14,662 research outputs found
A Constant-Factor Approximation for Multi-Covering with Disks
We consider variants of the following multi-covering problem with disks. We
are given two point sets (servers) and (clients) in the plane, a
coverage function , and a constant . Centered at each server is a single disk whose radius we are free to
set. The requirement is that each client be covered by at least
of the server disks. The objective function we wish to minimize is
the sum of the -th powers of the disk radii. We present a polynomial
time algorithm for this problem achieving an approximation
Constant-Factor Approximation for TSP with Disks
We revisit the traveling salesman problem with neighborhoods (TSPN) and
present the first constant-ratio approximation for disks in the plane: Given a
set of disks in the plane, a TSP tour whose length is at most times
the optimal can be computed in time that is polynomial in . Our result is
the first constant-ratio approximation for a class of planar convex bodies of
arbitrary size and arbitrary intersections. In order to achieve a
-approximation, we reduce the traveling salesman problem with disks, up
to constant factors, to a minimum weight hitting set problem in a geometric
hypergraph. The connection between TSPN and hitting sets in geometric
hypergraphs, established here, is likely to have future applications.Comment: 14 pages, 3 figure
Approximation Algorithms for Polynomial-Expansion and Low-Density Graphs
We study the family of intersection graphs of low density objects in low
dimensional Euclidean space. This family is quite general, and includes planar
graphs. We prove that such graphs have small separators. Next, we present
efficient -approximation algorithms for these graphs, for
Independent Set, Set Cover, and Dominating Set problems, among others. We also
prove corresponding hardness of approximation for some of these optimization
problems, providing a characterization of their intractability in terms of
density
Tidal Barrier and the Asymptotic Mass of Proto Gas-Giant Planets
Extrasolar planets found with radial velocity surveys have masses ranging
from several Earth to several Jupiter masses. While mass accretion onto
protoplanetary cores in weak-line T-Tauri disks may eventually be quenched by a
global depletion of gas, such a mechanism is unlikely to have stalled the
growth of some known planetary systems which contain relatively low-mass and
close-in planets along with more massive and longer period companions. Here, we
suggest a potential solution for this conundrum. In general, supersonic infall
of surrounding gas onto a protoplanet is only possible interior to both of its
Bondi and Roche radii. At a critical mass, a protoplanet's Bondi and Roche
radii are equal to the disk thickness. Above this mass, the protoplanets' tidal
perturbation induces the formation of a gap. Although the disk gas may continue
to diffuse into the gap, the azimuthal flux across the protoplanets' Roche lobe
is quenched. Using two different schemes, we present the results of numerical
simulations and analysis to show that the accretion rate increases rapidly with
the ratio of the protoplanet's Roche to Bondi radii or equivalently to the disk
thickness. In regions with low geometric aspect ratios, gas accretion is
quenched with relatively low protoplanetary masses. This effect is important
for determining the gas-giant planets' mass function, the distribution of their
masses within multiple planet systems around solar type stars, and for
suppressing the emergence of gas-giants around low mass stars
Absorption line systems in simulated galaxies fed by cold streams
Hydro cosmological simulations reveal that massive galaxies at high redshift
are fed by long narrow streams of merging galaxies and a smoother component of
cold gas. We post-process seven high-resolution simulated galaxies with
radiative transfer to study the absorption characteristics of the gas in
galaxies and streams, in comparison with the statistics of observed
absorption-line systems. We find that much of the stream gas is ionized by UV
radiation from background and local stellar sources, but still optically thick
(N_HI > 10^17 cm^-2) so that the streams appear as Lyman-limit systems (LLSs).
At z>3, the fraction of neutral gas in streams becomes non-negligible, giving
rise to damped Lyman-alpha (DLA) absorbers as well. The gas in the central and
incoming galaxies remains mostly neutral, responsible for DLAs. Within one
(two) virial radii, the covering factor of optically thick gas is <25% (10%)
for LLSs and <5% (1%) for DLAs, slowly declining with time following the
universal expansion. Nevertheless, galaxies and their cold streams in the
studied mass range, M_vir = 10^10 - 10^12 Msun, account for >30% of the
observed absorbers in the foreground of quasars, the rest possibly arising from
smaller galaxies or the intergalactic medium. The mean metallicity in the
streams is ~1% solar, much lower than in the galaxies. The simulated galaxies
reproduce the Lyalpha-absorption equivalent widths observed around Lyman-break
galaxies, but they severely underpredict the equivalent widths in metal lines,
suggesting that the latter may arise from outflows. We conclude that the
observed metal-poor LLSs are likely detections of the predicted cold streams.
Revised analysis of the observed LLSs kinematics and simulations with more
massive outflows in conjunction with the inflows may enable a clearer
distinction between the signatures of the various gas modes.Comment: 27 pages, 20 figures, 7 tables. MNRAS accepte
- âŠ