48 research outputs found
Polyhedral Analysis using Parametric Objectives
The abstract domain of polyhedra lies at the heart of many program analysis techniques. However, its operations can be expensive, precluding their application to polyhedra that involve many variables. This paper describes a new approach to computing polyhedral domain operations. The core of this approach is an algorithm to calculate variable elimination (projection) based on parametric linear programming. The algorithm enumerates only non-redundant inequalities of the projection space, hence permits anytime approximation of the output
Quivers, Tilings, Branes and Rhombi
We describe a simple algorithm that computes the recently discovered brane
tilings for a given generic toric singular Calabi-Yau threefold. This therefore
gives AdS/CFT dual quiver gauge theories for D3-branes probing the given
non-compact manifold. The algorithm solves a longstanding problem by computing
superpotentials for these theories directly from the toric diagram of the
singularity. We study the parameter space of a-maximization; this study is made
possible by identifying the R-charges of bifundamental fields as angles in the
brane tiling. We also study Seiberg duality from a new perspective.Comment: 36 pages, 40 figures, JHEP
Gravitational Collapse and Disk Formation in Magnetized Cores
We discuss the effects of the magnetic field observed in molecular clouds on
the process of star formation, concentrating on the phase of gravitational
collapse of low-mass dense cores, cradles of sunlike stars. We summarize recent
analytic work and numerical simulations showing that a substantial level of
magnetic field diffusion at high densities has to occur in order to form
rotationally supported disks. Furthermore, newly formed accretion disks are
threaded by the magnetic field dragged from the parent core during the
gravitational collapse. These disks are expected to rotate with a sub-Keplerian
speed because they are partially supported by magnetic tension against the
gravity of the central star. We discuss how sub-Keplerian rotation makes it
difficult to eject disk winds and accelerates the process of planet migration.
Moreover, magnetic fields modify the Toomre criterion for gravitational
instability via two opposing effects: magnetic tension and pressure increase
the disk local stability, but sub-Keplerian rotation makes the disk more
unstable. In general, magnetized disks are more stable than their nonmagnetic
counterparts; thus, they can be more massive and less prone to the formation of
giant planets by gravitational instability.Comment: Chapter 16 in "Magnetic Fields in Diffuse Media", Springer-Verlag,
eds. de Gouveia Dal Pino, E., Lazarian, A., Melioli,
Design and construction of the MicroBooNE detector
This paper describes the design and construction of the MicroBooNE liquid
argon time projection chamber and associated systems. MicroBooNE is the first
phase of the Short Baseline Neutrino program, located at Fermilab, and will
utilize the capabilities of liquid argon detectors to examine a rich assortment
of physics topics. In this document details of design specifications, assembly
procedures, and acceptance tests are reported
Whole-genome sequencing reveals host factors underlying critical COVID-19
Critical COVID-19 is caused by immune-mediated inflammatory lung injury. Host genetic variation influences the development of illness requiring critical care1 or hospitalization2,3,4 after infection with SARS-CoV-2. The GenOMICC (Genetics of Mortality in Critical Care) study enables the comparison of genomes from individuals who are critically ill with those of population controls to find underlying disease mechanisms. Here we use whole-genome sequencing in 7,491 critically ill individuals compared with 48,400 controls to discover and replicate 23 independent variants that significantly predispose to critical COVID-19. We identify 16 new independent associations, including variants within genes that are involved in interferon signalling (IL10RB and PLSCR1), leucocyte differentiation (BCL11A) and blood-type antigen secretor status (FUT2). Using transcriptome-wide association and colocalization to infer the effect of gene expression on disease severity, we find evidence that implicates multiple genesâincluding reduced expression of a membrane flippase (ATP11A), and increased expression of a mucin (MUC1)âin critical disease. Mendelian randomization provides evidence in support of causal roles for myeloid cell adhesion molecules (SELE, ICAM5 and CD209) and the coagulation factor F8, all of which are potentially druggable targets. Our results are broadly consistent with a multi-component model of COVID-19 pathophysiology, in which at least two distinct mechanisms can predispose to life-threatening disease: failure to control viral replication; or an enhanced tendency towards pulmonary inflammation and intravascular coagulation. We show that comparison between cases of critical illness and population controls is highly efficient for the detection of therapeutically relevant mechanisms of disease
A search for understanding
We discuss an important breakthrough for us in our search for a technical meaning of âunderstandingâ in mathematics education. In this article, we describe the background to this discovery, the catalyst for the breakthrough, and a concise explanation of our new definition. In discussing the consequences of this definition, both in terms of the theoretical implications for the internal characteristics and external manifestations of understanding with some initial practical consequences for a teacher's attempts to model a learner's understanding, we begin to describe our ongoing search for a more comprehensive theory. Much of our work during the past 5 years has been devoted to developing a theory of learning which accounts for our own experiences as learners, teachers, and researchers. The theory we have developed in which we identify learning experiences as natural, conflicting, or alien, and consider the ways in which learners might respond to them (Duffin & Simpson, 1993) has been used to analyze a number of learning incidents we have encountered (Duffin & Simpson, 1995). In the course of such analyses, we found that the word âunderstandingâ often entered our discussions and eventually, we felt compelled to seek a definition of the word, which would fit within the basic framework of our theory and might enable us to make more sense of the incidents we encountered. The quest for such a definition has occupied us for more than 3 years and we will use this article to explore how we came to our current position. The article is not focussed on understanding, its focus is on the journey we undertook to make sense of a particular term (which happens to be âunderstandingâ) in the context of our on-going development of a personal theory of learning. As such, the article contains almost as much about what we later came to see as mistaken (or to replace) as it does about the terminus of our journey: our current definition of understanding
Shape transition in very large germanium islands on Si(111)
Ge islands with areas up to hundreds of ÎŒm2 were grown on Si(111). These islands, grown above 750â°C and at a deposition rate of 1 monolayer/min, become decreasingly compact with increasing size and can have nonuniform cross sections with heights reaching over 500 nm. The largest islands are ramified, often comprising multiple discrete parts. X-rayphotoemission electron microscopy absorption maps show that the islands have a higher concentration of Ge at their centers, with more Si near the edges. We propose that the shape transformation is driven by strain relief at the island perimeters
The electronic properties of Si(001)âBi(2 Ă n)
A Bi 2 Ă n surface net was grown on the Si(001) surface and studied with inverse photoemission, scanning tunnelling microscopy and ab initio and empirical pseudopotential calculations. The experiments demonstrated that Bi adsorption eliminates the dimer related Ï1* and Ï2* surface states, produced by correlated dimer buckling, leaving the bulk bandgap clear of unoccupied surface states. Ab initio calculations support this observation and demonstrate that the surface states derived from the formation of symmetric Bi dimers do not penetrate the fundamental bandgap of bulk Si. Since symmetric Bi dimers are an important structural component of the recently discovered Bi nanolines, that self-organize on Si(001) above the Bi desorption temperature, a connection will be made between our findings and the electronic structure of the nanolines