1,546 research outputs found
The kernel Kalman rule: efficient nonparametric inference with recursive least squares
Nonparametric inference techniques provide promising tools
for probabilistic reasoning in high-dimensional nonlinear systems.
Most of these techniques embed distributions into reproducing
kernel Hilbert spaces (RKHS) and rely on the kernel
Bayesâ rule (KBR) to manipulate the embeddings. However,
the computational demands of the KBR scale poorly
with the number of samples and the KBR often suffers from
numerical instabilities. In this paper, we present the kernel
Kalman rule (KKR) as an alternative to the KBR. The derivation
of the KKR is based on recursive least squares, inspired
by the derivation of the Kalman innovation update. We apply
the KKR to filtering tasks where we use RKHS embeddings
to represent the belief state, resulting in the kernel Kalman filter
(KKF). We show on a nonlinear state estimation task with
high dimensional observations that our approach provides a
significantly improved estimation accuracy while the computational
demands are significantly decreased
Learning robust policies for object manipulation with robot swarms
Swarm robotics investigates how a large population of robots with simple actuation and limited sensors can collectively solve complex tasks. One particular interesting application with robot swarms is autonomous object assembly.
Such tasks have been solved successfully with robot swarms that are controlled by a human operator using a light source.
In this paper, we present a method to solve such assembly tasks autonomously based on policy search methods. We split the assembly process in two subtasks: generating a high-level assembly plan and learning a low-level object movement policy. The assembly policy plans the trajectories for each object and the object movement policy controls the trajectory execution. Learning the object movement policy is challenging as it depends on the complex state of the swarm which consists of an individual state for each agent. To approach this problem, we introduce a representation of the swarm which is based on Hilbert space embeddings of distributions. This representation is invariant to the number of agents in the swarm as well as to the allocation of an agent to its position in the swarm. These invariances make the learned policy robust to changes in the swarm and also reduce the search space for the policy search method significantly. We show that the resulting system is able to solve assembly tasks with varying object shapes in multiple simulation scenarios and evaluate the robustness of our representation to changes in the swarm size. Furthermore, we demonstrate that the policies learned in simulation are robust enough to be transferred to real robots
Robust learning of object assembly tasks with an invariant representation of robot swarms
â Swarm robotics investigates how a large population of robots with simple actuation and limited sensors can collectively solve complex tasks. One particular interesting application with robot swarms is autonomous object assembly. Such tasks have been solved successfully with robot swarms that are controlled by a human operator using a light source. In this paper, we present a method to solve such assembly tasks autonomously based on policy search methods. We split the assembly process in two subtasks: generating a high-level assembly plan and learning a low-level object movement policy. The assembly policy plans the trajectories for each object and the object movement policy controls the trajectory execution.
Learning the object movement policy is challenging as it depends on the complex state of the swarm which consists of an individual state for each agent. To approach this problem, we introduce a representation of the swarm which is based on Hilbert space embeddings of distributions. This representation is invariant to the number of agents in the swarm as well as to the allocation of an agent to its position in the swarm. These invariances make the learned policy robust to changes in the swarm and also reduce the search space for the policy search method significantly. We show that the resulting system is able to solve assembly tasks with varying object shapes in multiple simulation scenarios and evaluate the robustness of our representation to changes in the swarm size. Furthermore, we demonstrate that the policies learned in simulation are robust enough to be transferred to real robots
Hydrodynamical evolution near the QCD critical end point
Hydrodynamical calculations have been successful in describing global
observables in ultrarelativistic heavy ion collisions, which aim to observe the
production of the quark-gluon plasma. On the other hand, recently, a lot of
evidence that there exists a critical end point (CEP) in the QCD phase diagram
has been accumulating. Nevertheless, so far, no equation of state with the CEP
has been employed in hydrodynamical calculations. In this paper, we construct
the equation of state with the CEP on the basis of the universality hypothesis
and show that the CEP acts as an attractor of isentropic trajectories. We also
consider the time evolution in the case with the CEP and discuss how the CEP
affects the final state observables, such as the correlation length,
fluctuation, chemical freezeout, kinetic freezeout, and so on. Finally, we
argue that the anomalously low kinetic freezeout temperature at the BNL
Relativistic Heavy Ion Collider suggests the possibility of the existence of
the CEP.Comment: 13 pages, 12 figures, accepted for publication in Physical Review
The Spin of M87 as measured from the Rotation of its Globular Clusters
We revisit the kinematical data for 204 globular clusters in the halo of M87.
