5,744 research outputs found
Optimal model-free prediction from multivariate time series
Forecasting a time series from multivariate predictors constitutes a
challenging problem, especially using model-free approaches. Most techniques,
such as nearest-neighbor prediction, quickly suffer from the curse of
dimensionality and overfitting for more than a few predictors which has limited
their application mostly to the univariate case. Therefore, selection
strategies are needed that harness the available information as efficiently as
possible. Since often the right combination of predictors matters, ideally all
subsets of possible predictors should be tested for their predictive power, but
the exponentially growing number of combinations makes such an approach
computationally prohibitive. Here a prediction scheme that overcomes this
strong limitation is introduced utilizing a causal pre-selection step which
drastically reduces the number of possible predictors to the most predictive
set of causal drivers making a globally optimal search scheme tractable. The
information-theoretic optimality is derived and practical selection criteria
are discussed. As demonstrated for multivariate nonlinear stochastic delay
processes, the optimal scheme can even be less computationally expensive than
commonly used sub-optimal schemes like forward selection. The method suggests a
general framework to apply the optimal model-free approach to select variables
and subsequently fit a model to further improve a prediction or learn
statistical dependencies. The performance of this framework is illustrated on a
climatological index of El Ni\~no Southern Oscillation.Comment: 14 pages, 9 figure
Disentangling different types of El Ni\~no episodes by evolving climate network analysis
Complex network theory provides a powerful toolbox for studying the structure
of statistical interrelationships between multiple time series in various
scientific disciplines. In this work, we apply the recently proposed climate
network approach for characterizing the evolving correlation structure of the
Earth's climate system based on reanalysis data of surface air temperatures. We
provide a detailed study on the temporal variability of several global climate
network characteristics. Based on a simple conceptual view on red climate
networks (i.e., networks with a comparably low number of edges), we give a
thorough interpretation of our evolving climate network characteristics, which
allows a functional discrimination between recently recognized different types
of El Ni\~no episodes. Our analysis provides deep insights into the Earth's
climate system, particularly its global response to strong volcanic eruptions
and large-scale impacts of different phases of the El Ni\~no Southern
Oscillation (ENSO).Comment: 20 pages, 12 figure
Fast energy transfer mediated by multi-quanta bound states in a nonlinear quantum lattice
By using a Generalized Hubbard model for bosons, the energy transfer in a
nonlinear quantum lattice is studied, with special emphasis on the interplay
between local and nonlocal nonlinearity. For a strong local nonlinearity, it is
shown that the creation of v quanta on one site excites a soliton band formed
by bound states involving v quanta trapped on the same site. The energy is
first localized on the excited site over a significant timescale and then
slowly delocalizes along the lattice. As when increasing the nonlocal
nonlinearity, a faster dynamics occurs and the energy propagates more rapidly
along the lattice. Nevertheless, the larger is the number of quanta, the slower
is the dynamics. However, it is shown that when the nonlocal nonlinearity
reaches a critical value, the lattice suddenly supports a very fast energy
propagation whose dynamics is almost independent on the number of quanta. The
energy is transfered by specific bound states formed by the superimposition of
states involving v-p quanta trapped on one site and p quanta trapped on the
nearest neighbour sites, with p=0,..,v-1. These bound states behave as
independent quanta and they exhibit a dynamics which is insensitive to the
nonlinearity and controlled by the single quantum hopping constant.Comment: 28 pages, 8 figure
Two-vibron bound states in alpha-helix proteins : the interplay between the intramolecular anharmonicity and the strong vibron-phonon coupling
The influence of the intramolecular anharmonicity and the strong
vibron-phonon coupling on the two-vibron dynamics in an -helix protein
is studied within a modified Davydov model. The intramolecular anharmonicity of
each amide-I vibration is considered and the vibron dynamics is described
according to the small polaron approach. A unitary transformation is performed
to remove the intramolecular anharmonicity and a modified Lang-Firsov
transformation is applied to renormalize the vibron-phonon interaction. Then, a
mean field procedure is realized to obtain the dressed anharmonic vibron
Hamiltonian. It is shown that the anharmonicity modifies the vibron-phonon
interaction which results in an enhancement of the dressing effect. In
addition, both the anharmonicity and the dressing favor the occurrence of two
different bound states which the properties strongly depend on the interplay
between the anharmonicity and the dressing. Such a dependence was summarized in
a phase diagram which characterizes the number and the nature of the bound
states as a function of the relevant parameters of the problem. For a
significant anharmonicity, the low frequency bound states describe two vibrons
trapped onto the same amide-I vibration whereas the high frequency bound states
refer to the trapping of the two vibrons onto nearest neighbor amide-I
vibrations.Comment: may 2003 submitted to Phys. Rev.
