3,869 research outputs found
Computing Shapley Values in the Plane
We consider the problem of computing Shapley values for points in the plane, where each point is interpreted as a player, and the value of a coalition is defined by the area of usual geometric objects, such as the convex hull or the minimum axis-parallel bounding box.
For sets of n points in the plane, we show how to compute in roughly O(n^{3/2}) time the Shapley values for the area of the minimum axis-parallel bounding box and the area of the union of the rectangles spanned by the origin and the input points. When the points form an increasing or decreasing chain, the running time can be improved to near-linear. In all these cases, we use linearity of the Shapley values and algebraic methods.
We also show that Shapley values for the area of the convex hull or the minimum enclosing disk can be computed in O(n^2) and O(n^3) time, respectively. These problems are closely related to the model of stochastic point sets considered in computational geometry, but here we have to consider random insertion orders of the points instead of a probabilistic existence of points
Computing Shapley Values for Mean Width in 3-D
The Shapley value is a common tool in game theory to evaluate the importance
of a player in a cooperative setting. In a geometric context, it provides a way
to measure the contribution of a geometric object in a set towards some
function on the set. Recently, Cabello and Chan (SoCG 2019) presented
algorithms for computing Shapley values for a number of functions for point
sets in the plane. More formally, a coalition game consists of a set of players
and a characteristic function with . Let be a uniformly random permutation of , and be
the set of players in that appear before player in the permutation
. The Shapley value of the game is defined to be . More intuitively,
the Shapley value represents the impact of player 's appearance over all
insertion orders. We present an algorithm to compute Shapley values in 3-D,
where we treat points as players and use the mean width of the convex hull as
the characteristic function. Our algorithm runs in time and
space. Our approach is based on a new data structure for a variant of
the dynamic convolution problem , where we want to answer
dynamically. Our data structure supports updating at position ,
incrementing and decrementing and rotating by . We present a data
structure that supports operations in time and
space. Moreover, the same approach can be used to compute the Shapley values
for the mean volume of the convex hull projection onto a uniformly random -subspace in time and space for a point set in
-dimensional space ()
The Structure of the Outer Halo of the Galaxy and its Relationship to Nearby Large-Scale Structure
We present evidence to support an earlier indication that the Galaxy is
embedded in an extended, highly inclined, triaxial halo outlined by the spatial
distribution of companion galaxies to the Milky Way. Signatures of this spatial
distribution are seen in 1) the angular variation of the radial-velocity
dispersion of the companion galaxies, 2) the spatial distribution of the M~31
sub-group of galaxies, 3) the spatial distribution of the isolated, mainly
dwarf irregular, galaxies of the Local Group, 4) the velocity anisotropy
quadrupole of a sub-group of high-velocity clouds, and 5) the spatial
distribution of galaxies in the Coma-Sculptor cloud. Tidal effects of M~31 and
surrounding galaxies on the Galaxy are not strong enough to have affected the
observed structure. We conclude that this distribution is a reflection of
initial conditions. A simple galaxy formation scenario is proposed which ties
together the results found here with those of Holmberg (1969) and Zaritsky et
al. (1997) on the peculiar distribution of satellites around a large sample of
spiral galaxies.Comment: Accepted for publication in the Astron J., March 2000, 12 pages with
1 figur
Photometric Properties of Lyman-break Galaxies at z=3 in Cosmological SPH Simulations
We study the photometric properties of Lyman-break galaxies (LBGs) formed by
redshift z=3 in a set of large cosmological smoothed-particle hydrodynamics
simulations of the Lambda cold dark matter (CDM) model. Our numerical
simulations include radiative cooling and heating with a uniform UV background,
star formation, supernova feedback, and a phenomenological model for galactic
winds. Analysing a series of simulations of varying boxsize and particle number
allows us to isolate the impact of numerical resolution on our results. We
compute spectra of simulated galaxies using a population synthesis model, and
derive colours and luminosity functions of galaxies at z=3 after applying local
dust extinction and absorption by the intergalactic medium (IGM). We find that
the simulated galaxies have U-G and G-R colours consistent with observations,
provided that intervening absorption by the IGM is applied. The observed
properties of LBGs, including their number density, colours, and luminosity
functions, can be explained if LBGs are identified with the most massive
galaxies at z=3, having typical stellar mass of M_{star} ~ 1e10 Msun/h, a
conclusion broadly consistent with earlier studies based on hydrodynamic
simulations of the Lamda CDM model. We also find that most simulated LBGs were
continuously forming stars at a high rate for more than one Gyr up until z=3,
but with numerous starbursts lying on top of the continuous component.
