336 research outputs found
The convexification effect of Minkowski summation
Let us define for a compact set the sequence It was independently proved by Shapley, Folkman and Starr (1969)
and by Emerson and Greenleaf (1969) that approaches the convex hull of
in the Hausdorff distance induced by the Euclidean norm as goes to
. We explore in this survey how exactly approaches the convex
hull of , and more generally, how a Minkowski sum of possibly different
compact sets approaches convexity, as measured by various indices of
non-convexity. The non-convexity indices considered include the Hausdorff
distance induced by any norm on , the volume deficit (the
difference of volumes), a non-convexity index introduced by Schneider (1975),
and the effective standard deviation or inner radius. After first clarifying
the interrelationships between these various indices of non-convexity, which
were previously either unknown or scattered in the literature, we show that the
volume deficit of does not monotonically decrease to 0 in dimension 12
or above, thus falsifying a conjecture of Bobkov et al. (2011), even though
their conjecture is proved to be true in dimension 1 and for certain sets
with special structure. On the other hand, Schneider's index possesses a strong
monotonicity property along the sequence , and both the Hausdorff
distance and effective standard deviation are eventually monotone (once
exceeds ). Along the way, we obtain new inequalities for the volume of the
Minkowski sum of compact sets, falsify a conjecture of Dyn and Farkhi (2004),
demonstrate applications of our results to combinatorial discrepancy theory,
and suggest some questions worthy of further investigation.Comment: 60 pages, 7 figures. v2: Title changed. v3: Added Section 7.2
resolving Dyn-Farkhi conjectur
Separable and tree-like asymptotic cones of groups
Using methods from nonstandard analysis, we will discuss which metric spaces
can be realized as asymptotic cones. Applying the results we will find in the
context of groups, we will prove that a group with "a few" separable asymptotic
cones is virtually nilpotent, and we will classify the real trees appearing as
asymptotic cones of (not necessarily hyperbolic) groups.Comment: The hypothesis of Theorem 1.2 had to be strengthene
Uncountable sets of unit vectors that are separated by more than 1
Let be a Banach space. We study the circumstances under which there
exists an uncountable set of unit vectors such that
for distinct . We prove that such a set exists
if is quasi-reflexive and non-separable; if is additionally
super-reflexive then one can have for some
that depends only on . If is a non-metrisable compact,
Hausdorff space, then the unit sphere of also contains such a subset;
if moreover is perfectly normal, then one can find such a set with
cardinality equal to the density of ; this solves a problem left open by S.
K. Mercourakis and G. Vassiliadis.Comment: to appear in Studia Mat
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