9,449 research outputs found
Finite Weil restriction of curves
Given number fields , smooth projective curves defined over
and defined over , and a non-constant -morphism ,we consider the curve defined over whose -rational points
parametrize the -rational points on whose images under are defined
over . Our construction provides a framework which includes as a special
case that used in Elliptic Curve Chabauty techniques and their higher genus
versions. The set can be infinite only when has genus at most 1;
we analyze completely the case when has genus 1.Comment: Comments are welcome
Descent via (3,3)-isogeny on Jacobians of genus 2 curves
We give parametrisation of curves C of genus 2 with a maximal isotropic
(ZZ/3)^2 in J[3], where J is the Jacobian variety of C, and develop the theory
required to perform descent via (3,3)-isogeny. We apply this to several
examples, where it can shown that non-reducible Jacobians have nontrivial
3-part of the Tate-Shafarevich group.Comment: 17 page
Slow work force growth: a challenge for the Midwest?
If today's tight labor market in the Midwest can be sustained, as now seems likely, the region's policymakers and businesses will face problems associated with labor-constrained growth rather than the underemployment of the recent past. An era of tight labor markets can be expected to add impetus to public policies that improve labor market efficiency, along with those that address perceived labor market imperfections.Labor market ; Labor supply ; Middle West ; Employment (Economic theory)
AGM-Like Paraconsistent Belief Change
Two systems of belief change based on paraconsistent logics are introduced in this article by means of AGM-like postulates. The first one, AGMp, is defined over any paraconsistent logic which extends classical logic such that the law of excluded middle holds w.r.t. the paraconsistent negation. The second one, AGMo , is specifically designed for paraconsistent logics known as Logics of Formal Inconsistency (LFIs), which have a formal consistency operator that allows to recover all the classical inferences. Besides the three usual operations over belief sets, namely expansion, contraction and revision (which is obtained from contraction by the Levi identity), the underlying paraconsistent logic allows us to define additional operations involving (non-explosive) contradictions. Thus, it is defined external revision (which is obtained from contraction by the reverse Levi identity), consolidation and semi-revision, all of them over belief sets. It is worth noting that the latter operations, introduced by S. Hansson, involve the temporary acceptance of contradictory beliefs, and so they were originally defined only for belief bases. Unlike to previous proposals in the literature, only defined for specific paraconsistent logics, the present approach can be applied to a general class of paraconsistent logics which are supraclassical, thus preserving the spirit of AGM. Moreover, representation theorems w.r.t. constructions based on selection functions are obtained for all the operations
Flux creep in Bi2Sr2Ca1Cu2O(8+x) single crystals
Dissipative effects were investigated in Bi2Sr2Ca1Cu2O(8+x) single crystals by critical current and magnetic relaxation measurements. Activation energies for the flux motion were determined from the temperature dependence of the critical current and from the time decay of the zero field cooled and the remanent magnetization. The effective activation energy was found to increase with temperature, in agreement with the existence of a distribution of activation energies (E sub o 20 meV at 4.2 K for H + 10 kOe applied parallel to the c-axis.)
Bright hot impacts by erupted fragments falling back on the Sun: UV redshifts in stellar accretion
A solar eruption after a flare on 7 Jun 2011 produced EUV-bright impacts of
fallbacks far from the eruption site, observed with the Solar Dynamics
Observatory. These impacts can be taken as a template for the impact of stellar
accretion flows. Broad red-shifted UV lines have been commonly observed in
young accreting stars. Here we study the emission from the impacts in the
Atmospheric Imaging Assembly's UV channels and compare the inferred velocity
distribution to stellar observations. We model the impacts with 2D hydrodynamic
simulations. We find that the localised UV 1600A emission and its timing with
respect to the EUV emission can be explained by the impact of a cloud of
fragments. The first impacts produce strong initial upflows. The following
fragments are hit and shocked by these upflows. The UV emission comes mostly
from the shocked front shell of the fragments while they are still falling, and
is therefore redshifted when observed from above. The EUV emission instead
continues from the hot surface layer that is fed by the impacts. Fragmented
accretion can therefore explain broad redshifted UV lines (e.g. C IV 1550A) to
speeds around 400 km/s observed in accreting young stellar objects.Comment: 12 pages, 4 figures (movies available upon request), accepted for
publicatio
The quenched generating functional for hadronic weak interactions
The ultraviolet behaviour of the generating functional for hadronic weak
interactions with is investigated to one loop for a generic
number of flavours and in the quenched approximation. New quenched chiral
logarithms generated by the weak interactions can be accounted for via a
redefinition of the weak mass term in the weak effective
Lagrangian at leading order. Finally, we illustrate how chiral logarithms are
modified by the quenched approximation in matrix elements with
and 3/2.Comment: LATTICE98(matrixelement), 3 page
X-ray Diagnostics of Grain Depletion in Matter Accreting onto T Tauri Stars
Recent analysis of high resolution Chandra X-ray spectra has shown that the
Ne/O abundance ratio is remarkably constant in stellar coronae. Based on this
result, we point out the utility of the Ne/O ratio as a discriminant for
accretion-related X-rays from T Tauri stars, and for probing the measure of
grain-depletion of the accreting material in the inner disk. We apply the Ne/O
diagnostic to the classical T Tauri stars BP Tau and TW Hya--the two stars
found to date whose X-ray emission appears to originate, at least in part, from
accretion activity. We show that TW Hya appears to be accreting material which
is significantly depleted in O relative to Ne. In constrast, BP Tau has an Ne/O
abundance ratio consistent with that observed for post-T Tauri stars. We
interpret this result in terms of the different ages and evolutionary states of
the circumstellar disks of these stars. In the young BP Tau disk (age 0.6 Myr)
dust is still present near the disk corotation radius and can be ionized and
accreted, re-releasing elements depleted onto grains. In the more evolved TW
Hya disk (age 10 Myr), evidence points to ongoing coagulation of grains into
much larger bodies, and possibly planets, that can resist the drag of
inward-migrating gas, and accreting gas is consequently depleted of
grain-forming elements.Comment: 13 pages, 1 Figure, ApJ Letters, in pres
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