810 research outputs found
On the chromatic roots of generalized theta graphs
The generalized theta graph \Theta_{s_1,...,s_k} consists of a pair of
endvertices joined by k internally disjoint paths of lengths s_1,...,s_k \ge 1.
We prove that the roots of the chromatic polynomial $pi(\Theta_{s_1,...,s_k},z)
of a k-ary generalized theta graph all lie in the disc |z-1| \le [1 + o(1)]
k/\log k, uniformly in the path lengths s_i. Moreover, we prove that
\Theta_{2,...,2} \simeq K_{2,k} indeed has a chromatic root of modulus [1 +
o(1)] k/\log k. Finally, for k \le 8 we prove that the generalized theta graph
with a chromatic root that maximizes |z-1| is the one with all path lengths
equal to 2; we conjecture that this holds for all k.Comment: LaTex2e, 25 pages including 2 figure
The flat phase of fixed-connectivity membranes
The statistical mechanics of flexible two-dimensional surfaces (membranes)
appears in a wide variety of physical settings. In this talk we discuss the
simplest case of fixed-connectivity surfaces. We first review the current
theoretical understanding of the remarkable flat phase of such membranes. We
then summarize the results of a recent large scale Monte Carlo simulation of
the simplest conceivable discrete realization of this system \cite{BCFTA}. We
verify the existence of long-range order, determine the associated critical
exponents of the flat phase and compare the results to the predictions of
various theoretical models.Comment: 7 pages, 5 figures, 3 tables. LaTeX w/epscrc2.sty, combined
contribution of M. Falcioni and M. Bowick to LATTICE96(gravity), to appear in
Nucl. Phys. B (proc. suppl.
CRF1 Receptor Activation Increases the Response of Neurons in the Basolateral Nucleus of the Amygdala to Afferent Stimulation
The basolateral nucleus (BLA) of the amygdala contributes to the consolidation of memories for emotional or stressful events. The nucleus contains a high density of CRF1 receptors that are activated by corticotropin-releasing factor (CRF). Modulation of the excitability of neurons in the BLA by CRF may regulate the immediate response to stressful events and the formation of associated memories. In the present study, CRF was found to increase the amplitude of field potentials recorded in the BLA following excitatory afferent stimulation, in vitro. The increase was mediated by CRF1 receptors, since it could be blocked by the selective, non-peptide antagonists, NBI30775 and NBI35583, but not by the CRF2-selective antagonist, astressin 2B. Furthermore, the CRF2-selective agonist, urocortin II had no effect on field potential amplitude. The increase induced by CRF was long-lasting, could not be reversed by subsequent administration of NBI35583, and required the activation of protein kinase C. This effect of CRF in the BLA may be important for increasing the salience of aversive stimuli under stressful conditions, and for enhancing the consolidation of associated memories. The results provide further justification for studying the efficacy of selective antagonists of the CRF1 receptor to reduce memory formation linked to emotional or traumatic events, and suggest that these compounds might be useful as prophylactic treatments for stress-related illnesses such as post-traumatic stress disorder
M.C.R.G. Study of Fixed-connectivity Surfaces
We apply Monte Carlo Renormalization group to the crumpling transition in
random surface models of fixed connectivity. This transition is notoriously
difficult to treat numerically. We employ here a Fourier accelerated Langevin
algorithm in conjunction with a novel blocking procedure in momentum space
which has proven extremely successful in . We perform two
successive renormalizations in lattices with up to sites. We obtain a
result for the critical exponent in general agreement with previous
estimates and similar error bars, but with much less computational effort. We
also measure with great accuracy . As a by-product we are able to
determine the fractal dimension of random surfaces at the crumpling
transition.Comment: 35 pages,Latex file, 6 Postscript figures uuencoded,uses psfig.sty 2
misspelled references corrected and one added. Paper unchange
An Investigation of the Steady-State Pharmacokinetics of Oral Valacyclovir in Immunocompromised Children
