5,132 research outputs found
Selective sampling for combined learning from labelled and unlabelled data
This paper examines the problem of selecting a suitable subset of data to be labelled when building pattern classifiers from labelled and unlabelled data. The selection of representative set is guided by a clustering information and various options of allocating a number of samples within clusters and their distributions are investigated. The experimental results show that hybrid methods like Semi-supervised clustering with selective sampling can result in building a classifier which requires much less labelled data in order to achieve a comparable classification performance to classifiers built only on the basis of labelled data
The current state and dynamics of regional disparities in Romania
The paper aims to highlight the dynamics and current state of regional disparities in Romania, discussing both inter-regional disparities -between the eight development regions, corresponding to the NUTS II level- and intra-regional disparities -between the counties (corresponding to the NUTS III level) included in each region. The analysis of regional disparities has been based on a series of data and indicators provided by Romanian official statistics for 2000 and 2005, processed by various statistical methods. We constructed a new variant of the relative distances ranking method adapted to the objective of our study, so as to make it possible to measure simultaneously the intra and inter-regional disparities. Based on a multidimensional index of inequality, the 41 Romanian counties plus Bucharest municipality have been ranked and included into four major development categories, according to their relative position in economic development. Gini index, Herfindahl index and Theil index have been also employed, showing a low level of concentration, which suggests a relatively low amplitude of both inter-regional and intra-regional disparities.disparities, ranking, concentration, region, county, Romania
Two-oscillator model of trapped-modes interaction in a nonlinear bilayer fish-scale metamaterial
We discuss the similarity between the nature of resonant oscillations in two
nonlinear systems, namely, a chain of coupled Duffing oscillators and a bilayer
fish-scale metamaterial. In such systems two different resonant states arise
which differ in their spectral lines. The spectral line of the first resonant
state has a Lorentzian form, while the second one has a Fano form. This
difference leads to a specific nonlinear response of the systems which
manifests itself in appearance of closed loops in spectral lines and bending
and overlapping of resonant curves. Conditions of achieving bistability and
multistability are found out.Comment: 14 pages, 6 figure
The Mott insulator phase of the one dimensional Bose-Hubbard model: a high order perturbative study
The one dimensional Bose-Hubbard model at a unit filling factor is studied by
means of a very high order symbolic perturbative expansion. Analytical
expressions are derived for the ground state quantities such as energy per
site, variance of on-site occupation, and different correlation functions.
These findings are compared to numerics and good agreement is found in the Mott
insulator phase. Our results provide analytical approximations to important
observables in the Mott phase, and are also of direct relevance to future
experiments with ultra cold atomic gases placed in optical lattices. We also
discuss the symmetry of the Bose-Hubbard model associated with the sign change
of the tunneling coupling.Comment: 7 pages, 4 figures, 1 table. Significantly expanded version with
respect to former submission (to appear in Phys. Rev. A
Shock waves in a one-dimensional Bose gas: from a Bose-Einstein condensate to a Tonks gas
We derive and analyze shock-wave solutions of hydrodynamic equations
describing repulsively interacting one dimensional Bose gas. We also use the
number-conserving Bogolubov approach to verify accuracy of the Gross-Pitaevskii
equation in shock wave problems. We show that quantum corrections to dynamics
of shocks (dark-shock-originated solitons) in a Bose-Einstein condensate are
negligible (important) for a realistic set of system parameters. We point out
possible signatures of a Bose-Einstein condensate -- Tonks crossover in shock
dynamics. Our findings can be directly verified in different experimental
setups.Comment: 10 pages, small corrections with respect to the last submission,
version accepted in Phys. Rev.
Onion-shell model for cosmic ray electrons and radio synchrotron emission in supernova remnants
The spectrum of cosmic ray electrons, accelerated in the shock front of a supernova remnant (SNR), is calculated in the test-particle approximation using an onion-shell model. Particle diffusion within the evolving remnant is explicity taken into account. The particle spectrum becomes steeper with increasing radius as well as SNR age. Simple models of the magnetic field distribution allow a prediction of the intensity and spectrum of radio synchrotron emission and their radial variation. The agreement with existing observations is satisfactory in several SNR's but fails in other cases. Radiative cooling may be an important effect, especially in SNR's exploding in a dense interstellar medium
Theoretical study of electronic transport through a small quantum dot with a magnetic impurity
We model a small quantum dot with a magnetic impurity by the Anderson
Hamiltonian with a supplementary exchange interaction term. The transport
calculations are performed by means of the Green functions within the equation
of motion scheme, in which two decoupling procedures are proposed, for high and
low temperatures, respectively. The paper focuses on the charge fluctuations
for such a system, aspect not addressed before, as well as on the Kondo
resonance. We show a specific role of the excited state, which can be observed
in transport and in spin-spin correlations. Our studies show on a new many-body
feature of the phase shift of transmitted electrons, which is manifested in a
specific dip. In the Kondo regime, our calculations complement existing
theoretical results. The system shows three Kondo peaks in the density of
states: one at the Fermi energy and two side peaks, at a distance corresponding
to the singlet-triplet level spacing. The existence of the central peak is
conditioned by a degenerate state (the triplet) below the Fermi energy.Comment: 12 pages, 4 figure
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