83 research outputs found
Echoes of the hexagon: remnants of hexagonal packing inside regular polygons
Based on numerical simulations that we have carried out, we provide evidence
that for regular polygons with sides (with ), (with ) congruent disks of appropriate size can be
nicely packed inside these polygons in highly symmetrical configurations which
apparently have maximal density for sufficiently small. These
configurations are invariant under rotations of and are closely related
to the configurations with perfect hexagonal packing in the regular hexagon and
to the configurations with {\sl curved hexagonal packing} (CHP) in the circle
found long time ago by Graham and Lubachevsky. At the basis of our explorations
are the algorithms that we have devised, which are very efficient in producing
the CHP and more general configurations inside regular polygons. We have used
these algorithms to generate a large number of CHP configurations for different
regular polygons and numbers of disks; a careful study of these results has
made possible to fully characterize the general properties of the CHP
configurations and to devise a {\sl deterministic} algorithm that completely
ensembles a given CHP configuration once an appropriate input ("DNA") is
specified. Our analysis shows that the number of CHP configurations for a given
is highly degenerate in the packing fraction and it can be explicitly
calculated in terms of (number of shells), of the building block of the DNA
itself and of the number of vertices in the fundamental domain (because of the
symmetry we work in of the whole domain). With the help of our
deterministic algorithm we are able to build {\sl all} the CHP configurations
for a polygon with shells.Comment: 25 pages, 12 figures, 4 table
Circle packing in arbitrary domains
We describe an algorithm that allows one to find dense packing configurations
of a number of congruent disks in arbitrary domains in two or more dimensions.
We have applied it to a large class of two dimensional domains such as
rectangles, ellipses, crosses, multiply connected domains and even to the
cardioid. For many of the cases that we have studied no previous result was
available. The fundamental idea in our approach is the introduction of "image"
disks, which allows one to work with a fixed container, thus lifting the
limitations of the packing algorithms of \cite{Nurmela97,Amore21,Amore23}. We
believe that the extension of our algorithm to three (or higher) dimensional
containers (not considered here) can be done straightforwardly.Comment: 26 pages, 17 figure
Contact numbers for congruent sphere packings in Euclidean 3-space
Continuing the investigations of Harborth (1974) and the author (2002) we
study the following two rather basic problems on sphere packings. Recall that
the contact graph of an arbitrary finite packing of unit balls (i.e., of an
arbitrary finite family of non-overlapping unit balls) in Euclidean 3-space is
the (simple) graph whose vertices correspond to the packing elements and whose
two vertices are connected by an edge if the corresponding two packing elements
touch each other. One of the most basic questions on contact graphs is to find
the maximum number of edges that a contact graph of a packing of n unit balls
can have in Euclidean 3-space. Our method for finding lower and upper estimates
for the largest contact numbers is a combination of analytic and combinatorial
ideas and it is also based on some recent results on sphere packings. Finally,
we are interested also in the following more special version of the above
problem. Namely, let us imagine that we are given a lattice unit sphere packing
with the center points forming the lattice L in Euclidean 3-space (and with
certain pairs of unit balls touching each other) and then let us generate
packings of n unit balls such that each and every center of the n unit balls is
chosen from L. Just as in the general case we are interested in finding good
estimates for the largest contact number of the packings of n unit balls
obtained in this way.Comment: 18 page
Hyperbolic intersection graphs and (quasi)-polynomial time
We study unit ball graphs (and, more generally, so-called noisy uniform ball
graphs) in -dimensional hyperbolic space, which we denote by .
Using a new separator theorem, we show that unit ball graphs in
enjoy similar properties as their Euclidean counterparts, but in one dimension
lower: many standard graph problems, such as Independent Set, Dominating Set,
Steiner Tree, and Hamiltonian Cycle can be solved in
time for any fixed , while the same problems need
time in . We also show that these algorithms in
are optimal up to constant factors in the exponent under ETH.
This drop in dimension has the largest impact in , where we
introduce a new technique to bound the treewidth of noisy uniform disk graphs.
The bounds yield quasi-polynomial () algorithms for all of the
studied problems, while in the case of Hamiltonian Cycle and -Coloring we
even get polynomial time algorithms. Furthermore, if the underlying noisy disks
in have constant maximum degree, then all studied problems can
be solved in polynomial time. This contrasts with the fact that these problems
require time under ETH in constant maximum degree
Euclidean unit disk graphs.
Finally, we complement our quasi-polynomial algorithm for Independent Set in
noisy uniform disk graphs with a matching lower bound
under ETH. This shows that the hyperbolic plane is a potential source of
NP-intermediate problems.Comment: Short version appears in SODA 202
Collective Phenomena in Active Systems
This dissertation investigates collective phenomena in active systems of biological relavance across length scales, ranging from intracellular actin systems to bird flocks. The study has been conducted via theoretical modeling and computer simulations using tools from soft condensed matter physics and non-equilibrium statistical mechanics. The work has been organized into two parts through five chapters. In part one (chapter 2 to 3), continuum theories have been utilized to study pattern formation in bacteria suspensions, actomyosin systems and bird flocks, whose dynamics is described generically within the framework of polar active fluids. The continuum field equations have been written down phenomenogically and derived rigorously through explicit coarse-graining of corresponding microscopic equations of motion. We have investigated the effects of alignment interaction, active motility, non-conserved density, and rotational inertia on pattern formation in active systems. In part two (chapter 4 to 5), computer simulations have been performed to study the self-organization and mechanical properties of dense active systems. A minimal self-propelled particle (SPP) model has been utilized to understand the aggregation and segregation of active systems under confinement (Chapter 4), where an active pressure has been defined for the first time to characterize the mechanical state of the active system. The same model is utilized in Chapter 5 to understand the self-assembly of passive particles in an active bath
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