7,664 research outputs found
Axiomatic opportunities and obstacles for inferring a species tree from gene trees
The reconstruction of a central tendency `species tree' from a large number
of conflicting gene trees is a central problem in systematic biology. Moreover,
it becomes particularly problematic when taxon coverage is patchy, so that not
all taxa are present in every gene tree. Here, we list four apparently
desirable properties that a method for estimating a species tree from gene
trees could have (the strongest property states that building a species tree
from input gene trees and then pruning leaves gives a tree that is the same as,
or more resolved than, the tree obtained by first removing the taxa from the
input trees and then building the species tree). We show that while it is
technically possible to simultaneously satisfy these properties when taxon
coverage is complete, they cannot all be satisfied in the more general
supertree setting. In part two, we discuss a concordance-based consensus method
based on Baum's `plurality clusters', and an extension to concordance
supertrees.Comment: 19 pages, 2 figure
Manual measurement of retinal bifurcation features
This paper introduces a new computerized tool for
accurate manual measurement of features of retinal bifurcation
geometry, designed for use in investigating correlations between measurement features and clinical conditions. The tool uses user-placed rectangles to measure the vessel width, and lines placed along vessel center lines to measure the angles. An
analysis is presented of measurements taken from 435 bifurcations.
These are compared with theoretical predictions based on
optimality principles presented in the literature. The new tool shows better agreement with the theoretical predictions than a simpler manual method published in the literature, but there remains a significant discrepancy between current theory and measured geometry
Marine tethysuchian crocodyliform from the ?Aptian-Albian (Lower Cretaceous) of the Isle of Wight, UK
A marine tethysuchian crocodyliform from the Isle of Wight, most likely from the Upper Greensand Formation (upper Albian, Lower Cretaceous), is described. However, we cannot preclude it being from the Ferruginous Sands Formation (upper Aptian), or more remotely, the Sandrock Formation (upper Aptian-upper Albian). The specimen consists of the anterior region of the right dentary, from the tip of the dentary to the incomplete fourth alveolus. This specimen increases the known geological range of marine tethysuchians back into the late Lower Cretaceous. Although we refer it to Tethysuchia incertae sedis, there are seven anterior dentary characteristics that suggest a possible relationship with the Maastrichtian-Eocene clade Dyrosauridae. We also review ‘middle’ Cretaceous marine tethysuchians, including putative Cenomanian dyrosaurids. We conclude that there is insufficient evidence to be certain that any known Cenomanian specimen can be safely referred to Dyrosauridae, as there are some cranial similarities between basal dyrosaurids and Cenomanian–Turonian marine ‘pholidosaurids’. Future study of middle Cretaceous tethysuchians could help unlock the origins of Dyrosauridae and improve our understanding of tethysuchian macroevolutionary trends
Adiabatic optical entanglement between electron spins in separate quantum dots
We present an adiabatic approach to the design of entangling quantum
operations with two electron spins localized in separate InAs/GaAs quantum dots
via the Coulomb interaction between optically-excited localized states.
Slowly-varying optical pulses minimize the pulse noise and the relaxation of
the excited states. An analytic "dressed state" solution gives a clear physical
picture of the entangling process, and a numerical solution is used to
investigate the error dynamics. For two vertically-stacked quantum dots we show
that, for a broad range of dot parameters, a two-spin state with concurrence
can be obtained by four optical pulses with durations
ns.Comment: 7 pages, 5 figure
A Southern Hemisphere radar meteor orbit survey
A meteor radar system has been operated on a routine basis near Christchurch, New Zealand, to determine the orbits of Earth-impacting interplanetary dust and meteoroids. The system sensitivity is +13 visual magnitude, corresponding to approximately 100 micron sized meteoroids. With an orbital precision of 2 degrees in angular elements and 10 percent in orbital energy (1/a), the operation yields an average of 1500 orbits daily with a total to date in excess of 10(exp 5). The use of pc's and automated data reduction permit the large orbital data sets we collect to be routinely reduced. Some illustrative examples are presented of the signal formats/processing and the results of data reduction, giving the individual orbital elements and hence the overall distributions. Current studies include the distribution of dust in the inner solar system; the influx of meteoroids associated with near-Earth asteroids; and the orbital structure existing in comet-produced streams
In Defense of the Permanent Resident: Alleged Defects Relating to Alien Labor Certifications
This Article addresses the defense of permanent resident aliens whose status is being challenged due to alleged defects or ineligibility surrounding the original labor certification process. These issues include such matters as not reporting to the sponsoring employer, terminating employment with the sponsor shortly after immigrating, not having the claimed or required experience, not being paid the required wage, or not having a validly issued alien labor certification. The analysis pf these issues and the possible defenses available vary depending on the procedural context in which the issue was raised. The article discusses these various factual patterns and possible defenses available. The state of the law in the area is far from precise and has not been crystallized by the Board of immigration appeals. Thus, it appears that a good faith intention at the time of immigration is crucial. Lastly, the Article emphasizes the importance of practicing preventative law to prevent the issues developed in this article from arising
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