5,870 research outputs found
On the noise-induced passage through an unstable periodic orbit II: General case
Consider a dynamical system given by a planar differential equation, which
exhibits an unstable periodic orbit surrounding a stable periodic orbit. It is
known that under random perturbations, the distribution of locations where the
system's first exit from the interior of the unstable orbit occurs, typically
displays the phenomenon of cycling: The distribution of first-exit locations is
translated along the unstable periodic orbit proportionally to the logarithm of
the noise intensity as the noise intensity goes to zero. We show that for a
large class of such systems, the cycling profile is given, up to a
model-dependent change of coordinates, by a universal function given by a
periodicised Gumbel distribution. Our techniques combine action-functional or
large-deviation results with properties of random Poincar\'e maps described by
continuous-space discrete-time Markov chains.Comment: 44 pages, 4 figure
Universality of residence-time distributions in non-adiabatic stochastic resonance
We present mathematically rigorous expressions for the residence-time and
first-passage-time distributions of a periodically forced Brownian particle in
a bistable potential. For a broad range of forcing frequencies and amplitudes,
the distributions are close to periodically modulated exponential ones.
Remarkably, the periodic modulations are governed by universal functions,
depending on a single parameter related to the forcing period. The behaviour of
the distributions and their moments is analysed, in particular in the low- and
high-frequency limits.Comment: 8 pages, 1 figure New version includes distinction between
first-passage-time and residence-time distribution
Metastability in Interacting Nonlinear Stochastic Differential Equations II: Large-N Behaviour
We consider the dynamics of a periodic chain of N coupled overdamped
particles under the influence of noise, in the limit of large N. Each particle
is subjected to a bistable local potential, to a linear coupling with its
nearest neighbours, and to an independent source of white noise. For strong
coupling (of the order N^2), the system synchronises, in the sense that all
oscillators assume almost the same position in their respective local potential
most of the time. In a previous paper, we showed that the transition from
strong to weak coupling involves a sequence of symmetry-breaking bifurcations
of the system's stationary configurations, and analysed in particular the
behaviour for coupling intensities slightly below the synchronisation
threshold, for arbitrary N. Here we describe the behaviour for any positive
coupling intensity \gamma of order N^2, provided the particle number N is
sufficiently large (as a function of \gamma/N^2). In particular, we determine
the transition time between synchronised states, as well as the shape of the
"critical droplet", to leading order in 1/N. Our techniques involve the control
of the exact number of periodic orbits of a near-integrable twist map, allowing
us to give a detailed description of the system's potential landscape, in which
the metastable behaviour is encoded
On the Rational Type 0f Moment Angle Complexes
In this note it is shown that the moment angle complexes Z(K;(D^2,,S^1))
which are rationally elliptic are a product of odd spheres and a diskComment: This version avoids the use of an incorrect result from the
literature in the proof of Theorem 1.3. There is some text overlap with
arXiv:1410.645
Patterns of quark mass matrices in a class of Calabi-Yau models
We study a class of superstring models compactified in the 3-generation
Calabi-Yau manifold of Tian and Yau. Our analysis includes the complete
-singlet sector, which has been recently evaluated using techniques of
spectral and exact sequences. We use the discrete symmetries of the models to
find flat directions of symmetry breaking that leave unbroken a low energy
matter parity and make all leptoquarks heavy while preserving light Higgs
fields. Then we classify the patterns of ordinary quark mass matrices and show
that (without invoking effects due to nonrenormalizable terms) only one
structure can accommodate the observed value of fermion masses and mixing
angles, with preference for a heavy {\it top} quark ( GeV for
). The model, which unifies perturbatively and predicts a
realistic structure of quark mass matrices with texture zeroes, is one of the
many possible string vacua. However, in contrast with what is often assumed in
the search for realistic unified scenarios, it is highly nonminimal near the
unification scale and the predicted mass matrices have no simple symmetry
properties.Comment: 30 (including Tables and Figures), UG-FT-38/9
Relating the Cosmological Constant and Supersymmetry Breaking in Warped Compactifications of IIB String Theory
It has been suggested that the observed value of the cosmological constant is
