13,300 research outputs found
Maximal antichains of minimum size
Let be a natural number, and let be a set . We study the problem to find the smallest possible size of a
maximal family of subsets of such that
contains only sets whose size is in , and for all
, i.e. is an antichain. We present a
general construction of such antichains for sets containing 2, but not 1.
If our construction asymptotically yields the smallest possible size
of such a family, up to an error. We conjecture our construction to be
asymptotically optimal also for , and we prove a weaker bound for
the case . Our asymptotic results are straightforward applications of
the graph removal lemma to an equivalent reformulation of the problem in
extremal graph theory which is interesting in its own right.Comment: fixed faulty argument in Section 2, added reference
Minimizing the regularity of maximal regular antichains of 2- and 3-sets
Let be a natural number. We study the problem to find the
smallest such that there is a family of 2-subsets and
3-subsets of with the following properties: (1)
is an antichain, i.e. no member of is a subset of
any other member of , (2) is maximal, i.e. for every
there is an with or , and (3) is -regular, i.e. every point
is contained in exactly members of . We prove lower
bounds on , and we describe constructions for regular maximal antichains
with small regularity.Comment: 7 pages, updated reference
Assessment of the environmental toxicity and carcinogenicity of tungsten-based shot.
The toxicity of elemental tungsten released from discharged shot was assessed against previous studies that established a 1% toxic threshold for soil organisms. Extremely heavy theoretical shot loadings of 69,000 shot/ha were used to generate estimated environmental concentrations (EEC) for two brands of tungsten-based shot containing 51% and 95% tungsten. The corresponding tungsten EEC values were 6.5–13.5 mg W/kg soil, far below the 1% toxic threshold. The same shot loading in water produced tungsten EEC values of 2.1–4.4 mg W/L, levels that are not toxic under experimental conditions. Pure tungsten has not been shown to exhibit carcinogenic properties when ingested or embedded in animal tissues, but nickel, with which it is often alloyed, has known carcinogenicity. Given the large number of waterfowl that carry shot embedded in their body, it is advisable to screen lead shot substitutes for their carcinogenic potential through intra-muscular implantation
Minimum Weight Flat Antichains of Subsets
Building on classical theorems of Sperner and Kruskal-Katona, we investigate
antichains in the Boolean lattice of all subsets of
, where is flat, meaning that it contains
sets of at most two consecutive sizes, say , where contains only -subsets,
while contains only -subsets. Moreover, we assume
consists of the first -subsets in squashed
(colexicographic) order, while consists of all -subsets
not contained in the subsets in . Given reals , we
say the weight of is
. We characterize the minimum
weight antichains for any given , and we do the
same when in addition is a maximal antichain. We can then derive
asymptotic results on both the minimum size and the minimum Lubell function
New stability results for Einstein scalar gravity
We consider asymptotically anti de Sitter gravity coupled to a scalar field
with mass slightly above the Breitenlohner-Freedman bound. This theory admits a
large class of consistent boundary conditions characterized by an arbitrary
function . An important open question is to determine which admit stable
ground states. It has previously been shown that the total energy is bounded
from below if is bounded from below and the bulk scalar potential
admits a suitable superpotential. We extend this result and show that the
energy remains bounded even in some cases where can become arbitrarily
negative. As one application, this leads to the possibility that in
gauge/gravity duality, one can add a double trace operator with negative
coefficient to the dual field theory and still have a stable vacuum
Video-Mediated Opportunities for Self-Directed Learning in Undergraduate Research Methodology Courses
Effect of Light-Cure Initiation Time on Polymerization Efficiency and Orthodontic Bond Strength with a Resin-Modified Glass-Ionomer
Objectives –  The polymerization and acid–base reactions in resin-modified glass-ionomers (RMGI) are thought to compete with and inhibit one another. To examine the effect of visible light-cure (VLC) delay on the polymerization efficiency and orthodontic bond strength of a dual-cured RMGI.
Setting and Sample Population –  The Orthodontics Graduate Program at Marquette University. An in vitro study utilizing 72 freshly extracted human bicuspid teeth.
Materials and Methods –  A RMGI light-cured immediately, 2.5, 5, or 10 min after mixing comprised the experimental groups. Isothermal and dynamic temperature scan differential scanning calorimetry (DSC) analysis of the RMGI was performed to determine extents of VLC polymerization and acid–base reaction exotherms. Human premolars (n = 18/group) were bonded with the RMGI. Shear bond strength and adhesive remnant index (ARI) scores were determined.
Results –  Differential scanning calorimetry results showed the 10-min-delay RMGI group experienced significantly (p \u3c 0.05) lower VLC polymerization compared with the other groups. Acid–base reaction exotherms were undetected in all groups except the 10-min delay group. No significant differences (p \u3e 0.05) were noted among the groups for mean shear bond strength. A chi-square test showed no significant difference (p = 0.428) in ARI scores between groups.
