On coverings of plane convex sets by translates of strips

In the euclidean planeE ² letS ₁,S ₂, ... be a sequence of strips of widthsw ₁,w ₂, .... It is shown thatE ² can be covered by translates of the stripsS _i if w ₁ ^3/2 = . Further results concern conditions in order that a compact convex domain inE ² can be covered by translates ofS ₁,S ₂, ....This research was supported by National Science Foundation Research Grant MCS 76-06111.     

On space coverings by unbounded convex sets

Let (C_i) be a sequence of closed convex subsets of Euclidean n-space Eⁿ. This paper is concerned with the problem of finding necessary and sufficient conditions that the sets C_i can be rearranged (by the application of rigid motions or translations) so as to cover all or almost all Eⁿ. Particular attention is paid to the problems that arise if the sets C_i are permitted to be unbounded. It is shown that under certain conditions this covering problem can be reduced to the already thoroughly investigated case of compact sets with bounded diameter set{d(C_i)}, and it is also proved that there are two additional covering possibilities if such a reduction is not possible.     

On the discrepancy of convex plane sets

LetD ₂ (N) be the discrepancy function of the class of convex sets in the unit square [0, 1)² as defined in the introduction. A well known result of W. M. Schmidt states thatD ₂ f(N)>constN ^1/3. In this paper it is shown that Schmidt's bound is nearly best possible, more precisely, $$D_2 (N)< const N^{1/3} (\log N)^4 .$$      

On convex bodies that permit packings of high density

It is well known that ann-dimensional convex body permits a lattice packing of density 1 only if it is a centrally symmetric polytope of at most 2(2 ⁿ –1) facets. This article concerns itself with the associated stability problem whether a convex body that permits a packing of high density is in some sense close to such a polytope. Several inequalities that address this stability problem are proved. A corresponding theorem for coverings by two-dimensional convex bodies is also proved.Supported by National Science Foundation Research Grants DMS 8300825 and DMS 8701893.     

Generalized matching theorems for closed coverings of convex sets

In this paper we use fixed point and coincidence theorems due to Park [8] to give matching theorems concerning closed coverings of nonempty convex sets in a real topological vector space. Our new results extend previously given ones due to Ky Fan [2], [3], Shih [10], Shih and Tan [11], and Park [7].     

On the addition of convex sets in the hyperbolic plane

Analogue to the definition $K + L := \bigcup_{x\in K}(x + L)$ of the Minkowski addition in the euclidean geometry it is proposed to define the (noncommutative) addition $K \vdash L := \bigcup_{0\, \leqsl\, \rho\,\leqsl\, a(\varphi),0\,\leqsl\,\varphi\,<\, 2\pi}T_{\rho}^{(\varphi)}(L)$ for compact, convex and smoothly bounded sets K and L in the hyperbolic plane $\Omega$ (Kleins model). Here $\rho = a(\varphi)$ is the representation of the boundary $\partial$ K in geodesic polar coordinates and $T_{\rho}^{(\varphi)}$ is the hyperbolic translation of $\Omega$ of length $\rho$ along the line through the origin o of direction $\varphi$. In general this addition does not preserve convexity but nevertheless we may prove as main results: (1) $o \in$ int $K, o \in$ int L and K,L horocyclic convex imply the strict convexity of $K \vdash L$, and (2) in this case there exists a hyperbolic mixed volume $V_h(K,L)$ of K and L which has a representation by a suitable integral over the unit circle.     

Separating convex sets in the plane

Given a setA inR ² and a collectionS of plane sets, we say that a lineL separatesA fromS ifA is contained in one of the closed half-planes defined byL, while every set inS is contained in the complementary closed half-plane.We prove that, for any collectionF ofn disjoint disks inR ², there is a lineL that separates a disk inF from a subcollection ofF with at least (n–7)/4 disks. We produce configurationsH _n andG _n , withn and 2n disks, respectively, such that no pair of disks inH _n can be simultaneously separated from any set with more than one disk ofH _n , and no disk inG _n can be separated from any subset ofG _n with more thann disks.We also present a setJ _m with 3m line segments inR ², such that no segment inJ _m can be separated from a subset ofJ _m with more thanm+1 elements. This disproves a conjecture by N. Alonet al. Finally we show that ifF is a set ofn disjoint line segments in the plane such that they can be extended to be disjoint semilines, then there is a lineL that separates one of the segments from at least n/3+1 elements ofF.     

Embedding theorems for classes of convex sets

Rådström's embedding theorem states that the nonempty compact convex subsets of a normed vector space can be identified with points of another normed vector space such that the embedding map is additive, positively homogeneous, and isometric. In the present paper, extensions of Rådström's embedding theorem are proven which provide additional information on the embedding space. These results include those of Hörmander who proved a similar embedding theorem for the nonempty closed bounded convex subsets of a Hausdorff locally convex vector space. In contrast to Hörmander's approach via support functionals, all embedding theorems of the present paper are proven by a refinement of Rådström's original method which is constructive and does not rely on Zorn's lemma. This paper also includes a brief discussion of some actual or potential applications of embedding theorems for classes of convex sets in probability theory, mathematical economics, interval mathematics, and related areas.     

A pre-hilbert space consisting of classes of convex sets

An equivalence relation is defined in the set of all bounded closed convex sets in Euclidean spaceE ⁿ. The equivalence classes are shown to be elements of a pre-Hilbert spaceA ⁿ, and geometrical relationships betweenA ⁿ andE ⁿ are investigated.     

Isoperimetric deficit and convex plane sets of maximum translative discrepancy

The paper deals with the following question: Among the convex plane sets of fixed isoperimetric deficit, which are the sets of maximum translative deviation from the circular shape? The answer is given for the cases in which the deviation is measured either by the translative Hausdorff metric or by the translative symmetric difference metric.