7,034 research outputs found
Bylaag 3 tot „Koer&’’, April en Junie, 1955: Die belang van toegepaste geologie en die werk van die geoloog in ons mineral afhanklike besawing
Die toekomstigc ontwikkeling, welvaart en sekuriteit van ons land en die handhawing en verbetering van ons huidige lewenstandaard hang tot’n groot mate af van die doeltreffendste gebruik van ons oorvloedige mineraalhulpbronne, maar beperkte watervoorrade
Afrika-Seminaar: Die geologie van Afrika in oorsig
Dit is vanselfsprekend dat in ’n artikel soos hierdie die geologie van Afrika slegs in hooftrekke aangedui kan word en dat daar geen sprake van besonderhede kan wees nie. Wat hierop volg kan dus eintlik maar net as’n selektiewe uittreksel uit die publikasies van sekere skrywers oor Afrika beskou word, en met die oog hierop is dit miskien goed om eers kortliks die vernaamste bronne van inligting aan te dui
A tight lower bound instance for k-means++ in constant dimension
The k-means++ seeding algorithm is one of the most popular algorithms that is
used for finding the initial centers when using the k-means heuristic. The
algorithm is a simple sampling procedure and can be described as follows: Pick
the first center randomly from the given points. For , pick a point to
be the center with probability proportional to the square of the
Euclidean distance of this point to the closest previously chosen
centers.
The k-means++ seeding algorithm is not only simple and fast but also gives an
approximation in expectation as shown by Arthur and Vassilvitskii.
There are datasets on which this seeding algorithm gives an approximation
factor of in expectation. However, it is not clear from these
results if the algorithm achieves good approximation factor with reasonably
high probability (say ). Brunsch and R\"{o}glin gave a dataset where
the k-means++ seeding algorithm achieves an approximation ratio
with probability that is exponentially small in . However, this and all
other known lower-bound examples are high dimensional. So, an open problem was
to understand the behavior of the algorithm on low dimensional datasets. In
this work, we give a simple two dimensional dataset on which the seeding
algorithm achieves an approximation ratio with probability
exponentially small in . This solves open problems posed by Mahajan et al.
and by Brunsch and R\"{o}glin.Comment: To appear in TAMC 2014. arXiv admin note: text overlap with
arXiv:1306.420
Failure to meet aerobic fitness standards among urban elementary students
The aim of this study was to explore the relationship of aerobic fitness with the elementary school environment and student characteristics among 4th and 5th grade children attending urban public schools in St. Louis, MO, USA. This cross-sectional study was conducted during 2012–2015 and included 2381 children (mean age 10.5 y) who completed the FITNESSGRAM® 20-m Progressive Aerobic Cardiovascular Endurance Run. Healthy Fitness Zone (HFZ) was defined according to FITNESSGRAM® aerobic capacity criteria. Other student-level variables included age, race, National School Lunch Program eligibility, BMI z-score, weight status, and daily pedometer steps. School environment variables included playground features and playground safety, physical education and recess practices, and school census tract data on vacant houses and median household income. Bivariate analyses with sex stratification were used to identify student-level and school-level predictors of failure to achieve the aerobic HFZ; predictors were then included in a multivariable logistic regression model. Failure to meet the aerobic HFZ was observed among 33% of boys and 57% of girls. School environment was not predictive, but higher age and fewer daily steps were: each additional year of age was associated with 41% higher odds of failing to meet the aerobic HFZ among boys and 100% higher odds among girls. Conversely, each additional 1000 daily steps was associated with 15% (boys) and 13% (girls) lower odds of failure. Obesity posed a 60% higher risk of failure to meet HFZ among girls. These results highlight the importance of childhood physical activity opportunities, especially for girls residing in low-resource areas. Keywords: Aerobic fitness, School, Environment, Student, Child, Urban, Low-resourc
An adaptive-binning method for generating constant-uncertainty/constant-significance light curves with Fermi-LAT data
We present a method enabling the creation of
constant-uncertainty/constant-significance light curves with the data of the
Fermi-Large Area Telescope (LAT). The adaptive-binning method enables more
information to be encapsulated within the light curve than with the
fixed-binning method. Although primarily developed for blazar studies, it can
be applied to any sources. This method allows the starting and ending times of
each interval to be calculated in a simple and quick way during a first step.
The reported mean flux and spectral index (assuming the spectrum is a power-law
distribution) in the interval are calculated via the standard LAT analysis
during a second step. The absence of major caveats associated with this method
has been established by means of Monte-Carlo simulations. We present the
performance of this method in determining duty cycles as well as power-density
spectra relative to the traditional fixed-binning method.Comment: 17 pages, 13 figures, 5 tables. Submitted to A&
Early Epithelial Invasion by Salmonella enterica Serovar Typhimurium DT104 in the Swine Ileum
Salmonella enterica serovar Typhimurium is an important intestinal pathogen in swine. This study was performed to document the early cellular invasion of Salmonellaserovar Typhimurium in swine ileum. Ileal gut-loops were surgically prepared in ten 4- to 5-week-old mixed-breed pigs and inoculated for 0-60 minutes. Loops were harvested and prepared for both scanning and transmission electron microscopy (SEM and TEM, respectively). Preferential bacterial adherence to microfold cells (M cells) was seen within 5 minutes, and by 10 minutes bacterial invasion of the apical membrane was seen in M cells, goblet cells, and enterocytes. This multicellular invasion was observed throughout the course of infection. In addition, SEM revealed a specific affinity of Salmonella serovar Typhimurium to sites of cell extrusion. Using TEM, bacteria in these areas were focused in the crevices formed by the extruding cell and the adjacent cells and in the cytoplasm immediately beneath the extruding cell. Our results suggest that early cellular invasion by Salmonella serovar Typhimurium is nonspecific and rapid in swine. Furthermore, the combination of SEM and TEM data suggests that Salmonella serovar Typhimurium may use sites of cell extrusion as an additional mechanism for early invasion
On Existence and Properties of Approximate Pure Nash Equilibria in Bandwidth Allocation Games
In \emph{bandwidth allocation games} (BAGs), the strategy of a player
consists of various demands on different resources. The player's utility is at
most the sum of these demands, provided they are fully satisfied. Every
resource has a limited capacity and if it is exceeded by the total demand, it
has to be split between the players. Since these games generally do not have
pure Nash equilibria, we consider approximate pure Nash equilibria, in which no
player can improve her utility by more than some fixed factor through
unilateral strategy changes. There is a threshold (where
is a parameter that limits the demand of each player on a specific
resource) such that -approximate pure Nash equilibria always exist for
, but not for . We give both
upper and lower bounds on this threshold and show that the
corresponding decision problem is -hard. We also show that the
-approximate price of anarchy for BAGs is . For a restricted
version of the game, where demands of players only differ slightly from each
other (e.g. symmetric games), we show that approximate Nash equilibria can be
reached (and thus also be computed) in polynomial time using the best-response
dynamic. Finally, we show that a broader class of utility-maximization games
(which includes BAGs) converges quickly towards states whose social welfare is
close to the optimum
Simple solutions of fireball hydrodynamics for self-similar elliptic flows
Simple, self-similar, elliptic solutions of non-relativistic fireball
hydrodynamics are presented, generalizing earlier results for spherically
symmetric fireballs with Hubble flows and homogeneous temperature profiles. The
transition from one dimensional to three dimensional expansions is investigated
in an efficient manner.Comment: 12 pages, 4 figures in 8 .eps files, references to recent data added,
accepted in Physics Letters
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