550 research outputs found
Contact graphs of ball packings
A contact graph of a packing of closed balls is a graph with balls as
vertices and pairs of tangent balls as edges. We prove that the average degree
of the contact graph of a packing of balls (with possibly different radii) in
is not greater than . We also find new upper bounds for
the average degree of contact graphs in and
Repeated minimizers of -frame energies
For a collection of unit vectors , define the
-frame energy of as the quantity . In this paper, we connect the problem of minimizing this
value to another optimization problem, so giving new lower bounds for such
energies. In particular, for , we prove that this energy is at least
which is sharp for and . We prove that for , a repeated orthonormal basis
construction of vectors minimizes the energy over an interval,
, and demonstrate an analogous result for all in the case
. Finally, in connection, we give conjectures on these and other energies
Lower bounds for the simplexity of the n-cube
In this paper we prove a new asymptotic lower bound for the minimal number of
simplices in simplicial dissections of -dimensional cubes. In particular we
show that the number of simplices in dissections of -cubes without
additional vertices is at least .Comment: 10 page
Revisiting spin state crossover in (MgFe)O by means of high resolution X-ray diffraction from a single crystal
(MgFe)O is a solid solution with ferrous iron undergoing the high to low spin
state (HS-LS) crossover under high pressure. The exact state of the material in
the region of the crossover is still a mystery, as domains with different spin
states may coexist over a wide pressure range without changing the crystal
structure neither from the symmetry nor from the atomic positions point of
view. At the conditions of the crossover, (MgFe)O is a special type of
microscopic disorder system. We explore the influences of (a) stress-strain
relations in a diamond anvil cell, (b) time relaxation processes, and (c) the
crossover itself on the characteristic features of a single crystal (111) Bragg
spot before, during and after the transformation. Using high resolution X-ray
diffraction as a novel method for studies of unconventional processes at the
conditions of suppressed diffusion, we detect and discuss subtle changes of the
(111) Bragg spot projections which we measure and analyze as a function of
pressure. We report changes of the spot shape which can be correlated with the
HS-LS relative abundance. In addition, we report the formation of structural
defects as an intrinsic material response. These static defects are accumulated
during transformation of the material from HS to LS.Comment: 28 pages, 11 Figure
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