11,777 research outputs found
Topological Stability of Kinetic -Centers
We study the -center problem in a kinetic setting: given a set of
continuously moving points in the plane, determine a set of (moving)
disks that cover at every time step, such that the disks are as small as
possible at any point in time. Whereas the optimal solution over time may
exhibit discontinuous changes, many practical applications require the solution
to be stable: the disks must move smoothly over time. Existing results on this
problem require the disks to move with a bounded speed, but this model is very
hard to work with. Hence, the results are limited and offer little theoretical
insight. Instead, we study the topological stability of -centers.
Topological stability was recently introduced and simply requires the solution
to change continuously, but may do so arbitrarily fast. We prove upper and
lower bounds on the ratio between the radii of an optimal but unstable solution
and the radii of a topologically stable solution---the topological stability
ratio---considering various metrics and various optimization criteria. For we provide tight bounds, and for small we can obtain nontrivial
lower and upper bounds. Finally, we provide an algorithm to compute the
topological stability ratio in polynomial time for constant
A Framework for Algorithm Stability
We say that an algorithm is stable if small changes in the input result in
small changes in the output. This kind of algorithm stability is particularly
relevant when analyzing and visualizing time-varying data. Stability in general
plays an important role in a wide variety of areas, such as numerical analysis,
machine learning, and topology, but is poorly understood in the context of
(combinatorial) algorithms. In this paper we present a framework for analyzing
the stability of algorithms. We focus in particular on the tradeoff between the
stability of an algorithm and the quality of the solution it computes. Our
framework allows for three types of stability analysis with increasing degrees
of complexity: event stability, topological stability, and Lipschitz stability.
We demonstrate the use of our stability framework by applying it to kinetic
Euclidean minimum spanning trees
On the nonlinear dynamics of topological solitons in DNA
Dynamics of topological solitons describing open states in the DNA double
helix are studied in the frameworks of the model which takes into account
asymmetry of the helix. It is shown that three types of topological solitons
can occur in the DNA double chain. Interaction between the solitons, their
interactions with the chain inhomogeneities and stability of the solitons with
respect to thermal oscillations are investigated.Comment: 16 pages, 16 figure
Macromolecular crowding modulates folding mechanism of alpha/beta protein apoflavodoxin
Protein dynamics in cells may be different from that in dilute solutions in
vitro since the environment in cells is highly concentrated with other
macromolecules. This volume exclusion due to macromolecular crowding is
predicted to affect both equilibrium and kinetic processes involving protein
conformational changes. To quantify macromolecular crowding effects on protein
folding mechanisms, here we have investigated the folding energy landscape of
an alpha/beta protein, apoflavodoxin, in the presence of inert macromolecular
crowding agents using in silico and in vitro approaches. By coarse-grained
molecular simulations and topology-based potential interactions, we probed the
effects of increased volume fraction of crowding agents (phi_c) as well as of
crowding agent geometry (sphere or spherocylinder) at high phi_c. Parallel
kinetic folding experiments with purified Desulfovibro desulfuricans
apoflavodoxin in vitro were performed in the presence of Ficoll (sphere) and
Dextran (spherocylinder) synthetic crowding agents. In conclusion, we have
identified in silico crowding conditions that best enhance protein stability
and discovered that upon manipulation of the crowding conditions, folding
routes experiencing topological frustrations can be either enhanced or
relieved. The test-tube experiments confirmed that apoflavodoxin's
time-resolved folding path is modulated by crowding agent geometry. We propose
that macromolecular crowding effects may be a tool for manipulation of protein
folding and function in living cells.Comment: to appear in Biophysical Journal (2009). to appear in Biophysical
Journal (2009
Water and molecular chaperones act as weak links of protein folding networks: energy landscape and punctuated equilibrium changes point towards a game theory of proteins
Water molecules and molecular chaperones efficiently help the protein folding
process. Here we describe their action in the context of the energy and
topological networks of proteins. In energy terms water and chaperones were
suggested to decrease the activation energy between various local energy minima
smoothing the energy landscape, rescuing misfolded proteins from conformational
traps and stabilizing their native structure. In kinetic terms water and
chaperones may make the punctuated equilibrium of conformational changes less
punctuated and help protein relaxation. Finally, water and chaperones may help
the convergence of multiple energy landscapes during protein-macromolecule
interactions. We also discuss the possibility of the introduction of protein
games to narrow the multitude of the energy landscapes when a protein binds to
another macromolecule. Both water and chaperones provide a diffuse set of
rapidly fluctuating weak links (low affinity and low probability interactions),
which allow the generalization of all these statements to a multitude of
networks.Comment: 9 pages, 1 figur
Two Skyrmion Dynamics with Omega Mesons
We present our first results of numerical simulations of two skyrmion
dynamics using an -meson stabilized effective Lagrangian. We consider
skyrmion-skyrmion scattering with a fixed initial velocity of , for
various impact parameters and groomings. The physical picture that emerges is
surprisingly rich, while consistent with previous results and general
conservation laws. We find meson radiation, skyrmion scattering out of the
scattering plane, orbiting and capture to bound states.Comment: 19 pages, 22 figure
Topological phase separation in 2D hard-core Bose-Hubbard system away from half-filling
We suppose that the doping of the 2D hard-core boson system away from
half-filling may result in the formation of multi-center topological defect
such as charge order (CO) bubble domain(s) with Bose superfluid (BS) and extra
bosons both localized in domain wall(s), or a {\it topological} CO+BS {\it
phase separation}, rather than an uniform mixed CO+BS supersolid phase.
Starting from the classical model we predict the properties of the respective
quantum system. The long-wavelength behavior of the system is believed to
remind that of granular superconductors, CDW materials, Wigner crystals, and
multi-skyrmion system akin in a quantum Hall ferromagnetic state of a 2D
electron gas.Comment: 6 pages, 1 figur
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