403 research outputs found
Quicksort, Largest Bucket, and Min-Wise Hashing with Limited Independence
Randomized algorithms and data structures are often analyzed under the
assumption of access to a perfect source of randomness. The most fundamental
metric used to measure how "random" a hash function or a random number
generator is, is its independence: a sequence of random variables is said to be
-independent if every variable is uniform and every size subset is
independent. In this paper we consider three classic algorithms under limited
independence. We provide new bounds for randomized quicksort, min-wise hashing
and largest bucket size under limited independence. Our results can be
summarized as follows.
-Randomized quicksort. When pivot elements are computed using a
-independent hash function, Karloff and Raghavan, J.ACM'93 showed expected worst-case running time for a special version of quicksort.
We improve upon this, showing that the same running time is achieved with only
-independence.
-Min-wise hashing. For a set , consider the probability of a particular
element being mapped to the smallest hash value. It is known that
-independence implies the optimal probability . Broder et al.,
STOC'98 showed that -independence implies it is . We show
a matching lower bound as well as new tight bounds for - and -independent
hash functions.
-Largest bucket. We consider the case where balls are distributed to
buckets using a -independent hash function and analyze the largest bucket
size. Alon et. al, STOC'97 showed that there exists a -independent hash
function implying a bucket of size . We generalize the
bound, providing a -independent family of functions that imply size .Comment: Submitted to ICALP 201
Hamiltonian of Tensionless Strings with Tensor Central Charge Coordinates
A new class of twistor-like string models in four-dimensional space-time
extended by the addition of six tensorial central charge (TCC) coordinates
is studied. The Hamiltonian of tensionless string in the extended
space-time is derived and its symmetries are investigated. We establish that
the string constraints reduce the number of independent TCC coordinates
to one real effective coordinate which composes an effective
5-dimensional target space together with the coordinates. We construct
the P.B. algebra of the first class constraints and discover that it coincides
with the P.B. algebra of tensionless strings. The Lorentz covariant
antisymmetric Dirac -matrix of the P.B. of the second class
constraints is constructed and its algebraic structure is further presented.Comment: 18 pages, Latex, no figure
Dynamical Systems approach to Saffman-Taylor fingering. A Dynamical Solvability Scenario
A dynamical systems approach to competition of Saffman-Taylor fingers in a
channel is developed. This is based on the global study of the phase space
structure of the low-dimensional ODE's defined by the classes of exact
solutions of the problem without surface tension. Some simple examples are
studied in detail, and general proofs concerning properties of fixed points and
existence of finite-time singularities for broad classes of solutions are
given. The existence of a continuum of multifinger fixed points and its
dynamical implications are discussed. The main conclusion is that exact
zero-surface tension solutions taken in a global sense as families of
trajectories in phase space spanning a sufficiently large set of initial
conditions, are unphysical because the multifinger fixed points are
nonhyperbolic, and an unfolding of them does not exist within the same class of
solutions. Hyperbolicity (saddle-point structure) of the multifinger fixed
points is argued to be essential to the physically correct qualitative
description of finger competition. The restoring of hyperbolicity by surface
tension is discussed as the key point for a generic Dynamical Solvability
Scenario which is proposed for a general context of interfacial pattern
selection.Comment: 3 figures added, major rewriting of some sections, submitted to Phys.
Rev.
A phase-field model of Hele-Shaw flows in the high viscosity contrast regime
A one-sided phase-field model is proposed to study the dynamics of unstable
interfaces of Hele-Shaw flows in the high viscosity contrast regime. The
corresponding macroscopic equations are obtained by means of an asymptotic
expansion from the phase-field model. Numerical integrations of the phase-field
model in a rectangular Hele-Shaw cell reproduce finger competition with the
final evolution to a steady state finger the width of which goes to one half of
the channel width as the velocity increases
Vertex Operators for Closed Superstrings
We construct an iterative procedure to compute the vertex operators of the
closed superstring in the covariant formalism given a solution of IIA/IIB
supergravity. The manifest supersymmetry allows us to construct vertex
operators for any generic background in presence of Ramond-Ramond (RR) fields.
