Minkowski decomposition of convex sets

A setK is decomposable if it can be written as the Minkowski sumA+B where neitherA norB is homothetic toK. In this paper, it is shown that a wide class of convex sets is decomposable including those which contain a sufficiently smooth neighborhood on their boundary.     

Nonoscillation theorems in convex sets

This paper presents a new and simple technique for a certain class of variational problems which includes many of the important problems of mathematical physics, e.g., the Brachistochrone, geodesies, and minimal surface of revolution problems. The technique uses Caratheodory's equivalent problems approach but combines two equivalent problems at the same time to get the sufficiency and uniqueness results. It does not use any of the classical sufficiency conditions such as the Weierstrass condition. The equations that we are led to by this new approach turn out to be the Hamilton-Jacobi and Euler-Lagrange equations for the problem, but here we have not had to use any of the classical Hamilton-Jacobi theory nor derivations to get its results (e.g., orthogonality of the extremals to the wave fronts) for this class of problems. The cases for one and n dependent variables are presented and illustrated. Implications and generalizations of the method are discussed.     

Fixed point theorems for convex sets

A conjecture of the author is verified by setting up a fundamental fixed point theorem so that some earlier results are unified and strengthened. Supported by National Foundation of Natural Science.     

Support theorems for unbounded convex sets

We present some generalizations of Helgason support theorem for functions with unbounded convex support.     

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.     

Fixed point theorems for paracompact convex sets

In the present paper a few fixed point theorems are given for upper hemi-continuous mappings from a paracompact convex set to its embracing space, a real, locally convex, Hausdorff topological vector space.     

Stability theorems for projections of convex sets

LetH ^n?1 denote the set of all (n ? 1)-dimensional linear subspaces of euclideann-dimensional spaceE ⁿ (n≧3), and letJ andK be two compact convex subsets ofE ⁿ. It is well-known thatJ andK are translation equivalent (or homothetic) if for allH ∈H ^n?1 the respective orthogonal projections, sayJ _H, K_H, are translation equivalent (or homothetic). This fact gives rise to the following stability problem: Ifε≧0 is given, and if for everyH ∈H ^n?1 a translate (or homothetic copy) ofK _H is within Hausdorff distanceε ofJ _H, how close isJ to a nearest translate (or homothetic copy) ofK? In the case of translations it is shown that under the above assumptions there is always a translate ofK within Hausdorff distance (1 + 2√2)ε ofJ. Similar results for homothetic transformations are proved and applications regarding the stability of characterizations of centrally symmetric sets and spheres are given. Furthermore, it is shown that these results hold even ifH ^n?1 is replaced by a rather small (but explicitly specified) subset ofH ^n?1.     

Separation theorems for convex polytopes and finitely-generated cones derived from theorems of the alternative

We derive from Motzkin’s Theorem that a point can be strongly separated by a hyperplane from a convex polytope and a finitely-generated convex cone. We state a similar result for Tucker’s Theorem of the alternative. A generalisation of the residual existence theorem for linear equations which has recently been proved by Rohn [8] is a corollary. We state all the results in the setting of a general vector space over a linearly ordered (possibly skew) field.     

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].     

Fixed point theorems on closed sets through abstract cones

In this paper the authors consider the problem of the existence, and iteration to a fixed point or a zero, of an operator on a closed subset of an abstract space. The results generalize the construction mapping principle. A generalized or cone norm is used.     

Fixed point theorems for paracompact convex sets (II)

The present paper is the continuation of [1]. Some further generalizations of the fixed point theorems in [2] are obtained by means of the results in [1]. Project supported by the National Natural Science Foundation of China     

Helly-type theorems for intersections of sets starshaped via orthogonally convex paths

Let ${\mathcal{K}}$ be a family of simply connected sets in the plane. If every countable subfamily of ${\mathcal{K}}$ has an intersection that is starshaped via orthogonally convex paths, then ${\mathcal{K}}$ itself has such an intersection. For the d-dimensional case, let ${\mathcal{K}}$ be a family of compact sets in ${\mathbb{R}^d}$ . If every finite subfamily of ${\mathcal{K}}$ has an intersection that is starshaped via orthogonally convex paths, again ${\mathcal{K}}$ itself has such an intersection.     

Fixed point theorems for better admissible multimaps on almost convex sets

We obtain new fixed point theorems on multimaps in the class Bp defined on almost convex subsets of topological vector spaces. Our main results are applied to deduce various fixed point theorems, coincidence theorems, almost fixed point theorems, intersection theorems, and minimax theorems. Consequently, our new results generalize well-known works of Kakutani, Fan, Browder, Himmelberg, Lassonde, and others.     

Fixed point theorems for multifunctions having KKM property on almost convex sets

In this paper, for two nonempty subsets X and Y of a linear space E, we define the class KKM(X,Y) and investigate the fixed point problem for T∈KKM(X,X) with X an almost convex subset of a locally convex space. Our fixed point theorem contains Lassonde fixed point theorem for Kakutani factorizable multifunctions as special case.