6,755 research outputs found
The gas of elastic quantum strings in 2+1 dimensions: finite temperatures
The finite temperature physics of the gas of elastic quantum strings as
introduced in J. Zaanen, Phys. Rev. Lett. 84, 753 is investigated. This model
is inspired on the stripes in the high Tc superconductors. We analyze in detail
how the kinetic interactions of the zero temperature quantum problem crossover
into the entropic interactions of the high temperature limit.Comment: 14 pages, 2 figure
Taking apart the dynamical clock. Fat-tailed dynamical kicks shape the blue-straggler star bimodality
In globular clusters, blue straggler stars are heavier than the average star,
so dynamical friction strongly affects them. The radial distribution of BSS,
normalized to a reference population, appears bimodal in a fraction of Galactic
GCs, with a density peak in the core, a prominent zone of avoidance at
intermediate radii, and again higher density in the outskirts. The zone of
avoidance appears to be located at larger radii the more relaxed the host
cluster, acting as a sort of dynamical clock. We use a new method to compute
the evolution of the BSS radial distribution under dynamical friction and
diffusion. We evolve our BSS in the mean cluster potential under dynamical
friction plus a random fluctuating force, solving the Langevin equation with
the Mannella quasi symplectic scheme. This amounts to a new simulation method
which is much faster and simpler than direct N-body codes but retains their
main feature: diffusion powered by strong, if infrequent, kicks. We compute the
radial distribution of initially unsegregated BSS normalized to a reference
population as a function of time. We trace the evolution of its minimum,
corresponding to the zone of avoidance. We compare the evolution under kicks
extracted from a Gaussian distribution to that obtained using a Holtsmark
distribution. The latter is a fat tailed distribution which correctly models
the effects of close gravitational encounters. We find that the zone of
avoidance moves outwards over time, as expected based on observations, only
when using the Holtsmark distribution. Thus the correct representation of near
encounters is crucial to reproduce the dynamics of the system. We confirm and
extend earlier results that showed how the dynamical clock indicator depends
both on dynamical friction and effective diffusion powered by dynamical
encounters.Comment: 8 pages, 6 figures. Version accepted in Astronomy & Astrophysic
Interpretable Categorization of Heterogeneous Time Series Data
Understanding heterogeneous multivariate time series data is important in
many applications ranging from smart homes to aviation. Learning models of
heterogeneous multivariate time series that are also human-interpretable is
challenging and not adequately addressed by the existing literature. We propose
grammar-based decision trees (GBDTs) and an algorithm for learning them. GBDTs
extend decision trees with a grammar framework. Logical expressions derived
from a context-free grammar are used for branching in place of simple
thresholds on attributes. The added expressivity enables support for a wide
range of data types while retaining the interpretability of decision trees. In
particular, when a grammar based on temporal logic is used, we show that GBDTs
can be used for the interpretable classi cation of high-dimensional and
heterogeneous time series data. Furthermore, we show how GBDTs can also be used
for categorization, which is a combination of clustering and generating
interpretable explanations for each cluster. We apply GBDTs to analyze the
classic Australian Sign Language dataset as well as data on near mid-air
collisions (NMACs). The NMAC data comes from aircraft simulations used in the
development of the next-generation Airborne Collision Avoidance System (ACAS
X).Comment: 9 pages, 5 figures, 2 tables, SIAM International Conference on Data
Mining (SDM) 201
Safe, Remote-Access Swarm Robotics Research on the Robotarium
This paper describes the development of the Robotarium -- a remotely
accessible, multi-robot research facility. The impetus behind the Robotarium is
that multi-robot testbeds constitute an integral and essential part of the
multi-agent research cycle, yet they are expensive, complex, and time-consuming
to develop, operate, and maintain. These resource constraints, in turn, limit
access for large groups of researchers and students, which is what the
Robotarium is remedying by providing users with remote access to a
state-of-the-art multi-robot test facility. This paper details the design and
operation of the Robotarium as well as connects these to the particular
considerations one must take when making complex hardware remotely accessible.
In particular, safety must be built in already at the design phase without
overly constraining which coordinated control programs the users can upload and
execute, which calls for minimally invasive safety routines with provable
performance guarantees.Comment: 13 pages, 7 figures, 3 code samples, 72 reference
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