We extend the procedure to all massive states of open and closed superstrings
and we identify two new nilpotent charges which are used to impose the gauge
fixing on the physical states. We solve iteratively the equations of the vertex
for linear x-dependent RR field strengths. This vertex plays a role in studying
non-constant C-deformations of superspace. Finally, we construct an action for
the free massless sector of closed strings, and we propose a form for the
kinetic term for closed string field theory in the pure spinor formalism.Comment: TeX, harvmac, amssym.tex, 41 pp; references adde
Comparison of immune response to lipopolysaccharide of rabbit does selected for litter size at weaning or founded for reproductive longevity
To evaluate differences in maternal lines to the immune response of reproductive rabbit does, a total of 64 animals of two different lines: (1) founded for hyper-longevity and litter size criteria (LP) and (2) selected for litter size at weaning (V) were used. Females were subjected to three different reproductive efforts: post-partum (PP) mating at first lactation and 9 kits during the second; post-weaning (PW) mating at first lactation and 9 kits during the second; and PW mating at first lactation and 5 kits during the second. At second weaning (30 days PP), an acute response was induced by intravenous infusion of lipopolysaccharide (LPS). LP females seemed to be lower affected during the hyper-acute phase than V females, showing lower plasma glucose content at 1.5 h post infusion (pi) and rectal temperature at 6 h pi; and showed higher ulterior immune response, with higher levels of C-reactive protein at 48 h pi and haptoglobin in plasma from 24 h pi. Survival test conferred a higher risk of culling for V than for LP females during the first hours after challenge. These results may suggest that, regarding immune response to LPS challenge, foundation by hyper-longevity productive criteria lead to obtain a more robust population of rabbit does, characterized by improved response ability. (C) 2013 Elsevier Ltd. All rights reserved.This study has been supported by the Interministerial Commission for Science and Technology (CICYT) from the Spanish Government. Grants Number: AGL2011-30170-C02-01; AGL2011-30170-C02-02), is gratefully acknowledged.Ferrian, S.; Blas Ferrer, E.; Larsen, T.; Sánchez Serrano, JP.; Friggens, NC.; Corpa, JM.; Baselga Izquierdo, M.... (2013). Comparison of immune response to lipopolysaccharide of rabbit does selected for litter size at weaning or founded for reproductive longevity. Research in Veterinary Science. 94(3):518-525. doi:10.1016/j.rvsc.2013.01.008S51852594
Cost calculation and prediction in adult intensive care: A ground-up utilization study
Publisher's copy made available with the permission of the publisherThe ability of various proxy cost measures, including therapeutic activity scores (TISS and Omega) and cumulative daily severity of illness scores, to predict individual ICU patient costs was assessed in a prospective “ground-up” utilization costing study over a six month period in 1991. Daily activity (TISS and Omega scores) and utilization in consecutive admissions to three adult university associated ICUs was recorded by dedicated data collectors. Cost prediction used linear regression with determination (80%) and validation (20%) data sets. The cohort, 1333 patients, had a mean (SD) age 57.5 (19.4) years, (41% female) and admission APACHE III score of 58 (27). ICU length of stay and mortality were 3.9 (6.1) days and 17.6% respectively. Mean total TISS and Omega scores were 117 (157) and 72 (113) respectively. Mean patient costs per ICU episode (1991 6801 (2534, range 95,602. Dominant cost fractions were nursing 43.3% and overheads 16.9%. Inflation adjusted year 2002 (mean) costs were AUS). Total costs in survivors were predicted by Omega score, summed APACHE III score and ICU length of stay; determination R2, 0.91; validation 0.88. Omega was the preferred activity score. Without the Omega score, predictors were age, summed APACHE III score and ICU length of stay; determination R2, 0.73; validation 0.73. In non-survivors, predictors were age and ICU length of stay (plus interaction), and Omega score (determination R2, 0.97; validation 0.91). Patient costs may be predicted by a combination of ICU activity indices and severity scores.J. L. Moran, A. R. Peisach, P. J. Solomon, J. Martinhttp://www.aaic.net.au/Article.asp?D=200403
Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy
We review HB stars in a broad astrophysical context, including both variable
and non-variable stars. A reassessment of the Oosterhoff dichotomy is
presented, which provides unprecedented detail regarding its origin and
systematics. We show that the Oosterhoff dichotomy and the distribution of
globular clusters (GCs) in the HB morphology-metallicity plane both exclude,
with high statistical significance, the possibility that the Galactic halo may
have formed from the accretion of dwarf galaxies resembling present-day Milky
Way satellites such as Fornax, Sagittarius, and the LMC. A rediscussion of the
second-parameter problem is presented. A technique is proposed to estimate the
HB types of extragalactic GCs on the basis of integrated far-UV photometry. The
relationship between the absolute V magnitude of the HB at the RR Lyrae level
and metallicity, as obtained on the basis of trigonometric parallax
measurements for the star RR Lyrae, is also revisited, giving a distance
modulus to the LMC of (m-M)_0 = 18.44+/-0.11. RR Lyrae period change rates are
studied. Finally, the conductive opacities used in evolutionary calculations of
low-mass stars are investigated. [ABRIDGED]Comment: 56 pages, 22 figures. Invited review, to appear in Astrophysics and
Space Scienc
Glucose utilization via glycogen phosphorylase sustains proliferation and prevents premature senescence in cancer cells.
Metabolic reprogramming of cancer cells provides energy and multiple intermediates critical for cell growth. Hypoxia in tumors represents a hostile environment that can encourage these transformations. We report that glycogen metabolism is upregulated in tumors in vivo and in cancer cells in vitro in response to hypoxia. In vitro, hypoxia induced an early accumulation of glycogen, followed by a gradual decline. Concordantly, glycogen synthase (GYS1) showed a rapid induction, followed by a later increase of glycogen phosphorylase (PYGL). PYGL depletion and the consequent glycogen accumulation led to increased reactive oxygen species (ROS) levels that contributed to a p53-dependent induction of senescence and markedly impaired tumorigenesis in vivo. Metabolic analyses indicated that glycogen degradation by PYGL is important for the optimal function of the pentose phosphate pathway. Thus, glycogen metabolism is a key pathway induced by hypoxia, necessary for optimal glucose utilization, which represents a targetable mechanism of metabolic adaptation
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