convex bodies

Karoly Bezdek, Muhammad A. Khan

摘要:

用同形覆盖凸体是离散几何中的一个经典概念,它导致了许多有趣的和长期存在的问题。Swanepoel引入了凸体的覆盖参数作为量化其覆盖性质的手段。本文引入了覆盖参数的两个相关概念--覆盖指数和弱覆盖指数,它们是凸体的光照数、光照参数和覆盖参数的上界.直观地说,这两个指数衡量了一个凸体可以被相对较少数量的具有相同的相对较小的同素比的同素覆盖的程度。证明了覆盖指数是凸体的Banach-Mazur空间上的下连续泛函。我们进一步表明,仿射d-立方体最小化覆盖指数在任何维度d,而圆盘最大化它在平面上。此外,覆盖指数满足与向量直接和和运算的良好兼容性。事实上,我们得到了凸体的向量直和的覆盖指数的一个精确公式,它适用于无限多个情形。这与最小化性质一起可以用来确定无限多个凸体的覆盖指数。顾名思义,弱覆盖指数失去了覆盖指数的一些重要性质。最后,我们得到了覆盖指数和弱覆盖指数的上界。

英文摘要:

Covering a convex body by its homothets is a classical notion in discrete geometry that has resulted in a number of interesting and long-standing problems. Swanepoel introduced the covering parameter of a convex body as a means of quantifying its covering properties. In this paper, we introduce two relatives of the covering parameter called covering index and weak covering index, which upper bound well-studied quantities like the illumination number, the illumination parameter and the covering parameter of a convex body. Intuitively, the two indices measure how well a convex body can be covered by a relatively small number of homothets having the same relatively small homothety ratio. We show that the covering index is a lower semicontinuous functional on the Banach-Mazur space of convex bodies. We further show that the affine d-cubes minimize covering index in any dimension d, while circular disks maximize it in the plane. Furthermore, the covering index satisfies a nice compatibility with the operations of direct vector sum and vector sum. In fact, we obtain an exact formula for the covering index of a direct vector sum of convex bodies that works in infinitely many instances. This together with a minimization property can be used to determine the covering index of infinitely many convex bodies. As the name suggests, the weak covering index loses some of the important properties of the covering index. Finally, we obtain upper bounds on the covering and weak covering index.

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