Interestingly, our simulations suggest that more than 50% of the total stellar
mass and star formation rate in the Universe are accounted for by galaxies that
are not detected in the current generation of LBG surveys.Comment: 12 pages, 8 figures, Error in AB magnitude calculation corrected.
Figures in the original published version in MNRAS contain error except Fig.5
& 6, but the basic conclusions are unchanged. Higher resolution version
available at http://cfa-www.harvard.edu/~knagamine/lbg.ps.g
Escape of Ionizing Radiation from High Redshift Galaxies
We model the escape of ionizing radiation from high-redshift galaxies using
high-resolution Adaptive Mesh Refinement N-body + hydrodynamics simulations.
Our simulations include time-dependent and spatially-resolved transfer of
ionizing radiation in three dimensions, including effects of dust absorption.
For galaxies of total mass M > 10^11 Msun and star formation rates SFR ~ 1-5
Msun/yr, we find angular averaged escape fractions of 0.01-0.03 over the entire
redshift interval studied (3<z<9). In addition, we find that the escape
fraction varies by more than an order of magnitude along different
lines-of-sight within individual galaxies, from the largest values near
galactic poles to the smallest along the galactic disk. The escape fraction
declines steeply at lower masses and SFR. We show that the low values of escape
fractions are due to a small fraction of young stars located just outside the
edge of HI disk. We compare our predicted escape fraction of ionizing photons
with previous results, and find a general agreement with both other simulation
results and available direct detection measurements at z ~ 3. We also compare
our simulations with a novel method to estimate the escape fraction in galaxies
from the observed distribution of neutral hydrogen column densities along the
lines of sights to long duration gamma-ray bursts. Using this method we find
escape fractions of the GRB host galaxies of 2-3%, consistent with our
theoretical predictions. [abridged]Comment: submitted to Ap
Axiomatic Attribution for Multilinear Functions
We study the attribution problem, that is, the problem of attributing a
change in the value of a characteristic function to its independent variables.
We make three contributions. First, we propose a formalization of the problem
based on a standard cost sharing model. Second, we show that there is a unique
attribution method that satisfies Dummy, Additivity, Conditional Nonnegativity,
Affine Scale Invariance, and Anonymity for all characteristic functions that
are the sum of a multilinear function and an additive function. We term this
the Aumann-Shapley-Shubik method. Conversely, we show that such a uniqueness
result does not hold for characteristic functions outside this class. Third, we
study multilinear characteristic functions in detail; we describe a
computationally efficient implementation of the Aumann-Shapley-Shubik method
and discuss practical applications to pay-per-click advertising and portfolio
analysis.Comment: 21 pages, 2 figures, updated version for EC '1
Cosmic rays can drive strong outflows from gas-rich high-redshift disk galaxies
We present simulations of the magnetized interstellar medium (ISM) in models
of massive star forming (40 Msun / yr) disk galaxies with high gas surface
densities (~100 Msun / pc^2) similar to observed star forming high-redshift
disks. We assume that type II supernovae deposit 10 per cent of their energy
into the ISM as cosmic rays and neglect the additional deposition of thermal
energy or momentum. With a typical Galactic diffusion coefficient for CRs (3e28
cm^2 / s) we demonstrate that this process alone can trigger the local
formation of a strong low density galactic wind maintaining vertically open
field lines. Driven by the additional pressure gradient of the relativistic
fluid the wind speed can exceed 1000 km/s, much higher than the escape velocity
of the galaxy. The global mass loading, i.e. the ratio of the gas mass leaving
the galactic disk in a wind to the star formation rate becomes of order unity
once the system has settled into an equilibrium. We conclude that relativistic
particles accelerated in supernova remnants alone provide a natural and
efficient mechanism to trigger winds similar to observed mass-loaded galactic
winds in high-redshift galaxies. These winds also help explaining the low
efficiencies for the conversion of gas into stars in galaxies as well as the
early enrichment of the intergalactic medium with metals. This mechanism can be
at least of similar importance than the traditionally considered momentum
feedback from massive stars and thermal and kinetic feedback from supernova
explosions.Comment: 5 pages, 5 figures, accepted in ApJL; corrected titl
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