Valacyclovir was administered to 28 immunocompromised children (ages 5-12 years) to obtain preliminary pharmacokinetic and safety information. Patients were randomized to valacyclovir regimens of 250 mg (9.4-13.3 mg/kg) or 500 mg (13.9-27.0 mg/kg) twice daily or 500 mg (13.2-21.7 mg/kg) 3 times a day. Acyclovir pharmacokinetics were evaluated at steady state. Valacyclovir was rapidly absorbed and converted to acyclovir. Mean (±SD) acyclovir peak concentrations from 250 mg and 500 mg valacyclovir were 4.11±1.41 and 5.19±1.96 μg/mL, respectively. Corresponding single dose area-under-curve values were 12.14±6.60 and 14.49±4.69h×μg/mL. By using historical data for intravenous acyclovir as reference, the overall estimate of acyclovir bioavailability from valacyclovir was 48%, 2- to 4-fold greater than for oral acyclovir. In general, adverse events were not attributable to valacyclovir and were consistent with disease-related expectations and concomitant therapies. Dosage options for using valacyclovir in children are discusse
Evaluation of a community‐based early childhood caries (ECC) intervention in Cambodia
Objective: To critically evaluate an early childhood caries (ECC) intervention performed by non-dental primary healthcare providers. Methods: This mixed-methods investigation includes data from three sources: (a) a pilot non-randomized controlled trial to examine clinical outcomes at four health centres; (b) stakeholder focus group interviews; and (c) a survey of parents whose children were exposed to the intervention. The pilot study involved four Community Health Centres in rural Cambodia whereby mother-child (6-24 months of age)dyads received oral health education (OHE), toothbrushes, fluoride toothpaste and fluoride varnish on up to six occasions as part of the routine vaccination schedule. Outcomes were as follows: presence of ECC; impacts on oral health-related quality of life (OHRQoL); stakeholder perceptions of intervention delivery; and parental perceptions of fluoride varnish. Results: Participants in the intervention group had six times lower odds of developing ECC than those in the comparison group after controlling for socio-economic status (OR 0.13). Those in the intervention group also had a large reduction OHRQoL scale scores. Key knowledge and practice gaps were identified among stakeholders. Surveyed parents had favourable views of the fluoride varnish placement by medical professionals, and four out of five stated that they would recommend fluoride varnish for other children. Primary healthcare providers, commune council representatives and community health promoters supported oral health interventions being provided in CHCs. Conclusions: OHE and fluoride varnish interventions provided by non-dental primary health workers were feasible and acceptable for stakeholders in a Cambodian setting. The intervention group had lower ECC experience and better OHRQoL at 2 years of age
Two Ising Models Coupled to 2-Dimensional Gravity
To investigate the properties of matter coupled to d{--}gravity we
have performed large-scale simulations of two copies of the Ising Model on a
dynamical lattice. We measure spin susceptibility and percolation critical
exponents using finite-size scaling. We show explicitly how logarithmic
corrections are needed for a proper comparison with theoretical exponents. We
also exhibit correlations, mediated by gravity, between the energy and magnetic
properties of the two Ising species. The prospects for extending this work
beyond are addressed.Comment: revised version w/ typos corrected; standard latex w/ epsf and 9
figure
Heavy Quark Effective Theory beyond Perturbation Theory: Renormalons, the Pole Mass and the Residual Mass Term
We study the asymptotic behaviour of the perturbative series in the heavy
quark effective theory (HQET) using the expansion. We find that this
theory suffers from an {\it ultraviolet} renormalon problem, corresponding to a
non-Borel-summable behaviour of perturbation series in large orders, and
leading to a principal nonperturbative ambiguity in its definition. This
ambiguity is related to an {\it infrared} renormalon in the pole mass and can
be understood as the necessity to include the residual mass term in
the definition of HQET, which must be considered as ambiguous (and possibly
complex), and is required to cancel the ultraviolet renormalon singularity
generated by the perturbative expansion. The formal status of is
thus identical to that of condensates in the conventional short-distance
expansion of correlation functions in QCD. The status of the pole mass of a
heavy quark, the operator product expansion for inclusive decays, and QCD sum
rules in the HQET are discussed in this context.Comment: LATEX, 43 pages, 6 figures appended as uu-encoded file, MPI-PhT/94-9,
(Text as to appear in NPB, typing errors corrected [Eq.(3.24),(3.26)], some
statements in Sect.5 more precise
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