1,043 research outputs found
Classical Kloosterman sums: representation theory, magic squares, and Ramanujan multigraphs
We study the representation theory of a certain finite group for which
Kloosterman sums appear as character values. This leads us to consider a
concrete family of commuting hermitian matrices which have Kloosterman sums as
eigenvalues. These matrices satisfy a number of "magical" combinatorial
properties and they encode various arithmetic properties of Kloosterman sums.
These matrices can also be regarded as adjacency matrices for multigraphs which
display Ramanujan-like behavior.Comment: 20 page
Fusarium Mycotoxins and Metabolites that Modulate Their Production
The genus Fusarium is a group of fungi producing several types of toxins with toxicological effect in both humans and animals. Such fungi are commonly found in soils so it can contaminate various types of crops, preferably cereals, leading to significant economic losses. Relative humidity, storage temperature and various handling in cereales increase the possibility of contamination by Fusarium toxins. Cereals naturally have secondary metabolites that may help attenuate contamination by these toxins, but it is necessary to know strategies and mechanisms that generate inactivation mycotoxins. This chapter reviews relevant information about cereal mycotoxin contamination, as well as the production of cereal secondary metabolites as a strategy to reduce the possibility of mycotoxin contamination
Supercharacters, Exponential Sums, and the Uncertainty Principle
The theory of supercharacters, which generalizes classical character theory, was recently introduced by P. Diaconis and I.M. Isaacs, building upon earlier work of C. Andre. We study supercharacter theories on induced by the actions of certain matrix groups, demonstrating that a variety of exponential sums of interest in number theory (e.g., Gauss, Ramanujan, and Kloosterman sums) arise in this manner. We develop a generalization of the discrete Fourier transform, in which supercharacters play the role of the Fourier exponential basis. We provide a corresponding uncertainty principle and compute the associated constants in several cases
Oxygen abundances in unevolved metal-poor stars from near-UV OH lines
We have performed a detailed oxygen abundance analysis of 23 metal-poor
(-3.0<[Fe/H]<-0.3) unevolved halo stars and one giant through the OH bands in
the near UV, using high-resolution echelle spectra. Oxygen is found to be
overabundant with respect to iron in these stars, with the [O/Fe] ratio
increasing from 0.6 to 1 between [Fe/H]=-1.5 and -3.0. The behavior of the
oxygen overabundance with respect to [Fe/H] is similar to that seen in previous
works based on OI IR triplet data (Abia and Rebolo 1989; Tomkin et al. 1992;
Cavallo, Pilachowski, and Rebolo 1997). Contrary to the previously accepted
picture, our oxygen abundances, derived from low-excitation OH lines, agree
well with those derived from high-excitation lines of the triplet. For nine
stars in common with Tomkin et al. we obtain a mean difference of 0.00+/-0.11
dex with respect to the abundances determined from the triplet using the same
stellar parameters and model photospheres. For four stars in our sample we have
found measurements of the [OI] 6300 A line in the literature, from which we
derive oxygen abundances consistent (average difference 0.09 dex) with those
based on OH lines, showing that the long standing controversy between oxygen
abundances from forbidden and permitted lines in metal-poor unevolved stars can
be resolved. Our new oxygen abundances show a smooth extension of the
Edvardsson et al.'s (1993) [O/Fe] versus metallicity curve to much lower
abundances, with a slope -0.31+/- 0.11 (taking into account the error bars in
both oxygen abundances and metallicities) in the range -3<[Fe/H]<-1.Comment: 17 pages, 8 figures, 3 table
Spectroscopic Observations of Convective Patterns in the Atmospheres of Metal-Poor Stars
Convective line asymmetries in the optical spectrum of two metal-poor stars,
Gmb1830 and HD140283, are compared to those observed for solar metallicity
stars. The line bisectors of the most metal-poor star, the subgiant HD140283,
show a significantly larger velocity span that the expectations for a
solar-metallicity star of the same spectral type and luminosity class. The
enhanced line asymmetries are interpreted as the signature of the lower metal
content, and therefore opacity, in the convective photospheric patterns. These
findings point out the importance of three-dimensional convective velocity
fields in the interpretation of the observed line asymmetries in metal-poor
stars, and in particular, urge for caution when deriving isotopic ratios from
observed line shapes and shifts using one-dimensional model atmospheres.
