837 research outputs found
A sharp vanishing theorem for line bundles on K3 or Enriques surfaces
Let be a line bundle on a K3 or Enriques surface. We give a vanishing
theorem for that, unlike most vanishing theorems, gives necessary and
sufficient geometrical conditions for the vanishing. This result is essential
in our study of Brill-Noether theory of curves on Enriques surfaces (reference
[KL1]) and of Enriques-Fano threefolds (reference [KLM]).Comment: 4 pages, latex. Minor corrections. To appear on Proc. Amer. Math. So
Negative curves on algebraic surfaces
We study curves of negative self-intersection on algebraic surfaces. We
obtain results for smooth complex projective surfaces X on the number of
reduced, irreducible curves C of negative self-intersection C^2. The only known
examples of surfaces for which C^2 is not bounded below are in positive
characteristic, and the general expectation is that no examples can arise over
the complex numbers. Indeed, we show that the idea underlying the examples in
positive characteristic cannot produce examples over the complex number field.
The previous version of this paper claimed to give a counterexample to the
Bounded Negativity Conjecture. The idea of the counterexample was to use Hecke
translates of a smooth Shimura curve in order to create an infinite sequence of
curves violating the Bounded Negativity Conjecture. To this end we applied
Hirzebruch Proportionality to all Hecke translates, simultaneously
desingularized by a version of Jaffee's Lemma which exists in the literature
but which turns out to be false. Indeed, in the new version of the paper, we
show that only finitely many Hecke translates of a special subvariety of a
Hilbert modular surface remain smooth. This new result is based on work done
jointly with Xavier Roulleau, who has been added as an author. The other
results in the original posting of this paper remain unchanged.Comment: 14 pages, X. Roulleau added as author, counterexample to Bounded
Negativity Conjecture withdrawn and replaced by a proof that there are only
finitely many smooth Shimura curves on a compact Hilbert modular surface; the
other results in the original posting of this paper remain unchange
Understanding and Finding Solutions to the Problem of Sedimentation in the National Wildlife Refuge System
The National Wildlife Refuge System (Refuge System) is a collection of public lands maintained by the U.S. Fish and Wildlife Service for migratory birds and other wildlife. Wetlands on individual National Wildlife Refuges (Refuges) may be at risk of increased sedimentation because of land use and water management practices. Increased sedimentation can reduce wetland habitat quality by altering hydrologic function, degrading water quality, and inhibiting growth of vegetation and invertebrates. On Refuges negatively affected by increased sedimentation, managers have to address complex questions about how to best remediate and mitigate the negative effects. The best way to account for these complexities is often not clear. On other Refuges, managers may not know whether sedimentation is a problem. Decision makers in the Refuge System may need to allocate resources to studying which Refuges could be at risk. Such analyses would help them understand where to direct support for managing increased sedimentation. In this paper, we summarize a case study demonstrating the use of decision-analytic tools in the development of a sedimentation management plan for Agassiz National Wildlife Refuge, Minnesota. Using what we learned from that process, we surveyed other Refuges in U.S. Fish and Wildlife Service Region 3 (an area encompassing the states of Illinois, Indiana, Iowa, Ohio, Michigan, Minnesota, Missouri, and Wisconsin) and Region 6 (an area encompassing the states of Colorado, Kansas, Montana, Nebraska, North Dakota, South Dakota, Utah, and Wyoming) about whether they experience sediment-related impacts to management. Our results show that cases of management being negatively affected by increased sedimentation are not isolated. We suggest that the Refuge System conduct a comprehensive and systematic assessment of increased sedimentation among Refuges to understand the importance of sedimentation in context with other management problems that Refuges face. The results of such an assessment could guide how the Refuge System allocates resources to studying and managing widespread stressors
Isotropy, shear, symmetry and exact solutions for relativistic fluid spheres
The symmetry method is used to derive solutions of Einstein's equations for
fluid spheres using an isotropic metric and a velocity four vector that is
non-comoving. Initially the Lie, classical approach is used to review and
provide a connecting framework for many comoving and so shear free solutions.
