1,762 research outputs found
Maximum gradient embeddings and monotone clustering
Let (X,d_X) be an n-point metric space. We show that there exists a
distribution D over non-contractive embeddings into trees f:X-->T such that for
every x in X, the expectation with respect to D of the maximum over y in X of
the ratio d_T(f(x),f(y)) / d_X(x,y) is at most C (log n)^2, where C is a
universal constant. Conversely we show that the above quadratic dependence on
log n cannot be improved in general. Such embeddings, which we call maximum
gradient embeddings, yield a framework for the design of approximation
algorithms for a wide range of clustering problems with monotone costs,
including fault-tolerant versions of k-median and facility location.Comment: 25 pages, 2 figures. Final version, minor revision of the previous
one. To appear in "Combinatorica
Nonlinear spectral calculus and super-expanders
Nonlinear spectral gaps with respect to uniformly convex normed spaces are
shown to satisfy a spectral calculus inequality that establishes their decay
along Cesaro averages. Nonlinear spectral gaps of graphs are also shown to
behave sub-multiplicatively under zigzag products. These results yield a
combinatorial construction of super-expanders, i.e., a sequence of 3-regular
graphs that does not admit a coarse embedding into any uniformly convex normed
space.Comment: Typos fixed based on referee comments. Some of the results of this
paper were announced in arXiv:0910.2041. The corresponding parts of
arXiv:0910.2041 are subsumed by the current pape
The Savvidy ``ferromagnetic vacuum'' in three-dimensional lattice gauge theory
The vacuum effective potential of three-dimensional SU(2) lattice gauge
theory in an applied color-magnetic field is computed over a wide range of
field strengths. The background field is induced by an external current, as in
continuum field theory. Scaling and finite volume effects are analyzed
systematically. The first evidence from lattice simulations is obtained of the
existence of a nontrivial minimum in the effective potential. This supports a
``ferromagnetic'' picture of gluon condensation, proposed by Savvidy on the
basis of a one-loop calculation in (3+1)-dimensional QCD.Comment: 9pp (REVTEX manuscript). Postscript figures appende
Testing Rotational Mixing Predictions with New Boron Abundances in Main Sequence B-type Stars
(Abridged) New boron abundances for seven main-sequence B-type stars are
determined from HST STIS spectroscopy around the BIII 2066A line. Boron
abundances provide a unique and critical test of stellar evolution models that
include rotational mixing since boron is destroyed in the surface layers of
stars through shallow mixing long before other elements are mixed from the
stellar interior through deep mixing. Boron abundances range from 12+log(B/H) =
1.0 to 2.2. The boron abundances are compared to the published values of their
stellar nitrogen abundances (all have 12+log(N/H) < 7.8, i.e., they do not show
significant CNO-mixing) and to their host cluster ages (4 to 16 Myr) to
investigate the predictions from models of massive star evolution with
rotational mixing effects (Heger & Langer 2000). Only three stars (out of 34)
deviate from the model predictions, including HD36591, HD205021, and HD30836.
These three stars suggest that rotational mixing could be more efficient than
currently modelled at the highest rotation rates.Comment: 10 figures, 7 tables; accepted for publication in the Astrophysical
Journa
First results from the VIRIAL survey: the stellar content of -selected quiescent galaxies at from KMOS
We investigate the stellar populations of 25 massive, galaxies
() at using data obtained with
the K-band Multi-Object Spectrograph (KMOS) on the ESO VLT. Targets were
selected to be quiescent based on their broadband colors and redshifts using
data from the 3D-HST grism survey. The mean redshift of our sample is , where KMOS YJ-band data probe age- and metallicity-sensitive
absorption features in the rest-frame optical, including the band, Fe I,
and high-order Balmer lines. Fitting simple stellar population models to a
stack of our KMOS spectra, we derive a mean age of Gyr.
We confirm previous results suggesting a correlation between color and age for
quiescent galaxies, finding mean ages of Gyr and
Gyr for the reddest and bluest galaxies in our sample.
