1,650 research outputs found
Hyperbolic surface subgroups of one-ended doubles of free groups
Gromov asked whether every one-ended word-hyperbolic group contains a
hyperbolic surface group. We prove that every one-ended double of a free group
has a hyperbolic surface subgroup if (1) the free group has rank two, or (2)
every generator is used the same number of times in the amalgamating words. To
prove this, we formulate a stronger statement on Whitehead graphs and prove its
specialization by combinatorial induction for (1) and the characterization of
perfect matching polytopes by Edmonds for (2).Comment: 30 pages, 8 figures. This version has been accepted for publication
by the Journal of Topolog
An analysis of the far-field response to external forcing of a suspension in Stokes flow in a parallel-wall channel
The leading-order far-field scattered flow produced by a particle in a
parallel-wall channel under creeping flow conditions has a form of the
parabolic velocity field driven by a 2D dipolar pressure distribution. We show
that in a system of hydrodynamically interacting particles, the pressure
dipoles contribute to the macroscopic suspension flow in a similar way as the
induced electric dipoles contribute to the electrostatic displacement field.
Using this result we derive macroscopic equations governing suspension
transport under the action of a lateral force, a lateral torque or a
macroscopic pressure gradient in the channel. The matrix of linear transport
coefficients in the constitutive relations linking the external forcing to the
particle and fluid fluxes satisfies the Onsager reciprocal relation. The
transport coefficients are evaluated for square and hexagonal periodic arrays
of fixed and freely suspended particles, and a simple approximation in a
Clausius-Mossotti form is proposed for the channel permeability coefficient. We
also find explicit expressions for evaluating the periodic Green's functions
for Stokes flow between two parallel walls.Comment: 23 pages, 12 figure
Energy-Efficient Multiprocessor Scheduling for Flow Time and Makespan
We consider energy-efficient scheduling on multiprocessors, where the speed
of each processor can be individually scaled, and a processor consumes power
when running at speed , for . A scheduling algorithm
needs to decide at any time both processor allocations and processor speeds for
a set of parallel jobs with time-varying parallelism. The objective is to
minimize the sum of the total energy consumption and certain performance
metric, which in this paper includes total flow time and makespan. For both
objectives, we present instantaneous parallelism clairvoyant (IP-clairvoyant)
algorithms that are aware of the instantaneous parallelism of the jobs at any
time but not their future characteristics, such as remaining parallelism and
work. For total flow time plus energy, we present an -competitive
algorithm, which significantly improves upon the best known non-clairvoyant
algorithm and is the first constant competitive result on multiprocessor speed
scaling for parallel jobs. In the case of makespan plus energy, which is
considered for the first time in the literature, we present an
-competitive algorithm, where is the total number of
processors. We show that this algorithm is asymptotically optimal by providing
a matching lower bound. In addition, we also study non-clairvoyant scheduling
for total flow time plus energy, and present an algorithm that achieves -competitive for jobs with arbitrary release time and
-competitive for jobs with identical release time. Finally,
we prove an lower bound on the competitive ratio of
any non-clairvoyant algorithm, matching the upper bound of our algorithm for
jobs with identical release time
Capillary interactions in Pickering emulsions
The effective capillary interaction potentials for small colloidal particles
trapped at the surface of liquid droplets are calculated analytically. Pair
potentials between capillary monopoles and dipoles, corresponding to particles
floating on a droplet with a fixed center of mass and subjected to external
forces and torques, respectively, exhibit a repulsion at large angular
separations and an attraction at smaller separations, with the latter
resembling the typical behavior for flat interfaces. This change of character
is not observed for quadrupoles, corresponding to free particles on a
mechanically isolated droplet. The analytical results for quadrupoles are
compared with the numerical minimization of the surface free energy of the
droplet in the presence of ellipsoidal particles.Comment: twocolumn, 8 pages, 3 figures, submitted to Phys. Rev.
Chandra Multi-wavelength Plane (ChaMPlane) Survey: Design and Initial Results
The Chandra Multiwavength Plane (ChaMPlane) Survey of the galactic plane
incorporates serendipitous sources from selected Chandra pointings in or near
the galactic plane (b 20 ksec; lack of bright diffuse or point
sources) to measure or constrain the luminosity function of low-luminosity
accretion sources in the Galaxy. The primary goal is to detect and identify
accreting white dwarfs (cataclysmic variables, with space density still
uncertain by a factor of >10-100), neutron stars and black holes (quiescent low
mass X-ray binaries) to constrain their space densities and thus origin and
evolution. Secondary objectives are to identify Be stars in high mass X-ray
binaries and constrain their space densities, and to survey the H-R diagram for
stellar coronal sources. A parallel optical imaging under the NOAO Long Term
Survey program provides deep optical images using the Mosaic imager on the CTIO
and KPNO 4-m telescopes. The 36arcmin X 36arcmin optical images (Halpha, R, V
and I) cover ~5X the area of each enclosed Chandra ACIS FOV, providing an
extended survey of emission line objects for comparison with Chandra.
