12,215 research outputs found
Low Rank Approximation of Binary Matrices: Column Subset Selection and Generalizations
Low rank matrix approximation is an important tool in machine learning. Given
a data matrix, low rank approximation helps to find factors, patterns and
provides concise representations for the data. Research on low rank
approximation usually focus on real matrices. However, in many applications
data are binary (categorical) rather than continuous. This leads to the problem
of low rank approximation of binary matrix. Here we are given a
binary matrix and a small integer . The goal is to find two binary
matrices and of sizes and respectively, so
that the Frobenius norm of is minimized. There are two models of this
problem, depending on the definition of the dot product of binary vectors: The
model and the Boolean semiring model. Unlike low rank
approximation of real matrix which can be efficiently solved by Singular Value
Decomposition, approximation of binary matrix is -hard even for .
In this paper, we consider the problem of Column Subset Selection (CSS), in
which one low rank matrix must be formed by columns of the data matrix. We
characterize the approximation ratio of CSS for binary matrices. For
model, we show the approximation ratio of CSS is bounded by
and this bound is asymptotically tight. For
Boolean model, it turns out that CSS is no longer sufficient to obtain a bound.
We then develop a Generalized CSS (GCSS) procedure in which the columns of one
low rank matrix are generated from Boolean formulas operating bitwise on
columns of the data matrix. We show the approximation ratio of GCSS is bounded
by , and the exponential dependency on is inherent.Comment: 38 page
Daily partition of urinary nitrogen and nitrogen balance during treatment of protein-depleted infants
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A statistical-mechanical explanation of dark matter halo properties
Cosmological N-body simulations have revealed many empirical relationships of
dark matter halos, yet the physical origin of these halo properties still
remains unclear. On the other hand, the attempts to establish the statistical
mechanics for self-gravitating systems have encountered many formal
difficulties, and little progress has been made for about fifty years. The aim
of this work is to strengthen the validity of the statistical-mechanical
approach we have proposed previously to explain the dark matter halo
properties. By introducing an effective pressure instead of the radial pressure
to construct the specific entropy, we use the entropy principle and proceed in
a similar way as previously to obtain an entropy stationary equation. An
equation of state for equilibrated dark halos is derived from this entropy
stationary equation, by which the dark halo density profiles with finite mass
can be obtained. We also derive the anisotropy parameter and pseudo-phase-space
density profile. All these predictions agree well with numerical simulations in
the outer regions of dark halos. Our work provides further support to the idea
that statistical mechanics for self-gravitating systems is a viable tool for
investigation.Comment: 5 pages, 4 figures, Accepted by A&
Modelling the WMAP large-angle anomalies as an effect of a local density inhomogeneity
We investigate large-angle scale temperature anisotropy in the Cosmic
Microwave Background (CMB) with the Wilkinson Microwave Anisotropy Probe (WMAP)
data and model the large-angle anomalies as the effect of the CMB quadrupole
anisotropies caused by the local density inhomogeneities. The quadrupole caused
by the local density inhomogeneities is different from the special relativity
kinematic quadrupole. If the observer inhabits a strong inhomogeneous region,
the local quadrupole should not be neglected. We calculate such local
quadrupole under the assumption that there is a huge density fluctuation field
in direction , where the density fluctuation is
, and its center is away from us. After
removing such mock signals from WMAP data, the power in quadrupole, ,
increases from the range to .