Combining our KMOS measurements with those obtained from previous studies at
we find evidence for a Gyr spread in the formation epoch of
massive galaxies. At the measured stellar ages are consistent with
passive evolution, while at they appear to saturate at
1 Gyr, which likely reflects changing demographics of the (mean)
progenitor population. By comparing to star-formation histories inferred for
"normal" star-forming galaxies, we show that the timescales required to form
massive galaxies at are consistent with the enhanced
-element abundances found in massive local early-type galaxies.Comment: 6 pages, 5 figures, accepted for publication in ApJ
The KMOS^3D Survey: design, first results, and the evolution of galaxy kinematics from 0.7<z<2.7
We present the KMOS^3D survey, a new integral field survey of over 600
galaxies at 0.7<z<2.7 using KMOS at the Very Large Telescope (VLT). The KMOS^3D
survey utilizes synergies with multi-wavelength ground and space-based surveys
to trace the evolution of spatially-resolved kinematics and star formation from
a homogeneous sample over 5 Gyrs of cosmic history. Targets, drawn from a
mass-selected parent sample from the 3D-HST survey, cover the star
formation-stellar mass () and rest-frame planes uniformly. We
describe the selection of targets, the observations, and the data reduction. In
the first year of data we detect Halpha emission in 191
Msun galaxies at z=0.7-1.1 and z=1.9-2.7. In
the current sample 83% of the resolved galaxies are rotation-dominated,
determined from a continuous velocity gradient and , implying
that the star-forming 'main sequence' (MS) is primarily composed of rotating
galaxies at both redshift regimes. When considering additional stricter
criteria, the Halpha kinematic maps indicate at least ~70% of the resolved
galaxies are disk-like systems. Our high-quality KMOS data confirm the elevated
velocity dispersions reported in previous IFS studies at z>0.7. For
rotation-dominated disks, the average intrinsic velocity dispersion decreases
by a factor of two from 50 km/s at z~2.3 to 25 km/s at z~0.9 while the
rotational velocities at the two redshifts are comparable. Combined with
existing results spanning z~0-3, disk velocity dispersions follow an
approximate (1+z) evolution that is consistent with the dependence of velocity
dispersion on gas fractions predicted by marginally-stable disk theory.Comment: 20 pages, 11 figures, 1 Appendix; Accepted to ApJ November 2
“My First Thought was Croutons”: Perceptions of Cigarettes and Cigarette Smoke Constituents Among Adult Smokers and Nonsmokers
Understanding what people think about harmful and potentially harmful constituents in cigarettes and cigarette smoke has new urgency given legislation requiring US Food and Drug Administration (FDA) to disclose constituent information. Our study sought to obtain qualitative information on what people think about these constituents and the language they use to talk about them
Presence of the Eucalyptus gall wasp Ophelimus maskelli and its parasitoid Closterocerus chamaeleon in Portugal: first record, geographic distribution and host preference
The Eucalyptus gall wasp Ophelimus
maskelli (Hymenoptera: Eulophidae) and its parasitoid
Closterocerus chamaeleon (Hymenoptera:
Eulophidae) were observed for the first time in
Portugal, in 2006 and 2007, respectively. Data on
the distribution of O. maskelli in Portugal, differences
in the susceptibility of two host species, Eucalyptus
globulus and Eucalyptus camaldulensis, and parasitism
by C. chamaeleon are given
What explains the uneven take-up of ISO 14001 at the global level?: a panel-data analysis
Since its release in the mid-1990s, close to 37 000 facilities have been certified to ISO 14001, the international voluntary standard for environmental management systems. Yet, despite claims that the standard can be readily adapted to very different corporate and geographic settings, its take-up has been highly geographically variable. This paper contributes to a growing body of work concerned with explaining the uneven diffusion of ISO 14001 at the global level. Drawing from the existing theoretical and empirical literature we develop a series of hypotheses about how various economic, market, and regulatory factors influence the national count of ISO 14001 certifications. These hypotheses are then tested using econometric estimation techniques with data for a panel of 142 developed and developing countries. We find that per capita ISO 14001 counts are positively correlated with income per capita, stock of foreign direct investment, exports of goods and services to Europe and Japan, and pressure from civil society. Conversely, productivity and levels of state intervention are negatively correlated. The paper finishes by offering a number of recommendations to policymakers concerned with accelerating the diffusion of voluntary environmental standards
Large Tree Level CP Violation in in The Two Higgs Doublet Model
We find a large CP violation effect within the Two-Higgs-Doublet-Model for
the reaction at future linear colliders. The
CP-asymmetry arises already at the tree level as a result of interference
between diagrams with emission from (and ) and its emission
from a and can be about 10--20\%. In the best case one needs a few
hundred events to observe CP violation at the 3 level.Comment: UU encoded tar compressed tex file with postscript figure
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