This provides the basis for the derivation of the classical point symmetries
for the more general and mathematicaly less tractable description of Einstein's
equations in the non-comoving frame. Although the range of symmetries is
restrictive, existing and new symmetry solutions with non-zero shear are
derived. The range is then extended using the non-classical direct symmetry
approach of Clarkson and Kruskal and so additional new solutions with non-zero
shear are also presented. The kinematics and pressure, energy density, mass
function of these solutions are determined.Comment: To appear in Classical and Quantum Gravit
Invariant construction of solutions to Einstein's field equations - LRS perfect fluids II
The properties of LRS class II perfect fluid space-times are analyzed using
the description of geometries in terms of the Riemann tensor and a finite
number of its covariant derivatives. In this manner it is straightforward to
obtain the plane and hyperbolic analogues to the spherical symmetric case. For
spherically symmetric static models the set of equations is reduced to the
Tolman-Oppenheimer-Volkoff equation only. Some new non-stationary and
inhomogeneous solutions with shear, expansion, and acceleration of the fluid
are presented. Among these are a class of temporally self-similar solutions
with equation of state given by , and a class of
solutions characterized by . We give an example of geometry
where the Riemann tensor and the Ricci rotation coefficients are not sufficient
to give a complete description of the geometry. Using an extension of the
method, we find the full metric in terms of curvature quantities.Comment: 24 pages, 1 figur
Mycotoxin exposure and human cancer risk : a systematic review of epidemiological studies
In recent years, there has been an increasing interest in investigating the carcinogenicity of mycotoxins in humans. This systematic review aims to provide an overview of data linking exposure to different mycotoxins with human cancer risk. Publications (2019 and earlier) of caseâcontrol or longitudinal cohort studies were identified in PubMed and EMBASE. These articles were then screened by independent reviewers and their quality was assessed according to the NewcastleâOttawa scale. Animal, crossâsectional, and molecular studies satisfied criteria for exclusion. In total, 14 articles were included: 13 caseâcontrol studies and 1 longitudinal cohort study. Included articles focused on associations of mycotoxin exposure with primary liver, breast, and cervical cancer. Overall, a positive association between the consumption of aflatoxinâcontaminated foods and primary liver cancer risk was verified. Two caseâcontrol studies in Africa investigated the relationship between zearalenone and its metabolites and breast cancer risk, though conflicting results were reported. Two caseâcontrol studies investigated the association between hepatocellular carcinoma and fumonisin B1 exposure, but no significant associations were observed. This systematic review incorporates several clear observations of doseâdependent associations between aflatoxins and liver cancer risk, in keeping with IARC Monograph conclusions. Only few human epidemiological studies investigated the associations between mycotoxin exposures and cancer risk. To close this gap, more inâdepth research is needed to unravel evidence for other common mycotoxins, such as deoxynivalenol and ochratoxin A. The link between mycotoxin exposures and cancer risk has mainly been established in experimental studies, and needs to be confirmed in human epidemiological studies to support the evidenceâbased public health strategies
A few questions about curves on surfaces
In this note we address the following kind of question: let X be a smooth, irreducible, projective surface and D a divisor on X satisfying some sort of positivity hypothesis, then is there some multiple of D depending only on X which is effective or movable? We describe some examples, discuss some conjectures and prove some results that suggest that the answer should in general be negative, unless one puts some really strong hypotheses either on D or on X
NâSubstituted Nipecotic Acids as (S )âSNAPâ5114 Analogues with Modified Lipophilic Domains
Potential mGAT4 inhibitors derived from the lead substance (S )âSNAPâ5114 have been synthesized and characterized for their inhibitory potency. Variations from the parent compound included the substitution of one of its aromatic 4âmethoxy and 4âmethoxyphenyl groups, respectively, with a more polar moiety, including a carboxylic acid, alcohol, nitrile, carboxamide, sulfonamide, aldehyde or ketone function, or amino acid partial structures. Furthermore, it was investigated how the substitution of more than one of the aromatic 4âmethoxy groups affects the potency and selectivity of the resulting compounds. Among the synthesized test substances (S )â1â{2â[(4âformylphenyl)bis(4âmethoxyphenyl)âmethoxy]ethyl}piperidineâ3âcarboxylic acid, that features a carbaldehyde function in place of one of the aromatic 4âmethoxy moieties of (S )âSNAPâ5114, was found to have a pIC50 value of 5.89±0.07, hence constituting a slightly more potent mGAT4 inhibitor than the parent substance while showing comparable subtype selectivity
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