Zone diagrams in Euclidean spaces and in other normed spaces

Zone diagrams are a variation on the classical concept of Voronoi diagrams. Given n sites in a metric space that compete for territory, the zone diagram is an equilibrium state in the competition. Formally it is defined as a fixed point of a certain “dominance” map. Asano, Matou?ek, and Tokuyama proved the existence and uniqueness of a zone diagram for point sites in the Euclidean plane, and Reem and Reich showed existence for two arbitrary sites in an arbitrary metric space. We establish existence and uniqueness for n disjoint compact sites in a Euclidean space of arbitrary (finite) dimension, and more generally, in a finite-dimensional normed space with a smooth and rotund norm. The proof is considerably simpler than that of Asano et?al. We also provide an example of non-uniqueness for a norm that is rotund but not smooth. Finally, we prove existence and uniqueness for two point sites in the plane with a smooth (but not necessarily rotund) norm.     

Isometries of spaces of compact or compact convex subsets of metric manifolds

The isometries with respect to the Hausdorff metric of spaces of compact or compact convex subsets of certain compact metric spaces are precisely the mappings generated by isometries of the underlying spaces. In particular this holds when the underlying space is a finite dimensional torus or a sphere in a finite dimensional strictly convex smooth normed space.     

Weakly compact operators on non-complete normed spaces

We prove that weakly compact operators on a non-reflexive normed space cannot be bijective. We also show that, in the above result, bijectivity cannot be relaxed to surjectivity. Finally, we study the behaviour of surjective weakly compact operators on a non-reflexive normed space, when they are perturbed by small scalar multiples of the identity, and derive from this study the recent result of Spurný [A note on compact operators on normed linear spaces, Expo. Math. 25 (2007) 261–263] that compact operators on an infinite-dimensional normed space cannot be surjective.     

Littlewood-Paley inequalities in uniformly convex and uniformly smooth Banach spaces

It is proved that the inequality δX(ε)?cεp, p?2, where δX is the modulus of convexity of X, is sufficient and necessary for the inequality

     

Martingales with values in uniformly convex spaces

Using the techniques of martingale inequalities in the case of Banach space valued martingales, we give a new proof of a theorem of Enflo: every super-reflexive space admits an equivalent uniformly convex norm. Letr be a number in ]2, ∞[; we prove moreover that if a Banach spaceX is uniformly convex (resp. ifδ _x(?)/? ^r when? → 0) thenX admits for someq<∞ (resp. for someq<r) an equivalent norm for which the corresponding modulus of convexity satisfiesδ(?)/? ^q → ∞ when? → 0. These results have dual analogues concerning the modulus of smoothness. Our method is to study some inequalities for martingales with values in super-reflexive or uniformly convex spaces which are characteristic of the geometry of these spaces up to isomorphism.     

Two observations regarding embedding subsets of Euclidean spaces in normed spaces

This paper contains two results concerning linear embeddings of subsets of Euclidean space in low-dimensional normed spaces. The first is an improvement of the known dependence on ? in Dvoretzky's theorem from order of ?² to order of ? (except for log factors). The second is a joint generalization of (Milman's version of) Dvoretzky's theorem and (a recent generalization by Klartag and Mendelson of) the Johnson-Lindenstrauss Lemma.     

Continuity of metric projections in uniformly convex and uniformly smooth Banach spaces

The continuity of the metric projection onto an approximatively compact set in a uniformly convex and uniformly smooth Banach space is investigated. An explicit modulus of continuity for the metric projection which depends on the directional radius of curvature at a certain point of the set is obtained. The results generalize and improve those obtained by B. O. Björnest l.     

Inner and outerj-radii of convex bodies in finite-dimensional normed spaces

This paper is concerned with the various inner and outer radii of a convex bodyC in ad-dimensional normed space. The innerj-radiusr _j (C) is the radius of a largestj-ball contained inC, and the outerj-radiusR _j (C) measures how wellC can be approximated, in a minimax sense, by a (d —j)-flat. In particular,r _d (C) andR _d (C) are the usual inradius and circumradius ofC, while 2r ₁(C) and 2R ₁(C) areC's diameter and width.Motivation for the computation of polytope radii has arisen from problems in computer science and mathematical programming. The radii of polytopes are studied in [GK1] and [GK2] from the viewpoint of the theory of computational complexity. This present paper establishes the basic geometric and algebraic properties of radii that are needed in that study.Much of this paper was written when both authors were visiting the Institute for Mathematics and Its Applications, 206 Church Street S.E., Minneapolis, MN 55455, USA. The research of P. Gritzmann was supported in part by the Alexander-von-Humboldt Stiftung and the Deutsche Forschungsgemeinschaft. V. Klee's research was supported in part by the National Science Foundation.     

Farthest points of sets in uniformly convex banach spaces

LetS be a closed and bounded set in a uniformly convex Banach spaceX. It is shown that the set of all points inX which have a farthest point inS is dense. Letb(S) denote the set of all farthest points ofS, then a sufficient condition for to hold is thatX have the following property (I): Every closed and bounded convex set is the intersection of a family of closed balls.     

The differentiability of nonlinear semigroups in uniformly convex spaces

LetT(t) be a semigroup on a subset of Banach spaceX. T(t) is generated by a product integral of the resolventJ _λ of an accretive operatorA. IfX is a Hilbert space, it is known that forx in the domain ofA, ‖J _t x−T(t)x‖=o(t) ast decreases to zero. We show this is true whenX is uniformly convex, and deduce some consequences.     

Uniformly smooth renormings of uniformly convex Banach spaces

In this note we study the quantitative side of the famous Enflo-Pisier theorem on the possibility of equivalent uniformly smooth renormings of superreflexive Banach spaces (in particular, uniformly convex and uniformly nonsquare ones). Typical re result: let the modulus of convexity of the space X, which has a locally unconditional structure, satisfy the condition _x() C·^p. Then the space X admits an equivalent q-smooth renorming for any q, satisfying the inequality q<1n 2/1n (2× (1–C·2^–p/2)).Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova AN SSSR, Vol. 135, pp. 120–134, 1984.     

A note on compact operators on normed linear spaces

We show that there is no surjective compact operator on a normed linear infinite-dimensional space.