Conclusions –  Delay in light-curing may reduce polymerization efficiency and alter the structure of the RMGI, but orthodontic shear bond strength does not appear to be compromised
Astrophysics datamining in the classroom: Exploring real data with new software tools and robotic telescopes
Within the efforts to bring frontline interactive astrophysics and astronomy
to the classroom, the Hands on Universe (HOU) developed a set of exercises and
platform using real data obtained by some of the most advanced ground and space
observatories. The backbone of this endeavour is a new free software Web tool -
Such a Lovely Software for Astronomy based on Image J (Salsa J). It is
student-friendly and developed specifically for the HOU project and targets
middle and high schools. It allows students to display, analyze, and explore
professionally obtained astronomical images, while learning concepts on
gravitational dynamics, kinematics, nuclear fusion, electromagnetism. The
continuous evolving set of exercises and tutorials is being completed with real
(professionally obtained) data to download and detailed tutorials. The
flexibility of the Salsa J platform tool enables students and teachers to
extend the exercises with their own observations. The software developed for
the HOU program has been designed to be a multi-platform, multi-lingual
experience for image manipulation and analysis in the classroom. Its design
enables easy implementation of new facilities (extensions and plugins), minimal
in-situ maintenance and flexibility for exercise plugin. Here, we describe some
of the most advanced exercises about astrophysics in the classroom, addressing
particular examples on gravitational dynamics, concepts currently introduced in
most sciences curricula in middle and high schools.Comment: 10 pages, 12 images, submitted to the special theme issue Using
Astronomy and Space Science Research in Physics Courses of the American
Journal of Physic
Sizes of flat maximal antichains of subsets
This is the second of two papers investigating for which positive integers
there exists a maximal antichain of size in the Boolean lattice
(the power set of , ordered by inclusion). In the first
part, the sizes of maximal antichains have been characterized. Here we provide
an alternative construction with the benefit of showing that almost all sizes
of maximal antichains can be obtained using antichains containing only -sets
and -sets for some
Hydrogen bond rearrangements and the motion of charge defects in water viewed using multidimensional ultrafast infrared spectroscopy
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, February 2010."December 2009." Cataloged from PDF version of thesis. Vita.Includes bibliographical references.Compared with other molecular liquids, water is highly structured due to its ability to form up to four hydrogen bonds to its nearest neighbors, resulting in a tetrahedral network of molecules. However, this network is highly dynamic, exhibiting fluctuations and rearrangements that take place on femtosecond to picosecond time scales. The transport of excess protons and proton holes in water makes exclusive use of water's hydrogen bonding network. Compared to ions of similar size and charge density, both hydronium and hydroxide ions exhibit anomalously high diffusion rates due to the fact that water molecules that neighbor these ions can undergo a proton transfer reaction with the ion. This allows the structural diffusion of the ion to occur apart from the displacement of individual water molecules. In this thesis we adopt a joint experimental and theoretical approach to characterize the fluctuations of water's hydrogen bonding network and how these fluctuations act to drive the structural diffusion of the aqueous hydroxide ion. The experimental data that we present consists of a series of ultrafast nonlinear infrared spectroscopies, in particular two-dimensional infrared spectroscopy (2D IR), applied to the O-H stretching transition of a dilute solution of HOD in NaOD/D20. The frequency of the O-H stretch, (OH, is highly sensitive to the configuration of its hydrogen bonding partner. 2D IR spectroscopy allows us to measure rapid shifts in OH that reveal time scales for changes in the local environment of the HOD molecule.(cont.) The calculation of 2D IR spectra from molecular dynamics simulations then allows us to make a direct connection between the results of our experiments and the underlying dynamics of the system that drive both hydrogen bond exchange and the structural diffusion of the hydroxide ion. 2D IR spectra recorded for dilute HOD in D20 show a strong asymmetry, preferentially broadening in the frequency region indicative of strained or broken hydrogen bonds, indicating that these configurations are unstable and quickly return to a hydrogen bond. The time scale over which the 2D spectra broaden, ~60 fs, is similar to the librational period of water and suggests that molecules exchange hydrogen bonding partners though rapid, large amplitude rotations. Molecular dynamics simulations find that the transition state for hydrogen bond exchange resembles a bifurcated hydrogen bond. In roughly half of the examined exchange events, a second solvation shell water molecule inserts across the breaking hydrogen bond. This suggests that hydrogen bond rearrangements are tied to the restructuring of a water molecule's solvation shell. Upon the addition of NaOD to HOD/D20 solution, a large absorption continuum appears to the low frequency side of the O-H stretch due to the formation of strong hydrogen bonds between HOD molecules and OD ions. At early waiting times, 2D IR spectra show large, offdiagonal intensity in this frequency range that rapidly relaxes within ~110 fs.(cont.) Modeling using an empirical valence bond simulation (MS-EVB) model of aqueous NaOH suggests that as the 0-H stretching potential symmetrizes during proton transfer events, overtone transitions of the shared proton contribute strongly to 2D spectra. The rapid loss of offdiagonal intensity results from the spectral sweeping of these vibrational overtones as the solvent modulates the motion of the shared proton. The collective electric field of the solvent is found to be an appropriate reaction coordinate for the formation and modulation of shared proton states. Over picosecond waiting times, spectral features appear in the 2D IR spectra that are indicative of the exchange of population between OH~ ions and HOD molecules due to proton transfer. The construction of a spectral fitting model gives a lower bound of 3 ps for this exchange. Calculations of structural parameters following proton exchange using the MS-EVB simulation model suggest that the observed exchange process corresponds to the formation and breakage of hydrogen bonds donated by the HOD/OD~ pair formed as a result of the proton transfer. A full description of the structural diffusion of the hydroxide ion requires both a description of the local hydrogen bonding structure of the ion as well as the dielectric fluctuations of the surrounding solvent.by Sean T. Roberts.Ph.D
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