Beyond 3 r_eff along the major axis of the galaxy light, these globular
clusters exhibit substantial rotation (~ 300 +/- 70 km/s) that translates into
an equally substantial spin (lambda ~ 0.18). The present appearance of M87 is
most likely the product of a single major merger, since this event is best able
to account for so sizable a spin. A rotation this large makes improbable any
significant accretion of material after this merger, since that would have
diluted the rotation signature. We see weak evidence for a difference between
the kinematics of the metal-poor and metal-rich population, in the sense that
the metal-poor globular clusters appear to dominate the rotation. If, as we
suspect, the last major merger event of M87 was mainly dissipationless and did
not trigger the formation of a large number of globular clusters, the kinematic
difference between the two could reflect their orbital properties in the
progenitor galaxies; these differences would be compatible with these
progenitors having formed in dissipational mergers. However, to put strong
kinematic constraints on the origin of the globular clusters themselves is
difficult, given the complex history of the galaxy and its last dominant merger
event.Comment: 20 pages (AAS two column style, including 1 table and 7 figures)
accepted in the AJ (November issue), also available at
http://www.ucolick.org/~mkissler
On Shape Transformations and Shape Fluctuations of Cellular Compartments and Vesicles
We discuss the shape formation and shape transitions of simple bilayer vesicles in context with their role in biology. In the first part several classes of shape changes of vesicles of one lipid component are described and it is shown that these can be explained in terms of the bending energy concept in particular augmented by the bilayer coupling hypothesis. In the second
part shape changes and vesicle fission of vesicles composed of membranes of lipid mixtures are reported. These are explained in terms of coupling between local curvature and phase separation
Spatially resolved simulation of a radio frequency driven micro atmospheric pressure plasma jet and its effluent
Radio frequency driven plasma jets are frequently employed as efficient
plasma sources for surface modification and other processes at atmospheric
pressure. The radio-frequency driven micro atmospheric pressure plasma jet
(APPJ) is a particular variant of that concept whose geometry allows
direct optical access. In this work, the characteristics of the APPJ
operated with a helium-oxygen mixture and its interaction with a helium
environment are studied by numerical simulation. The density and temperature of
the electrons, as well as the concentration of all reactive species are studied
both in the jet itself and in its effluent. It is found that the effluent is
essentially free of charge carriers but contains a substantial amount of
activated oxygen (O, O and O). The simulation results are
verified by comparison with experimental data
UBVI Surface Photometry of the Spiral Galaxy NGC 300 in the Sculptor Group
We present UBVI surface photometry for 20.'5 X 20.'5 area of a late-type
spiral galaxy NGC 300. In order to understand the morphological properties and
luminosity distribution characteristics of NGC 300, we have derived isophotal
maps, surface brightness profiles, ellipticity profiles, position angle
profiles, and color profiles. By merging the I-band data of our surface
brightness measurements with those of Boeker et al. (2002) based on Hubble
Space Telescope observations, we have made combined I-band surface brightness
profiles for the region of 0."02 < r < 500" and decomposed the profiles into
three components: a nucleus, a bulge, and an exponential disk.Comment: 16 pages(cjaa209.sty), Accepted by the Chinese J. Astron. Astrophys.,
Fig 2 and 8 are degraded to reduce spac
Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in the Globular Cluster M15: II. Kinematical Analysis and Dynamical Modeling
We analyze HST/STIS spectra (see Paper I) of the central region of the dense
globular cluster M15. We infer the velocities of 64 individual stars,
two-thirds of which have their velocity measured for the first time. This
triples the number of stars with measured velocities in the central 1 arcsec of
M15 and doubles the number in the central 2 arcsec. Combined with existing
ground-based data we obtain the radial profiles of the projected kinematical
quantities. The RMS velocity sigma_RMS rises to 14 km/s in the central few
arcsec, somewhat higher than the values of 10-12 km/s inferred previously from
ground-based data. To interpret the results we construct dynamical models based
on the Jeans equation, which imply that M15 must have a central concentration
of non-luminous material. If this is due to a single black hole, then its mass
is M_BH = (3.9 +/- 2.2) x 10^3 solar masses. This is consistent with the
relation between M_BH and sigma_RMS that has been established for galaxies.
Also, the existence of intermediate-mass black holes in globular clusters is
consistent with several scenarios for globular cluster evolution proposed in
the literature. Therefore, these results may have important implications for
our understanding of the evolution of globular clusters, the growth of black
holes, the connection between globular cluster and galaxy formation, and the
nature of the recently discovered `ultra-luminous' X-ray sources in nearby
galaxies. Instead of a single black hole, M15 could have a central
concentration of dark remnants (e.g., neutron stars) due to mass segregation.
However, the best-fitting Fokker-Planck models that have previously been
constructed for M15 do not predict a central mass concentration that is
sufficient to explain the observed kinematics.[ABRIDGED]Comment: 43 pages, LaTeX, with 14 PostScript figures. Astronomical Journal, in
press (Dec 2002). Please note that the results reported here are modified by
the Addendum available at astro-ph/0210158 (Astronomical Journal, in press,
Jan 2003). This second version submitted to astro-ph is identical to first,
with the exception of the preceeding remar
SuperMassive Black Holes in Bulges
We present spatially extended gas kinematics at parsec-scale resolution for
the nuclear regions of four nearby disk galaxies, and model them as rotation of
a gas disk in the joint potential of the stellar bulge and a putative central
black hole. The targets were selected from a larger set of long-slit spectra
obtained with the Hubble Space Telescope as part of the Survey of Nearby Nuclei
with STIS (SUNNS). They represents the 4 galaxies (of 24) that display
symmetric gas velocity curves consistent with a rotating disk. We derive the
stellar mass distribution from the STIS acquisition images adopting the stellar
mass-to-light ratio normalized so as to match ground-based velocity dispersion
measurements over a large aperture. Subsequently, we constrain the mass of a
putative black hole by matching the gas rotation curve, following two distinct
approaches. In the most general case we explore all the possible disk
orientations, alternatively we constrain the gas disk orientation from the
dust-lane morphology at similar radii. In the latter case the kinematic data
indicate the presence of a central black hole for three of the four objects,
with masses of 10^7 - 10^8 solar masses, representing up to 0.025 % of the host
bulge mass. For one object (NGC2787) the kinematic data alone provide clear
evidence for the presence of a central black hole even without external
constraints on the disk orientation. These results illustrate directly the need
to determine black-hole masses by differing methods for a large number of
objects, demonstrate that the variance in black hole/bulge mass is much larger
than previously claimed, and reinforce the recent finding that the black-hole
mass is tightly correlated with the bulge stellar velocity dispersion.Comment: 26 pages, 11 Postscript figures, accepted for publication on Ap
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