7,182 research outputs found
Relativistic Positioning Systems: The Emission Coordinates
This paper introduces some general properties of the gravitational metric and
the natural basis of vectors and covectors in 4-dimensional emission
coordinates. Emission coordinates are a class of space-time coordinates defined
and generated by 4 emitters (satellites) broadcasting their proper time by
means of electromagnetic signals. They are a constitutive ingredient of the
simplest conceivable relativistic positioning systems. Their study is aimed to
develop a theory of these positioning systems, based on the framework and
concepts of general relativity, as opposed to introducing `relativistic
effects' in a classical framework. In particular, we characterize the causal
character of the coordinate vectors, covectors and 2-planes, which are of an
unusual type. We obtain the inequality conditions for the contravariant metric
to be Lorentzian, and the non-trivial and unexpected identities satisfied by
the angles formed by each pair of natural vectors. We also prove that the
metric can be naturally split in such a way that there appear 2 parameters
(scalar functions) dependent exclusively on the trajectory of the emitters,
hence independent of the time broadcast, and 4 parameters, one for each
emitter, scaling linearly with the time broadcast by the corresponding
satellite, hence independent of the others.Comment: 13 pages, 3 figures. Only format changed for a new submission.
Submitted to Class. Quantum Gra
Jet shapes in ep collisions at HERA
New measurements of the jet shape in ep collisions at HERA using the
k_T-cluster jet algorithm are presented.Comment: 7 pages, 3 figures; plenary talk given at the 3rd UK Phenomenology
Workshop on HERA Physics, Durham, UK, September 199
Emerging agents to combat complicated and resistant infections: focus on ceftobiprole
Antimicrobial resistance is a global concern. Over the past few years, considerable efforts and resources have been expended to detect, monitor, and understand at the basic level the many different facets of emerging and increasing resistance. Development of new antimicrobial agents has been matched by the development of new mechanisms of resistance by bacteria. Current antibiotics act at a variety of sites within the target bacteria, including the cross-linking enzymes in the cell wall, various ribosomal enzymes, nucleic acid polymerases, and folate synthesis. Ceftobiprole is a novel parenteral cephalosporin with high affinity for most penicillin-binding proteins, including the mecA product penicillin-binding protein 2a, rendering it active against methicillin-resistant staphylococci. Its in vitro activity against staphylococci and multiresistant pneumococci, combined with its Gram-negative spectrum comparable to that of other extended-spectrum cephalosporins, its stability against a wide range of beta-lactamases, and its pharmacokinetic and safety profiles make ceftobiprole an attractive and well tolerated new antimicrobial agent. The US Food and Drug Administration granted ceftobiprole medocaril fast-track status in 2003 for the treatment of complicated skin infections and skin structure infections due to methicillin-resistant staphylococci, and subsequently extended this to treatment of hospital-acquired pneumonia, including ventilator-associated pneumonia due to suspected or proven methicillin-resistant Staphylococcus aureus
Erratum to “Comparative study of models of impedance boundary conditions in acoustic problems”
Peer Reviewe
Erratum to “Comparative study of models of impedance boundary conditions in acoustic problems”
Peer Reviewe
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