15,157 research outputs found
Penalized Likelihood Methods for Estimation of Sparse High Dimensional Directed Acyclic Graphs
Directed acyclic graphs (DAGs) are commonly used to represent causal
relationships among random variables in graphical models. Applications of these
models arise in the study of physical, as well as biological systems, where
directed edges between nodes represent the influence of components of the
system on each other. The general problem of estimating DAGs from observed data
is computationally NP-hard, Moreover two directed graphs may be observationally
equivalent. When the nodes exhibit a natural ordering, the problem of
estimating directed graphs reduces to the problem of estimating the structure
of the network. In this paper, we propose a penalized likelihood approach that
directly estimates the adjacency matrix of DAGs. Both lasso and adaptive lasso
penalties are considered and an efficient algorithm is proposed for estimation
of high dimensional DAGs. We study variable selection consistency of the two
penalties when the number of variables grows to infinity with the sample size.
We show that although lasso can only consistently estimate the true network
under stringent assumptions, adaptive lasso achieves this task under mild
regularity conditions. The performance of the proposed methods is compared to
alternative methods in simulated, as well as real, data examples.Comment: 19 pages, 8 figure
Automated detection of filaments in the large scale structure of the universe
We present a new method to identify large scale filaments and apply it to a
cosmological simulation. Using positions of haloes above a given mass as node
tracers, we look for filaments between them using the positions and masses of
all the remaining dark-matter haloes. In order to detect a filament, the first
step consists in the construction of a backbone linking two nodes, which is
given by a skeleton-like path connecting the highest local dark matter (DM)
density traced by non-node haloes. The filament quality is defined by a density
and gap parameters characterising its skeleton, and filament members are
selected by their binding energy in the plane perpendicular to the filament.
This membership condition is associated to characteristic orbital times;
however if one assumes a fixed orbital timescale for all the filaments, the
resulting filament properties show only marginal changes, indicating that the
use of dynamical information is not critical for the method. We test the method
in the simulation using massive haloes(h) as
filament nodes. The main properties of the resulting high-quality filaments
(which corresponds to of the detected filaments) are, i) their
lengths cover a wide range of values of up to hMpc, but are mostly
concentrated below 50hMpc; ii) their distribution of thickness peaks at
hMpc and increases slightly with the filament length; iii) their
nodes are connected on average to filaments for nodes; this number increases with the node mass to filaments for nodes.Comment: 17 pages, 13 figures, MNRAS Accepte
Ornstein-Zernike Theory for the finite range Ising models above T_c
We derive precise Ornstein-Zernike asymptotic formula for the decay of the
two-point function in the general context of finite range Ising type models on
Z^d. The proof relies in an essential way on the a-priori knowledge of the
strict exponential decay of the two-point function and, by the sharp
characterization of phase transition due to Aizenman, Barsky and Fernandez,
goes through in the whole of the high temperature region T > T_c. As a
byproduct we obtain that for every T > T_c, the inverse correlation length is
an analytic and strictly convex function of direction.Comment: 36 pages, 5 figure
Radar and RGB-depth sensors for fall detection: a review
This paper reviews recent works in the literature on the use of systems based on radar and RGB-Depth (RGB-D) sensors for fall detection, and discusses outstanding research challenges and trends related to this research field. Systems to detect reliably fall events and promptly alert carers and first responders have gained significant interest in the past few years in order to address the societal issue of an increasing number of elderly people living alone, with the associated risk of them falling and the consequences in terms of health treatments, reduced well-being, and costs. The interest in radar and RGB-D sensors is related to their capability to enable contactless and non-intrusive monitoring, which is an advantage for practical deployment and users’ acceptance and compliance, compared with other sensor technologies, such as video-cameras, or wearables. Furthermore, the possibility of combining and fusing information from The heterogeneous types of sensors is expected to improve the overall performance of practical fall detection systems. Researchers from different fields can benefit from multidisciplinary knowledge and awareness of the latest developments in radar and RGB-D sensors that this paper is discussing
- …