related to the supersymmetry breaking scale M_{susy} through the formula Lambda
\sim M_p^4 (M_{susy}/M_p)^8. We point out that a similar relation naturally
arises in the codimension two solutions of warped space-time varying
compactifications of string theory in which non-isotropic stringy moduli induce
a small but positive cosmological constant.Comment: 7 pages, LaTeX, references added and minor changes made, (v3) map
between deSitter and global cosmic brane solutions clarified, supersymmetry
breaking discussion improved and references adde
Beyond the Fokker-Planck equation: Pathwise control of noisy bistable systems
We introduce a new method, allowing to describe slowly time-dependent
Langevin equations through the behaviour of individual paths. This approach
yields considerably more information than the computation of the probability
density. The main idea is to show that for sufficiently small noise intensity
and slow time dependence, the vast majority of paths remain in small space-time
sets, typically in the neighbourhood of potential wells. The size of these sets
often has a power-law dependence on the small parameters, with universal
exponents. The overall probability of exceptional paths is exponentially small,
with an exponent also showing power-law behaviour. The results cover time spans
up to the maximal Kramers time of the system. We apply our method to three
phenomena characteristic for bistable systems: stochastic resonance, dynamical
hysteresis and bifurcation delay, where it yields precise bounds on transition
probabilities, and the distribution of hysteresis areas and first-exit times.
We also discuss the effect of coloured noise.Comment: 37 pages, 11 figure
Towards Mirror Symmetry as Duality for Two-Dimensional Abelian Gauge Theories
Superconformal sigma models with Calabi--Yau target spaces described as
complete intersection subvarieties in toric varieties can be obtained as the
low-energy limit of certain abelian gauge theories in two dimensions. We
formulate mirror symmetry for this class of Calabi--Yau spaces as a duality in
the abelian gauge theory, giving the explicit mapping relating the two
Lagrangians. The duality relates inequivalent theories which lead to isomorphic
theories in the low-energy limit. This formulation suggests that mirror
symmetry could be derived using abelian duality. The application of duality in
this context is complicated by the presence of nontrivial dynamics and the
absence of a global symmetry. We propose a way to overcome these obstacles,
leading to a more symmetric Lagrangian. The argument, however, fails to produce
a derivation of the conjecture.Comment: 14 pages, latex, no figure
Decision-Making Frameworks For Using Sensor Data And Evolutionary Algorithms To Flush A Contaminated Water Distribution System
In the event that a contaminant enters a water distribution system, opening hydrants to flush contaminated water can protect consumers from becoming exposed. Strategies for operating hydrants can be developed to specify the selection of hydrants and the timing of operations to maximize the amount of contaminant that is removed from the system. As an event unfolds, however, sensor data may be the only information that is available to indicate the location and timing of the contaminant source, and ultimately, hydrant strategies must be selected in a highly uncertain environment. The decision-making framework for making real-time decisions to select hydrant strategies relies on computational and sensor technologies, including the accuracy and precision of sensor data; the timeliness of data availability (e.g., streaming data or data that is collected manually); and computational capabilities to execute search simulation-optimization frameworks in real-time. This research will explore and compare two decision-making frameworks. The first framework integrates real-time algorithms to identify potential source locations and develop hydrant strategies, using precise water quality data and high performance computation. The source identification problem is solved using a multi-population evolution strategies approach, and a genetic algorithm approach is applied to identify hydrant strategies for specified source locations. The second decision-making framework provides a library of response options that can be selected based on sensor data as an event unfolds. The library of hydrant strategies is developed a priori using a simulation-optimization framework. Potential sources are classified based on the order of sensors that are activated, and hydrant strategies are identified to maximize average performance for events within each class through the application of a genetic algorithm framework. The two decision-making frameworks are applied and compared for a set of events that are simulated for a virtual city, Mesopolis
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