Statistical Mechanics of Time Domain Ensemble Learning

Conventional ensemble learning combines students in the space domain. On the other hand, in this paper we combine students in the time domain and call it time domain ensemble learning. In this paper, we analyze the generalization performance of time domain ensemble learning in the framework of online learning using a statistical mechanical method. We treat a model in which both the teacher and the student are linear perceptrons with noises. Time domain ensemble learning is twice as effective as conventional space domain ensemble learning.Comment: 10 pages, 10 figure

Statistical Mechanics of Linear and Nonlinear Time-Domain Ensemble Learning

Conventional ensemble learning combines students in the space domain. In this paper, however, we combine students in the time domain and call it time-domain ensemble learning. We analyze, compare, and discuss the generalization performances regarding time-domain ensemble learning of both a linear model and a nonlinear model. Analyzing in the framework of online learning using a statistical mechanical method, we show the qualitatively different behaviors between the two models. In a linear model, the dynamical behaviors of the generalization error are monotonic. We analytically show that time-domain ensemble learning is twice as effective as conventional ensemble learning. Furthermore, the generalization error of a nonlinear model features nonmonotonic dynamical behaviors when the learning rate is small. We numerically show that the generalization performance can be improved remarkably by using this phenomenon and the divergence of students in the time domain.Comment: 11 pages, 7 figure

Thermal and Electrical Properties of gamma-NaxCoO2 (0.70 < x < 0.78)

We have performed specific heat and electric resistivity measurements of Na$_{x}$CoO$_{2}$ ($x=0.70$-0.78). Two anomalies have been observed in the specific heat data for $x=0.78$, corresponding to magnetic transitions at $T_{c}=22$ K and $T_{k}\simeq 9$ K reported previously. In the electrical resistivity, a steep decrease at $T_{c}$ and a bending-like variation at $T_{b}$(=120K for $x=0.78$) have been observed. Moreover, we have investigated the $x$-dependence of these parameters in detail. The physical properties of this system are very sensitive to $x$, and the inconsistent results of previous reports can be explained by a small difference in $x$. Furthermore, for a higher $x$ value, a phase separation into Na-rich and Na-poor domains occurs as we previously proposed, while for a lower $x$ value, from characteristic behaviors of the specific heat and the electrical resistivity at the low-temperature region, the system is expected to be in the vicinity of the magnetic instability which virtually exists below $x=0.70$.Comment: 4 pages (3 figures included) and an extra figure (gif), to be published in J. Phys. Soc. Jpn. 73 (9) with possible minor revision

Synthesis, Characterization, and Magnetic Properties of gamma-NaxCoO2 (0.70 < x <0.84)

Powder Na$_{x}$CoO$_{2}$ ($0.70\leq x\leq 0.84$) samples were synthesized and characterized carefully by X-ray diffraction analysis, inductive-coupled plasma atomic emission spectroscopy, and redox titration. It was proved that $\gamma$-Na$_{x}$CoO$_{2}$ is formed only in the narrow range of $0.70\leq x\leq 0.78$. Nevertheless, the magnetic properties depend strongly on $x$. We found, for the first time, two characteristic features in the magnetic susceptibility of Na$_{0.78}$CoO$_{2}$, a sharp peak at $T_{p}=16$ K and an anomaly at $T_{k}=9$ K, as well as the transition at $T_{c}=22$ K and the broad maximum at $T_{m}=50$ K which had already been reported. A type of weak ferromagnetic transition seems to occur at $T_{k}$. The transition at $T_{c}$, which is believed to be caused by spin density wave formation, was observed clearly for $x\geq 0.74$ with constant $T_{c}$ and $T_{p}$ independent of $x$. On the other hand, ferromagnetic moment varies systematically depending on $x$. These facts suggest the occurrence of a phase separation at the microscopic level, such as the separation into Na-rich and Na-poor domains due to the segregation of Na ions. The magnetic phase diagram and transition mechanism proposed previously should be reconsidered.Comment: 4 pages (2 figures included) and 2 extra figures (gif), to be published in J. Phys. Soc. Jpn. 73 (8) with possible minor revision