The quantity S, which is used to estimate the alignment between the quadrupole
and the octopole, decreases from to , while the
model predict that , . So our local density
inhomogeneity model can, in part, explain the WMAP low- anomalies.Comment: 7 pages, 5 figures, accepted by RA
Observational Constraints on First-Star Nucleosynthesis. I. Evidence for Multiple Progenitors of CEMP-no Stars
We investigate anew the distribution of absolute carbon abundance, (C) (C), for carbon-enhanced metal-poor (CEMP) stars in the halo of
the Milky Way, based on high-resolution spectroscopic data for a total sample
of 305 CEMP stars. The sample includes 147 CEMP- (and CEMP-r/s) stars, 127
CEMP-no stars, and 31 CEMP stars that are unclassified, based on the currently
employed [Ba/Fe] criterion. We confirm previous claims that the distribution of
(C) for CEMP stars is (at least) bimodal, with newly determined peaks
centered on (C) (the high-C region) and (C) (the low-C
region). A very high fraction of CEMP- (and CEMP-r/s) stars belong to the
high-C region, while the great majority of CEMP-no stars reside in the low-C
region. However, there exists complexity in the morphology of the (C)-[Fe/H]
space for the CEMP-no stars, a first indication that more than one class of
first-generation stellar progenitors may be required to account for their
observed abundances. The two groups of CEMP-no stars we identify exhibit
clearly different locations in the (Na)-(C) and (Mg)-(C) spaces,
also suggesting multiple progenitors. The clear distinction in (C) between
the CEMP- (and CEMP-) stars and the CEMP-no stars appears to be $as\
successfullikely\ more\ astrophysically\ fundamental$, for the
separation of these sub-classes as the previously recommended criterion based
on [Ba/Fe] (and [Ba/Eu]) abundance ratios. This result opens the window for its
application to present and future large-scale low- and medium-resolution
spectroscopic surveys.Comment: 26pages, 7 figures, and 3 Tables ; Accepted for publication in ApJ;
added more data and corrected minor inconsistencies existed in the compiled
data of the previous studie
An attractor for dark matter structures
Cosmological simulations of dark matter structures have identified a set of
universal profiles, and similar characteristics have been seen in
non-cosmological simulations. It has therefore been speculated whether these
profiles of collisionless systems relate to accretion and merger history, or if
there is an attractor for the dark matter systems. Here we identify such a
1-dimensional attractor in the 3-dimensional space spanned by the 2 radial
slopes of the density and velocity dispersion, and the velocity anisotropy.
This attractor effectively removes one degree of freedom from the Jeans
equation. It also allows us to speculate on a new fluid interpretation for the
Jeans equation, with an effective polytropic index for the dark matter
particles between 1/2 and 3/4. If this attractor solution holds for other
collisionless structures, then it may hold the key to break the mass-anisotropy
degeneracy, which presently prevents us from measuring the mass profiles in
dwarf galaxies uniquely.Comment: 7 pages, 2 figures, comments welcom
Distribution Function in Center of Dark Matter Halo
N-body simulations of dark matter halos show that the density profiles of
halos behave as , where the density logarithmic
slope in the center and in the
outer parts of halos. However, some observations are not in agreement with
simulations in the very central region of halos. The simulations also show that
velocity dispersion anisotropy parameter in the inner part of
the halo and the so called "pseudo phase-space density" behaves
as a power-law in radius . With these results in mind, we study the
distribution function and the pseudo phase-space density of the
center of dark matter halos and find that they are closely-related.Comment: 8 pages, accepted by Int. J. Mod. Phys. D, add a quotation and remove
the appendi
A combined SEM, CV and EIS study of multi-layered porous ceramic reactors for flue gas purification
The effect of sintering temperature of 12-layered porous ceramic reactors (comprising 5 cells) was studied using scanning electron microscopy (SEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The difference in microstructures of the reactors was evaluated by SEM. Additional information on the influence of sintering temperature on the properties of the reactors could be gained by the use of EIS. The present work has provided the first set of fundamental electrochemical data and their interpretation in terms of fabrication conditions, for the multi-layered porous ceramic reactors
Limits on the parameters of the equation of state for interacting dark energy
Under the assumption that cold dark matter and dark energy interact with each
other through a small coupling term, , we constrain the parameter space of
the equation of state of those dark energy fields whose variation of the
field since last scattering do not exceed Planck's mass. We use three
parameterizations of and two different expressions for . Our work
extends previous ones.Comment: 18 pages, 11 figures, accepted for publication on Physics Letters
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