930 research outputs found
Induction in a von Karman flow driven by ferromagnetic impellers
We study magnetohydrodynamics in a von K\'arm\'an flow driven by the rotation
of impellers made of material with varying electrical conductivity and magnetic
permeability. Gallium is the working fluid and magnetic Reynolds numbers of
order unity are achieved. We find that specific induction effects arise when
the impeller's electric and magnetic characteristics differ from that of the
fluid. Implications in regards to the VKS dynamo are discussed.Comment: 14 pages, 7 figure
Analytic results and weighted Monte Carlo simulations for CDO pricing
We explore the possibilities of importance sampling in the Monte Carlo
pricing of a structured credit derivative referred to as Collateralized Debt
Obligation (CDO). Modeling a CDO contract is challenging, since it depends on a
pool of (typically about 100) assets, Monte Carlo simulations are often the
only feasible approach to pricing. Variance reduction techniques are therefore
of great importance. This paper presents an exact analytic solution using
Laplace-transform and MC importance sampling results for an easily tractable
intensity-based model of the CDO, namely the compound Poissonian. Furthermore
analytic formulae are derived for the reweighting efficiency. The computational
gain is appealing, nevertheless, even in this basic scheme, a phase transition
can be found, rendering some parameter regimes out of reach. A
model-independent transform approach is also presented for CDO pricing.Comment: 12 pages, 9 figure
A simple mean field model for social interactions: dynamics, fluctuations, criticality
We study the dynamics of a spin-flip model with a mean field interaction. The
system is non reversible, spacially inhomogeneous, and it is designed to model
social interactions. We obtain the limiting behavior of the empirical averages
in the limit of infinitely many interacting individuals, and show that phase
transition occurs. Then, after having obtained the dynamics of normal
fluctuations around this limit, we analize long time fluctuations for critical
values of the parameters. We show that random inhomogeneities produce critical
fluctuations at a shorter time scale compared to the homogeneous system.Comment: 37 pages, 2 figure
Terminus-driven retreat of a major southwest Greenland tidewater glacier during the early 19th century : insights from glacier reconstructions and numerical modelling
Peer reviewedPublisher PD
Counter-selectable marker for bacterial-based interaction trap systems
Counter-selectable markers can be used in two-hybrid systems to search libraries for a protein or compound that interferes with a macromolecular interaction or to identify macromolecules from a population that cannot mediate a particular interaction. In this report, we describe the adaptation of the yeast URA3/5-FOA counter-selection system for use in bacterial interaction trap experiments. Two different URA3 reporter systems were developed that allow robust counter-selection: (i) a single copy F\u27 episome reporter and (ii) a co-cistronic HIS3-URA3 reporter vector. The HIS3-URA3 reporter can be used for either positive or negative selections in appropriate bacterial strains. These reagents extend the utility of the bacterial two-hybrid system as an alternative to its yeast-based counterpart
New Drosophila circadian clock mutants affecting temperature compensation induced by targeted mutagenesis of Timeless
Drosophila melanogaster has served as an excellent genetic model to decipher the
molecular basis of the circadian clock. Two key proteins, PERIOD (PER) and TIMELESS
(TIM), are particularly well explored and a number of various arrhythmic, slow, and fast
clock mutants have been identified in classical genetic screens. Interestingly, the free
running period (tau, t) is influenced by temperature in some of these mutants, whereas
t is temperature-independent in other mutant lines as in wild-type flies. This, so-called
\u201ctemperature compensation\u201d ability is compromised in the mutant timeless allele \u201critsu\u201d
(tim rit), and, as we show here, also in the tim blind allele, mapping to the same region of
TIM. To test if this region of TIM is indeed important for temperature compensation, we
generated a collection of new mutants and mapped functional protein domains involved
in the regulation of t and in general clock function. We developed a protocol for targeted
mutagenesis of specific gene regions utilizing the CRISPR/Cas9 technology, followed by
behavioral screening. In this pilot study, we identified 20 new timeless mutant alleles with
various impairments of temperature compensation. Molecular characterization revealed
that the mutations included short in-frame insertions, deletions, or substitutions of a
few amino acids resulting from the non-homologous end joining repair process. Our
protocol is a fast and cost-efficient systematic approach for functional analysis of
protein-coding genes and promoter analysis in vivo. Interestingly, several mutations with
a strong temperatur
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