Athena: A New Code for Astrophysical MHD

A new code for astrophysical magnetohydrodynamics (MHD) is described. The code has been designed to be easily extensible for use with static and adaptive mesh refinement. It combines higher-order Godunov methods with the constrained transport (CT) technique to enforce the divergence-free constraint on the magnetic field. Discretization is based on cell-centered volume-averages for mass, momentum, and energy, and face-centered area-averages for the magnetic field. Novel features of the algorithm include (1) a consistent framework for computing the time- and edge-averaged electric fields used by CT to evolve the magnetic field from the time- and area-averaged Godunov fluxes, (2) the extension to MHD of spatial reconstruction schemes that involve a dimensionally-split time advance, and (3) the extension to MHD of two different dimensionally-unsplit integration methods. Implementation of the algorithm in both C and Fortran95 is detailed, including strategies for parallelization using domain decomposition. Results from a test suite which includes problems in one-, two-, and three-dimensions for both hydrodynamics and MHD are given, not only to demonstrate the fidelity of the algorithms, but also to enable comparisons to other methods. The source code is freely available for download on the web.Comment: 61 pages, 36 figures. accepted by ApJ

LHR band emissions at mid-latitude and their relationship to ionospheric ELF hiss and relativistic electrons

LHR band emissions observed at mid-latitude were investigated using data from the EXOS-C (Ohzora) satellite. A typical feature of the LHR band emissions is a continuous banded structure without burst-like and cut-off features whose center frequency decreases as the satellite moves to higher latitudes. A statistical analysis of the occurrence characteristics of the phenomena showed that mid-latitude LHR emissions are distributed inside the plasmapause during magnetically quiet periods, and the poleward boundary of the emission region moves to lower latitudes as the magnetic activity increases. The altitude distribution of the waves suggests that the propagation in the LHR duct formed horizontally in the mid-latitude upper-ionosphere. The emission is closely related to the occurrence of ionospheric ELF hiss. It is also shown that LHR emissions are commonly observed in the slot region of the radiation belt, and they sometimes accompany the enhancement of the ionospheric electron temperature. The generation of the LHR band emissions is discussed based on the observed characteristics

Mass-Enhanced Fermi Liquid Ground State in Na1.5_{1.5}Co2_2O4_4

Magnetic, transport, and specific heat measurements have been performed on layered metallic oxide Na$_{1.5}$Co$_2$O$_4$ as a function of temperature $T$. Below a characteristic temperature $T^*$=30$-$40 K, electrical resistivity shows a metallic conductivity with a $T^2$ behavior and magnetic susceptibility deviates from the Curie-Weiss behavior showing a broad peak at $\sim$14 K. The electronic specific heat coefficient $\gamma$ is $\sim$60 mJ/molK$^2$ at 2 K. No evidence for magnetic ordering is found. These behaviors suggest the formation of mass-enhanced Fermi liquid ground state analogous to that in $d$-electron heavy fermion compound LiV$_2$O$_4$.Comment: 4 pages, 4 figures, to be published in Phys. Rev. B 69 (2004

Low energy high angular resolution neutral atom detection by means of micro-shuttering techniques: the BepiColombo SERENA/ELENA sensor

The neutral sensor ELENA (Emitted Low-Energy Neutral Atoms) for the ESA cornerstone BepiColombo mission to Mercury (in the SERENA instrument package) is a new kind of low energetic neutral atoms instrument, mostly devoted to sputtering emission from planetary surfaces, from E ~20 eV up to E~5 keV, within 1-D (2x76 deg). ELENA is a Time-of-Flight (TOF) system, based on oscillating shutter (operated at frequencies up to a 100 kHz) and mechanical gratings: the incoming neutral particles directly impinge upon the entrance with a definite timing (START) and arrive to a STOP detector after a flight path. After a brief dissertation on the achievable scientific objectives, this paper describes the instrument, with the new design techniques approached for the neutral particles identification and the nano-techniques used for designing and manufacturing the nano-structure shuttering core of the ELENA sensor. The expected count-rates, based on the Hermean environment features, are shortly presented and discussed. Such design technologies could be fruitfully exported to different applications for planetary exploration.Comment: 11 page