431 research outputs found
The Noble Mission
Saving the diversity of life on Eart
On the independence ratio of distance graphs
A distance graph is an undirected graph on the integers where two integers
are adjacent if their difference is in a prescribed distance set. The
independence ratio of a distance graph is the maximum density of an
independent set in . Lih, Liu, and Zhu [Star extremal circulant graphs, SIAM
J. Discrete Math. 12 (1999) 491--499] showed that the independence ratio is
equal to the inverse of the fractional chromatic number, thus relating the
concept to the well studied question of finding the chromatic number of
distance graphs.
We prove that the independence ratio of a distance graph is achieved by a
periodic set, and we present a framework for discharging arguments to
demonstrate upper bounds on the independence ratio. With these tools, we
determine the exact independence ratio for several infinite families of
distance sets of size three, determine asymptotic values for others, and
present several conjectures.Comment: 39 pages, 12 figures, 6 table
Theoretical study of turbulent channel flow: Bulk properties, pressure fluctuations, and propagation of electromagnetic waves
In this paper, we apply two theoretical turbulence models, DIA and the recent GISS model, to study properties of a turbulent channel flow. Both models provide a turbulent kinetic energy spectral function E(k) as the solution of a non-linear equation; the two models employ the same source function but different closures. The source function is characterized by a rate n sub s (k) which is derived from the complex eigenvalues of the Orr--Sommerfeld (OS) equation in which the basic flow is taken to be of a Poiseuille type. The O--S equation is solved for a variety of Reynolds numbers corresponding to available experimental data. A physical argument is presented whereby the central line velocity characterizing the basic flow, U0 sup L, is not to be identified with the U0 appearing in the experimental Reynolds number. The theoretical results are compared with two types of experimental data: (1) turbulence bulk properties, and (2) properties that depend stongly on the structure of the turbulence spectrun at low wave numbers. The only existing analytical expression for Pi (k) cannot be used in the present case because it applies to the case of a flat plate, not a finite channel
Supermassive black hole mass in the massive elliptical galaxy M87 from integral-field stellar dynamics using OASIS and MUSE with adaptive optics: assessing systematic uncertainties
The massive elliptical galaxy M87 has been the subject of several
supermassive black hole mass measurements from stellar dynamics, gas dynamics,
and recently the black hole shadow by the Event Horizon Telescope (EHT). This
uniquely positions M87 as a benchmark for alternative black hole mass
determination methods. Here we use stellar kinematics extracted from
integral-field spectroscopy observations with Adaptive Optics (AO) using MUSE
and OASIS. We exploit our high-resolution integral field spectroscopy to
spectrally decompose the central AGN from the stars. We derive an accurate
inner stellar-density profile and find it is flatter than previously assumed.
We also use the spectrally-extracted AGN as a reference to accurately determine
the observed MUSE and OASIS AO PSF. We then perform Jeans Anisotropic Modelling
(JAM), with a new flexible spatially-variable anisotropy, and measure the
anisotropy profile, stellar mass-to-light variations, inner dark matter
fraction, and black hole mass. Our preferred black hole mass is . However, using the inner stellar
density from previous studies, we find a preferred black hole mass of , consistent with previous
work. We conduct numerous systematic tests of the kinematics and model
assumptions and conclude that uncertainties in the black hole mass of M87 from
previous determinations may have been underestimated and further analyses are
needed.Comment: 19 pages, 16 figures, 4 tables, Submitted to MNRA
Robustness of "cut and splice" genetic algorithms in the structural optimization of atomic clusters
We return to the geometry optimization problem of Lennard-Jones clusters to
analyze the performance dependence of "cut and splice" genetic algorithms (GAs)
on the employed population size. We generally find that admixing twinning
mutation moves leads to an improved robustness of the algorithm efficiency with
respect to this a priori unknown technical parameter. The resulting very stable
performance of the corresponding mutation+mating GA implementation over a wide
range of population sizes is an important feature when addressing unknown
systems with computationally involved first-principles based GA sampling.Comment: 5 pages including 3 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
TB163: Insect Predation of Seeds and Plant Population Dynamics
This review provides a framework for understanding the mechanisms of insect seed predation, the diversity of insects that prey on seeds, and the ecological and evolutionary consequences of insect seed predation. Insect seed predation can play significant roles in reducing plant population growth, modifying intraspecific and interspecific competition, shifting spatial and temporal distribution, affecting species evolution, and plant community structure, both in natural and agricultural ecosystems.https://digitalcommons.library.umaine.edu/aes_techbulletin/1039/thumbnail.jp
Spin Transport in a Mott Insulator of Ultracold Fermions
Strongly correlated materials are expected to feature unconventional
transport properties, such that charge, spin, and heat conduction are
potentially independent probes of the dynamics. In contrast to charge
transport, the measurement of spin transport in such materials is highly
challenging. We observed spin conduction and diffusion in a system of ultracold
fermionic atoms that realizes the half-filled Fermi-Hubbard model. For strong
interactions, spin diffusion is driven by super-exchange and
doublon-hole-assisted tunneling, and strongly violates the quantum limit of
charge diffusion. The technique developed in this work can be extended to
finite doping, which can shed light on the complex interplay between spin and
charge in the Hubbard model.Comment: 16 pages, 10 figure
Role of the (Mn)superoxide dismutase of Enterococcus faecalis in the in vitro interaction with microglia
Enterococcus faecalis is a significant human pathogen worldwide and is responsible for severenosocomial and community-acquired infections. Although enterococcal meningitis is rare,mortality is considerable, reaching 21 %. Nevertheless, the pathogenetic mechanisms of thisinfection remain poorly understood, even though the ability of E. faecalis to avoid or survivephagocytic attack in vivo may be very important during the infection process. We previouslyshowed that the manganese-cofactored superoxide dismutase (MnSOD) SodA of E. faecalis wasimplicated in oxidative stress responses and, interestingly, in the survival within mouse peritonealmacrophages using an in vivo\u2013in vitro infection model. In the present study, we investigated therole of MnSOD in the interaction of E. faecalis with microglia, the brain-resident macrophages. Byusing an in vitro infection model, murine microglial cells were challenged in parallel with the wildtypestrain JH2-2 and its isogenic sodA deletion mutant. While both strains were phagocytosedby microglia efficiently and to a similar extent, the DsodA mutant was found to be significantlymore susceptible to microglial killing than JH2-2, as assessed by the antimicrobial protectionassay. In addition, a significantly higher percentage of acidic DsodA-containing phagosomes wasfound and these also underwent enhanced maturation as determined by the expression ofendolysosomal markers. In conclusion, these results show that the MnSOD of E. faecaliscontributes to survival of the bacterium in microglial cells by influencing their antimicrobial activity,and this could even be important for intracellular killing in neutrophils and thus for E. faecalispathogenesis
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