Traceroute sampling makes random graphs appear to have power law degree distributions

The topology of the Internet has typically been measured by sampling traceroutes, which are roughly shortest paths from sources to destinations. The resulting measurements have been used to infer that the Internet's degree distribution is scale-free; however, many of these measurements have relied on sampling traceroutes from a small number of sources. It was recently argued that sampling in this way can introduce a fundamental bias in the degree distribution, for instance, causing random (Erdos-Renyi) graphs to appear to have power law degree distributions. We explain this phenomenon analytically using differential equations to model the growth of a breadth-first tree in a random graph G(n,p=c/n) of average degree c, and show that sampling from a single source gives an apparent power law degree distribution P(k) ~ 1/k for k < c

Exact solutions for models of evolving networks with addition and deletion of nodes

There has been considerable recent interest in the properties of networks, such as citation networks and the worldwide web, that grow by the addition of vertices, and a number of simple solvable models of network growth have been studied. In the real world, however, many networks, including the web, not only add vertices but also lose them. Here we formulate models of the time evolution of such networks and give exact solutions for a number of cases of particular interest. For the case of net growth and so-called preferential attachment -- in which newly appearing vertices attach to previously existing ones in proportion to vertex degree -- we show that the resulting networks have power-law degree distributions, but with an exponent that diverges as the growth rate vanishes. We conjecture that the low exponent values observed in real-world networks are thus the result of vigorous growth in which the rate of addition of vertices far exceeds the rate of removal. Were growth to slow in the future, for instance in a more mature future version of the web, we would expect to see exponents increase, potentially without bound.Comment: 9 pages, 3 figure

Native plants dominate understory vegetation following ponderosa pine forest restoration treatments

Dense ponderosa pine forests in the southwestern United States inhibit understory production and diversity and are susceptible to high-severity wildfire. Restoration treatments involving overstory thinning and prescribed burning are being implemented to increase understory productivity and diversity and to reduce the risk of severe wildfire. However, disturbances associated with treatments may favor invasion of nonnative species, and the severity of the disturbance may be related to the level of nonnative species establishment. We examined understory community composition, species richness, and plant cover responses to 3 stand-scale replicates of 4 different tree-thinning intensities. Restoration treatments altered the composition of the understory community regardless of thinning intensity. Understory richness and cover were highly variable among experimental blocks, but we observed strong trends of increasing richness and cover in the treated stands. Immediately following restoration treatments, nonnative species cover comprised 6% of the total cover where treatment-induced disturbances were the greatest. However, the initial increase in nonnative species did not persist and was reduced by half 6 years after treatment. Plant community composition was still in flux by the sixth year after treatment, indicating that continued monitoring is necessary for evaluating whether restoration targets are maintained over time

Variational approach to a class of nonlinear oscillators with several limit cycles

We study limit cycles of nonlinear oscillators described by the equation $\ddot x + \nu F(\dot x) + x =0$. Depending on the nonlinearity this equation may exhibit different number of limit cycles. We show that limit cycles correspond to relative extrema of a certain functional. Analytical results in the limits $\nu ->0$ and $\nu -> \infty$ are in agreement with previously known criteria. For intermediate $\nu$ numerical determination of the limit cycles can be obtained.Comment: 12 pages, 3 figure

Lung-protective ventilation initiated in the emergency department (LOV-ED): A study protocol for a quasi-experimental, before-after trial aimed at reducing pulmonary complications

INTRODUCTION: In critically ill patients, acute respiratory distress syndrome (ARDS) and ventilator-associated conditions (VACs) are associated with increased mortality, survivor morbidity and healthcare resource utilisation. Studies conclusively demonstrate that initial ventilator settings in patients with ARDS, and at risk for it, impact outcome. No studies have been conducted in the emergency department (ED) to determine if lung-protective ventilation in patients at risk for ARDS can reduce its incidence. Since the ED is the entry point to the intensive care unit for hundreds of thousands of mechanically ventilated patients annually in the USA, this represents a knowledge gap in this arena. A lung-protective ventilation strategy was instituted in our ED in 2014. It aims to address the parameters in need of quality improvement, as demonstrated by our previous research: (1) prevention of volutrauma; (2) appropriate positive end-expiratory pressure setting; (3) prevention of hyperoxia; and (4) aspiration precautions. METHODS AND ANALYSIS: The lung-protective ventilation initiated in the emergency department (LOV-ED) trial is a single-centre, quasi-experimental before-after study testing the hypothesis that lung-protective ventilation, initiated in the ED, is associated with reduced pulmonary complications. An intervention cohort of 513 mechanically ventilated adult ED patients will be compared with over 1000 preintervention control patients. The primary outcome is a composite outcome of pulmonary complications after admission (ARDS and VACs). Multivariable logistic regression with propensity score adjustment will test the hypothesis that ED lung-protective ventilation decreases the incidence of pulmonary complications. ETHICS AND DISSEMINATION: Approval of the study was obtained prior to data collection on the first patient. As the study is a before-after observational study, examining the effect of treatment changes over time, it is being conducted with waiver of informed consent. This work will be disseminated by publication of full-length manuscripts, presentation in abstract form at major scientific meetings and data sharing with other investigators through academically established means. TRIAL REGISTRATION NUMBER: NCT02543554

Rapid Mixing for Lattice Colorings with Fewer Colors

We provide an optimally mixing Markov chain for 6-colorings of the square lattice on rectangular regions with free, fixed, or toroidal boundary conditions. This implies that the uniform distribution on the set of such colorings has strong spatial mixing, so that the 6-state Potts antiferromagnet has a finite correlation length and a unique Gibbs measure at zero temperature. Four and five are now the only remaining values of q for which it is not known whether there exists a rapidly mixing Markov chain for q-colorings of the square lattice.Comment: Appeared in Proc. LATIN 2004, to appear in JSTA

Can I Be a Scientist?: Adolescent Exposure to STEM Literacy and Students’ Conceptions of Identity

There is a crisis of disenfranchised students dropping out of STEM majors and careers. The national STEM major retention rates in undergraduate institutions hovers around 40%, and this number is disproportionately smaller among marginalized students, especially women and students of color (Dagley et. al, 2015). These students, who struggle to identify within academic STEM communities, are failed by the inadequacy of texts that are unable to stimulate interest and engagement in their STEM identities. Based on student interviews, research of the literary market, and my own experiences, there is a severe de-emphasis on STEM nonfiction literature in the classroom for adolescent-aged students. Using a theoretical framework informed by Gholdy Muhammad’s Historically Responsive Literacy (HRL) framework, Pierre Bourdieu’s five major concepts, and Jean Lave & Etienne Wenger’s theory of situated learning informed by Claude Steele’s research on stereotype threat, I will argue for a need for more STEM nonfiction texts aimed at engaging a “young adult demographic” to further engage students that may otherwise decide to stop pursuing STEM

High-Frequency (> 100 GHz) Electronic Devices

Contains reports on four research projects, the research facility and a list of publications.Defense Advanced Research Projects Agency Contract MDA972-90-C-0021National Aeronautics and Space Administration Grant NAG2-693National Science Foundation Grant ECS 91-0933

High-Frequency (> 100 GHz) Electronic Devices

Contains reports on five research projects and a list of publications.MIT Lincoln LaboratoryNational Aeronautics and Space Administration Grant NAG2-693National Science Foundation Grant ECS 91-09330Defense Advanced Research Projects Agency Contract MDA972-90-C-0021U.S. Army - Research Office Grant DAAL03-92-G-025