SUCCESSIVE RADII OF FAMILIES OF CONVEX BODIES

2014 ◽  
Vol 91 (2) ◽  
pp. 331-344 ◽  
Author(s):  
BERNARDO GONZÁLEZ ◽  
MARÍA A. HERNÁNDEZ CIFRE ◽  
AICKE HINRICHS

AbstractWe study properties of the so-called inner and outer successive radii of special families of convex bodies. First we consider the balls of the $p$-norms, for which we show that the precise value of the outer (inner) radii when $p\geq 2$ ($1\leq p\leq 2$), as well as bounds in the contrary case $1\leq p\leq 2$ ($p\geq 2$), can be obtained as consequences of known results on Gelfand and Kolmogorov numbers of identity operators between finite-dimensional normed spaces. We also prove properties that successive radii satisfy when we restrict to the families of the constant width sets and the $p$-tangential bodies.

Mathematika ◽  
2013 ◽  
Vol 59 (2) ◽  
pp. 477-492 ◽  
Author(s):  
Horst Martini ◽  
Christian Richter ◽  
Margarita Spirova

Author(s):  
S. J. Dilworth

The purpose of this article is to extend certain results which are known to hold for convex bodies to a class of non-convex bodies occurring in the theory of topological vector spaces. In the first section after this introduction an analogue of F. John's Theorem on the distance of a finite dimensional space from Euclidean space is obtained, and the result is shown to be best possible. The Dvoretzky-Rogers Lemma on the points of contact of a symmetric convex body with the ellipsoid of maximum volume contained within it is discussed for certain non-convex bodies. In the next part the Dvoretzky Theorem on the existence of ellipsoidal sections is shown to hold with the best possible estimate for the dimension of the sections. It follows from estimates involving cotype constants that the finite dimensional subspaces of Lp (0 < p < 1) possess large almost Hilbertian subspaces. The final section extends the theorem of S. Szarek relating the volume of a body to the existence of ellipsoidal sections.


2014 ◽  
Vol 66 (3-4) ◽  
pp. 405-426 ◽  
Author(s):  
Marek Lassak ◽  
Horst Martini

Author(s):  
Paweł Wójcik

AbstractWe observe that every map between finite-dimensional normed spaces of the same dimension that respects fixed semi-inner products must be automatically a linear isometry. Moreover, we construct a uniformly smooth renorming of the Hilbert space $$\ell _2$$ ℓ 2 and a continuous injection acting thereon that respects the semi-inner products, yet it is non-linear. This demonstrates that there is no immediate extension of the former result to infinite dimensions, even under an extra assumption of uniform smoothness.


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