47,291 research outputs found
Artificial Contraception is Associated With Increased Numbers of Induced Abortions
Since the sexual revolution, it has been a widely accepted fact that contraceptives have been a necessity for our society. They seem to be the best way to prevent unwanted pregnancies that would otherwise result from sexual unions between two people unprepared for the commitment of raising any or additional children. It almost goes without saying that the more effective and available contraceptives become, the less likely women who wish to avoid pregnancy will become pregnant and the fewer induced abortions they will seek. In reality, the information available concerning abortions and the women who obtain them points to a different conclusion. The more effective and available contraceptives become in a society, the more widespread the practice of induced abortions becomes. On the other hand, natural family planning has been associated with fewer induced abortions when an unplanned pregnancy results. This paper will present a sample of the information available, attempt to draw conclusions, and discuss the possible reasons that contraceptives are unavoidably linked to increasing numbers of induced abortions and that, conversely, natural family planning is associated with very few or no induced abortions. </jats:p
The Hamiltonian Structure of Soliton Equations and Deformed W-Algebras
The Poisson bracket algebra corresponding to the second Hamiltonian structure
of a large class of generalized KdV and mKdV integrable hierarchies is
carefully analysed. These algebras are known to have conformal properties, and
their relation to -algebras has been previously investigated in some
particular cases. The class of equations that is considered includes
practically all the generalizations of the Drinfel'd-Sokolov hierarchies
constructed in the literature. In particular, it has been recently shown that
it includes matrix generalizations of the Gelfand-Dickey and the constrained KP
hierarchies. Therefore, our results provide a unified description of the
relation between the Hamiltonian structure of soliton equations and -algebras, and it comprises almost all the results formerly obtained by other
authors. The main result of this paper is an explicit general equation showing
that the second Poisson bracket algebra is a deformation of the Dirac bracket
algebra corresponding to the -algebras obtained through Hamiltonian
reduction.Comment: 41 pages, plain TeX, no figures. New introduction and references
added. Version to be published in Annals of Physics (N.Y.
Exciton Beats in GaAs Quantum Wells: Bosonic Representation and Collective Effects
We discuss light-heavy hole beats observed in transient optical experiments
in GaAs quantum wells in terms of a free-boson coherent state model. This
approach is compared with descriptions based on few-level representations.
Results lead to an interpretation of the beats as due to classical
electromagnetic interference. The boson picture correctly describes photon
excitation of extended states and accounts for experiments involving coherent
control of the exciton density and Rayleigh scattering beating.Comment: 4 pages, no figures. Accepted for publication in Solid State
Communication
Commutator methods for unitary operators
We present an improved version of commutator methods for unitary operators
under a weak regularity condition. Once applied to a unitary operator, the
method typically leads to the absence of singularly continuous spectrum and to
the local finiteness of point spectrum. Large families of locally smooth
operators are also exhibited. Half of the paper is dedicated to applications,
and a special emphasize is put on the study of cocycles over irrational
rotations. It is apparently the first time that commutator methods are applied
in the context of rotation algebras, for the study of their generators.Comment: 15 page
HighâSpeed Data Transmission Subsystem of the SEOSAR/PAZ Satellite
This paper analyzes a digital interface and bus system modeling and optimization of the SEOSAR/PAZ Earth Observation satellite. The important part of the satellite is an Xâband Synthetic Aperture Radar instrument that integrates 384 Transmit/Receive Modules located in 12 antenna panels 7.5 m away from the central processor and controlled by a synchronous 10 Mbps bidirectional serial protocol. This type of midârange pointâtoâmultipoint transmission is affected by bit errors due to crosstalk, transmission line attenuation and impedance mismatches. The highâspeed data communication network has been designed to optimize the transmission by using a simulation model of the data distribution system which takes into account the worstâcase scenario and by developing a labâscaled prototype which exhibits BER of 10-11 for an interfering signal of 10 Vpp. The result is a pointâtoâmultipoint bidirectional transmission network optimized in both directions with optimal values of loads and equalization resistors. This highâspeed data transmission subsystem provides a compact design through a simple solution
Nonleptonic decays and the nature of the orbitally excited charmed-strange mesons
The Belle Collaboration has recently reported a study of the decays and has given also estimates of relevant
ratios between branching fractions of decays
providing important information to check the structure of the
, and mesons. The
disagreement between experimental data and Heavy Quark Symmetry has been used
as an indication that and mesons could
have a more complex structure than the canonical one. We analyze
these ratios within the framework of a constituent quark model, which allows us
to incorporate the effects given by finite -quark mass corrections. Our
findings are that while the meson could have a sizable
non- component, the and mesons
seem to be well described by a pure structure.Comment: 13 pages, 1 figur
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