Thermal conductance of thin film YIG determined using Bayesian statistics
Thin film YIG (YFeO) is a prototypical material for
experiments on thermally generated pure spin currents and the spin Seebeck
effect. The 3-omega method is an established technique to measure the
cross-plane thermal conductance of thin films, but can not be used in YIG/GGG
(GaGdO) systems in its standard form. We use two-dimensional
modeling of heat transport and introduce a technique based on Bayesian
statistics to evaluate measurement data taken from the 3-omega method. Our
analysis method allows us to study materials systems that have not been
accessible with the conventionally used 3-omega analysis. Temperature dependent
thermal conductance data of thin film YIG are of major importance for
experiments in the field of spin-caloritronics. Here we show data between room
temperature and 10 K for films covering a wide thickness range as well as the
magnetic field effect on the thermal conductance between 10 K and 50 K
Skewed parton distributions and the scale dependence of the transverse size parameter
We discuss the scale dependence of a skewed parton distribution of the pion
obtained from a generalized light-cone wave function overlap formula. Using a
simple ansatz for the transverse momentum dependence of the light-cone wave
function and restricting ourselves to the case of a zero skewedness parameter,
the skewed parton distribution can be expressed through an ordinary parton
distribution multiplied by an exponential function. Matching the generalized
and ordinary DGLAP evolution equations of the skewed and ordinary parton
distributions, respectively, we derive a constraint for the scale dependence of
the transverse size parameter, which describes the width of the pion wave
function in transverse momentum space. This constraint has implications for the
Fock state probability and valence distribution. We apply our results to the
pion form factor.Comment: 10 pages, 4 figures; version to appear in Phys. Rev. D; Refs. added,
new discussion of results for pion form factor in view of new dat
MaGICC-WDM: the effects of warm dark matter in hydrodynamical simulations of disc galaxy formation
We study the effect of warm dark matter (WDM) on hydrodynamic simulations of
galaxy formation as part of the Making Galaxies in a Cosmological Context
(MaGICC) project. We simulate three different galaxies using three WDM
candidates of 1, 2 and 5 keV and compare results with pure cold dark matter
simulations. WDM slightly reduces star formation and produces less centrally
concentrated stellar profiles. These effects are most evident for the 1 keV
candidate but almost disappear for keV. All simulations
form similar stellar discs independent of WDM particle mass. In particular, the
disc scale length does not change when WDM is considered. The reduced amount of
star formation in the case of 1 keV particles is due to the effects of WDM on
merging satellites which are on average less concentrated and less gas rich.
The altered satellites cause a reduced starburst during mergers because they
trigger weaker disc instabilities in the main galaxy. Nevertheless we show that
disc galaxy evolution is much more sensitive to stellar feedback than it is to
WDM candidate mass. Overall we find that WDM, especially when restricted to
current observational constraints ( keV), has a minor
impact on disc galaxy formation.Comment: 13 pages, 9 figures, 2 tables; minor clarifications added in results
section, conclusions unchanged; accepted for publication in MNRA
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