Spectroscopic followup of optical counterparts is then conducted, thus far with
WIYN and Magellan. The X-ray preliminary results from both the Chandra and
optical surveys will be presented, including logN-logS vs. galactic position
(l,b) and optical idenifications.Comment: 4 pages, 7 figures (in 8 files), Astronomishe Nachrichten, in press
(Feb 2003). Proceedings of "X-ray Surveys, in the Light of New
Observatories", 4-6 September, Santander, Spain. Higher resolution figures
available at:
http://hea-www.harvard.edu/ChaMPlane/papers/champlane-santander.pd
A 2.15 Hour Orbital Period for the Low Mass X-Ray Binary XB 1832-330 in the Globular Cluster NGC 6652
We present a candidate orbital period for the low mass X-ray binary XB
1832-330 in the globular cluster NGC 6652 using a 6.5 hour Gemini South
observation of the optical counterpart of the system. Light curves in g' and r'
for two LMXBs in the cluster, sources A and B in previous literature, were
extracted and analyzed for periodicity using the ISIS image subtraction
package. A clear sinusoidal modulation is evident in both of A's curves, of
amplitude ~0.11 magnitudes in g' and ~0.065 magnitudes in r', while B's curves
exhibit rapid flickering, of amplitude ~1 magnitude in g' and ~0.5 magnitudes
in r'. A Lomb-Scargle test revealed a 2.15 hour periodic variation in the
magnitude of A with a false alarm probability less than 10^-11, and no
significant periodicity in the light curve for B. Though it is possible
saturated stars in the vicinity of our sources partially contaminated our
signal, the identification of A's binary period is nonetheless robust.Comment: 7 pages, 7 figures, ApJ in pres
RADCAP: a potential model tool for direct capture reactions
A computer program is presented aiming at the calculation of bound and
continuum states, reduced transition probabilities, phase-shifts,
photo-disintegration cross sections, radiative capture cross sections, and
astrophysical S-factors, for a two-body nuclear system. The code is based on a
potential model of a Woods-Saxon, a Gaussian, or a M3Y, type. It can be used to
calculate nuclear reaction rates in numerous astrophysical scenarios.Comment: 29 pages, 4 figures, accepted for publication in Computer Physics
Communication
Cerebral oximetry monitoring during aortic arch aneurysm replacement surgery in Jehovah's Witness patient -A case report-
Anesthetic management for aortic arch aneurysm (AAA) surgery employing deep hypothermic circulatory arrest in a Jehovah's Witness (JW) patient is a challenge to anesthesiologist due to its complexity of procedures and their refusal of allogeneic transfusion. Even in the strict application of intraoperative acute normovolemic hemodilution (ANH) and intraopertive cell salvage (ICS) technique, prompt timing of re-administration of salvaged blood is essential for successful operation without allogeneic transfusion or ischemic complication of major organs. Cerebral oximetery (rSO2) monitoring using near infrared spectroscopy is a useful modality for detecting cerebral ischemia during the AAA surgery requiring direct interruption of cerebral flow. The present case showed that rSO2 can be used as a trigger facilitating to find a better timing for the re-administration of salvaged blood acquired during the AAA surgery for JW patient
Hydrodynamic and Suspended Sediment Transport Controls on River Mouth Morphology
River mouths building into standing bodies of water have strikingly varied growth habits. This presents a compelling pattern formation problem that is also of great practical relevance for subsurface prediction and managing coastal wetlands. Here we present a generalized 2.5-dimensional potential vorticity (PV) theory that explains sedimentation patterns of a sediment-laden stationary jet by coupling an understanding of vorticity with suspended sediment concentration fields. We explore the physical meaning of this new sediment-PV definition, and its impact on outflow depositional patterns, by analyzing data from a shallow wall-bounded plane jet experiment and by discussing new theoretical insights. A key result is that lateral advection and diffusion of suspended sediment are directly proportional to jet vorticity, a feature that reveals the mechanistic process that forms elongated channels by focused levee deposition. The new PV theory constitutes a more generalized mathematical framework that expands the Rouse theory for the equilibrium of suspended sediment
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