4,692 research outputs found
Gluing of Branched Surfaces by Sewing of Fermionic String Vertices
We glue together two branched spheres by sewing of two Ramond (dual)
two-fermion string vertices and present a rigorous analytic derivation of the
closed expression for the four-fermion string vertex. This method treats all
oscillator levels collectively and the obtained answer verifies that the closed
form of the four vertex previously argued for on the basis of explicit results
restricted to the first two oscillator levels is the correct one.Comment: 20 pages + 5 figures as eps-file
The Twisted String Vertex Algorithm Applied to the -Twisted Scalar String Four Vertex
Recently an algorithm was found by means of which one can calculate terms at
arbitrary oscillator level in the four-Ramond vertex obtained by sewing. Here
we show that this algorithm is applicable also to the case of -twisted scalars and derive the full propagator for scalars on the Riemann
sphere with two branch cuts. The relation to similar results previously derived
in the literature by other means is discussed briefly.Comment: 18 pages (LaTeX), G\"oteborg ITP 94-
Theory of the Bloch Oscillating Transistor
The Bloch oscillating transistor (BOT) is a device, where single electron
current through a normal tunnel junction can be used to enhance Cooper pair
current in a mesoscopic Josephson junction leading to signal amplification. In
this paper we develop a theory, where the BOT dynamics is described as a
two-level system. The theory is used to predict current-voltage characteristics
and small-signal response. Transition from stable operation into hysteretic
regime is studied. By identifying the two-level switching noise as the main
source of fluctuations, the expressions for equivalent noise sources and the
noise temperature are derived. The validity of the model is tested by comparing
the results with simulations.Comment: 11 pages, 8 figure
Person or Place? Parametric and semiparametric estimates of intrametropolitan earnings variation
Some scholars have attributed earnings differences among locations to labor market conditions ("place effects") whereas others have focused on the skill level of residents ("person effects"). We estimate a variety of selection models in an effort to detect differences in labor market conditions while controlling for differences in skill levels. We maintain the assumption that there are no barriers to mobility within a metropolitan area for highly educated white men, which implies that intra-urban differences for this group reflect sorting by skill and earnings rather than real wage differences for equally productive workers. This prediction allows us to reject several conventional parametric selection models. We estimate a semiparametric selection model that yields strong evidence that, for less educated white men, the apparent suburban earnings premium is due to sorting rather than labor market differences.
Explicit optimization of plan quality measures in intensity-modulated radiation therapy treatment planning
Conventional planning objectives in optimization of intensity-modulated
radiotherapy treatment (IMRT) plans are designed to minimize the violation of
dose-volume histogram (DVH) thresholds using penalty functions. Although
successful in guiding the DVH curve towards these thresholds, conventional
planning objectives offer limited control of the individual points on the DVH
curve (doses-at-volume) used to evaluate plan quality. In this study, we
abandon the usual penalty-function framework and propose planning objectives
that more explicitly relate to DVH statistics. The proposed planning objectives
are based on mean-tail-dose, resulting in convex optimization. We also
demonstrate how to adapt a standard optimization method to the proposed
formulation in order to obtain a substantial reduction in computational cost.
We investigate the potential of the proposed planning objectives as tools for
optimizing DVH statistics through juxtaposition with the conventional planning
objectives on two patient cases. Sets of treatment plans with differently
balanced planning objectives are generated using either the proposed or the
conventional approach. Dominance in the sense of better distributed
doses-at-volume is observed in plans optimized within the proposed framework,
indicating that the DVH statistics are better optimized and more efficiently
balanced using the proposed planning objectives
- …