The mean line bisector of the photospheric atomic lines is compared with
those measured for the strong Mg I b1 and b2 features. The upper part of the
bisectors are similar, and assuming they overlap, the bottom end of the
stronger lines, which are formed higher in the atmosphere, goes much further to
the red. This is in agreement with the expected decreasing of the convective
blue-shifts in upper atmospheric layers, and compatible with the high velocity
redshifts observed in the chromosphere, transition region, and corona of
late-type stars.Comment: 27 pages, LaTeX; 10 Figures (14 PostScript files); to be published in
The Astrophysical Journa
Chronic kidney disease in Ecuador : an epidemiological and health system analysis of an emerging public health crisis
Funding: IT, AMSI, ML, SN, KB and RS were supported by a grant from Dialysis Clinic, Inc (DCI), award number 84232.The absence of a chronic kidney disease (CKD) registry in Ecuador makes it difficult to assess the burden of disease, but there is an anticipated increase in the incidence of CKD along with increasing diabetes, hypertension and population age. From 2012, augmented funding for renal replacement therapy expanded dialysis clinics and patient coverage. We conducted 73 in-depth sociological interviews with healthcare providers in eight provinces and collected quantitative epidemiological data on patients with CKD diagnoses from six national-level databases between 2015 and 2018. Datasets show a total of 17,484 dialysis patients in 2018, or 567 patients per million population (pmp), with an annual cost exceeding 11% of Ecuador’s public health budget. Each year, there were 139–162 pmp new dialysis patients, while doctors reported waiting lists. The number of patients on peritoneal dialysis was static; those on hemodialysis increased over time. Only 13 of 24 provinces were found to have dialysis services, and nephrologists were clustered in major cities, which limits access, delays medical attention, and adds a travel burden on patients. Prevention and screening programs are scarce, while hospitalization is an important reality for CKD patients. CKD is an emerging public health crisis that has increased dramatically over the last decade in Ecuador and is expected to continue, making coverage for all patients impossible and the current structure, unsustainable. A patient registry would help health policymakers and administrators estimate the demand and progression of patients with consideration for comorbidities, disease stage, requirements and costs, mortality and follow-up. This should be used to help identify where to focus prevention and improved treatment efforts. Organized monitoring of CKD patients would benefit from improvements in patient referral. Community-based education and prevention programs, the strengthening of primary healthcare capacity (including basic routine tests) and improved nephrology services are also urgently needed.Publisher PDFPeer reviewe
A Consistency Test of Spectroscopic Gravities for Late-Type Stars
Chemical analyses of late-type stars are usually carried out following the
classical recipe: LTE line formation and homogeneous, plane-parallel,
flux-constant, and LTE model atmospheres. We review different results in the
literature that have suggested significant inconsistencies in the spectroscopic
analyses, pointing out the difficulties in deriving independent estimates of
the stellar fundamental parameters and hence,detecting systematic errors.
The trigonometric parallaxes measured by the HIPPARCOS mission provide
accurate appraisals of the stellar surface gravity for nearby stars, which are
used here to check the gravities obtained from the photospheric iron ionization
balance. We find an approximate agreement for stars in the metallicity range -1
<= [Fe/H] <= 0, but the comparison shows that the differences between the
spectroscopic and trigonometric gravities decrease towards lower metallicities
for more metal-deficient dwarfs (-2.5 <= [Fe/H] <= -1.0), which casts a shadow
upon the abundance analyses for extreme metal-poor stars that make use of the
ionization equilibrium to constrain the gravity. The comparison with the
strong-line gravities derived by Edvardsson (1988) and Fuhrmann (1998a)
confirms that this method provides systematically larger gravities than the
ionization balance. The strong-line gravities get closer to the physical ones
for the stars analyzed by Fuhrmann, but they are even further away than the
iron ionization gravities for the stars of lower gravities in Edvardsson's
sample. The confrontation of the deviations of the iron ionization gravities in
metal-poor stars reported here with departures from the excitation balance
found in the literature, show that they are likely to be induced by the same
physical mechanism(s).Comment: AAS LaTeX v4.0, 35 pages, 10 PostScript files; to appear in The
Astrophysical Journa
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