30,406 research outputs found
Robust EM algorithm for model-based curve clustering
Model-based clustering approaches concern the paradigm of exploratory data
analysis relying on the finite mixture model to automatically find a latent
structure governing observed data. They are one of the most popular and
successful approaches in cluster analysis. The mixture density estimation is
generally performed by maximizing the observed-data log-likelihood by using the
expectation-maximization (EM) algorithm. However, it is well-known that the EM
algorithm initialization is crucial. In addition, the standard EM algorithm
requires the number of clusters to be known a priori. Some solutions have been
provided in [31, 12] for model-based clustering with Gaussian mixture models
for multivariate data. In this paper we focus on model-based curve clustering
approaches, when the data are curves rather than vectorial data, based on
regression mixtures. We propose a new robust EM algorithm for clustering
curves. We extend the model-based clustering approach presented in [31] for
Gaussian mixture models, to the case of curve clustering by regression
mixtures, including polynomial regression mixtures as well as spline or
B-spline regressions mixtures. Our approach both handles the problem of
initialization and the one of choosing the optimal number of clusters as the EM
learning proceeds, rather than in a two-fold scheme. This is achieved by
optimizing a penalized log-likelihood criterion. A simulation study confirms
the potential benefit of the proposed algorithm in terms of robustness
regarding initialization and funding the actual number of clusters.Comment: In Proceedings of the 2013 International Joint Conference on Neural
Networks (IJCNN), 2013, Dallas, TX, US
The impact of enhanced He and CNONa abundances on globular cluster relative age-dating methods
The impact that unrecognised differences in the chemical patterns of Galactic
globular clusters have on their relative age determinations is studied. The two
most widely used relative age-dating methods, horizontal and vertical, together
with the more recent relative MS-fitting method, were carefully analyzed on a
purely theoretical basis. The BaSTI library was adopted to perform the present
analysis. We find that relative ages derived using the horizontal and vertical
methods are largely dependent on the initial He content and heavy element
distribution. Unrecognized cluster-to-cluster chemical abundance differences
can lead to an error in the derived relative ages as large as ~0.5 (or ~6 Gyr
if an age of 12.8 Gyr is adopted for normalization), and even larger for some
extreme cases. It is shown that the relative MS-fitting method is by far the
age-dating technique for which undetected cluster-to-cluster differences in the
He abundance have less impact. Present results are used in order to pose
constraints on the maximum possible spread in the He and CNONa elements
abundances on the basis of the estimates - taken from the literature - of the
Galactic globular clusters relative age dispersion obtained with the various
relative age-dating techniques. Finally, it is shown that the age-metallicity
relation found for young Galactic globular clusters by the GC Treasury program
is a real age sequence and cannot be produced by variations in the He and/or
heavy element distribution.Comment: 26 pages, 8 figures, accepted for publication in ApJ
Proximity effect in granular superconductor-normal metal structures
We fabricated three-dimensional disordered Pb-Cu granular structures, with
various metal compositions. The typical grain size of both metals is smaller
than the superconductor and normal metal coherence lengths, thus satisfying the
Cooper limit. The critical temperature of the samples was measured and compared
with the critical temperature of bilayers. We show how the proximity effect
theories, developed for bilayers, can be modified for random mixtures and we
demonstrate that our experimental data fit well the de Gennes weak coupling
limit theory in the Cooper limit. Our results indicate that, in granular
structures, the Cooper limit can be satisfied over a wide range of
concentrations.Comment: 15 pages, 4 figure
Joint Clustering and Registration of Functional Data
Curve registration and clustering are fundamental tools in the analysis of
functional data. While several methods have been developed and explored for
either task individually, limited work has been done to infer functional
clusters and register curves simultaneously. We propose a hierarchical model
for joint curve clustering and registration. Our proposal combines a Dirichlet
process mixture model for clustering of common shapes, with a reproducing
kernel representation of phase variability for registration. We show how
inference can be carried out applying standard posterior simulation algorithms
and compare our method to several alternatives in both engineered data and a
benchmark analysis of the Berkeley growth data. We conclude our investigation
with an application to time course gene expression
Phase separation and critical percolation in bidimensional spin-exchange models
Binary mixtures prepared in an homogeneous phase and quenched into a
two-phase region phase-separate via a coarsening process whereby domains of the
two phases grow in time. With a numerical study of a spin-exchange model we
show that this dynamics first takes a system with equal density of the two
species to a critical percolation state. We prove this claim and we determine
the time-dependence of the growing length associated to this process with the
scaling analysis of the statistical and morphological properties of the
clusters of the two phases.Comment: 6 pages, 9 figure
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