George Mason University

• Fairfax, VA, United States
Recent publications
We prove that polyhedra $$P_1$$ and $$P_2$$ in $$\mathbb {R}^n$$, $$n \ge 3$$, are homothetic if and only if their orthogonal projections on every 2-dimensional plane of $$\mathbb {R}^n$$ are homothetic (this assertion does not hold for arbitrary closed convex sets). Also, we show that closed convex sets $$K_1$$ and $$K_2$$ in $$\mathbb {R}^n$$ are directly homothetic if and only if for any point $$p_1 \in \mathbb {R}^n {\setminus } K_1$$ there is a point $$p_2 \in \mathbb {R}^n {\setminus } K_2$$ such that the generated cone $$C_{p_1}(K_1)$$ is a translate of the respective cone $$C_{p_2}(K_2)$$.
Ocean carbon dioxide removal (OCDR) is rapidly attracting interest, as climate change is putting ecosystems at risk and endangering human communities globally. Due to the centrality of the ocean in the global carbon cycle, augmenting the carbon sequestration capacity of the ocean could be a powerful mechanism for the removal of legacy excess emissions. However, OCDR requires careful assessment due to the unique biophysical characteristics of the ocean and its centrality in the Earth system and many social systems. Using a sociotechnical system lens, this review identifies the sets of considerations that need to be included within robust assessments for OCDR decision-making. Specifically, it lays out the state of technical assessments of OCDR approaches along with key financial concerns, social issues (including public perceptions), and the underlying ethical debates and concerns that would need to be addressed if OCDR were to be deployed as a carbon dioxide removal strategy. Expected final online publication date for the Annual Review of Marine Science, Volume 15 is January 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
Load carriage (LC) is a contributing factor to musculoskeletal injury in many occupations. Given that falls are a common mechanism of injury for those frequently engaging in LC, understanding the effects of LC on postural stability (PS) is necessary. A systematic review and meta-analysis was conducted to examine effects of LC on PS. Sixteen and 9 studies were included in the qualitative and quantitative synthesis, respectively. In most studies, it was found that LC leads to a decrease in PS with significant effects on center of pressure (COP) sway area (standardized mean difference = 0.45; p < 0.005) and COP anterior-posterior excursion (standardized mean difference = 0.52; p < 0.05). Furthermore, load magnitude and load placement are factors which can significantly affect COP measures of PS. It is recommended to minimize load magnitude and equally distribute load when possible to minimize LC effects on PS. Future research should examine additional factors contributing to differences in individual PS responses to LC such as changes in muscle activation and prior LC experience.
The retina biological markers play a crucial role in managing chronic eye conditions, and optical coherence tomography (OCT) is widely used for ophthalmic diseases. However, the established biomarkers have variable shapes and sizes, and pixel-level medical imaged labels are hard to obtain in practice, both making challenges to the localization and classification of them. Therefore, it is essential to locate and classify retinal biomarkers simultaneously under weak supervision. In this paper, we propose a novel method for weakly supervised localization and classification of biomarkers in OCT images, named LCG-Net, based on generative adversarial network (GAN) and attention, using image-level labels for training. The structure-texture information is used to improve the quality of the reconstructed healthy-alike image. And erasure strategy is introduced to ensure the attention maps cover biomarker regions wholly and obtain the biomarker class information. Besides, we fused residual mask in reconstruction and attention mask to locate biomarkers. Our results demonstrate that the proposed LCG-Net achieved higher DSC, Recall and IoU than other comparison methods on the local AB dataset and the public Cell dataset. The proposed method achieved averaged Dice 0.504 for seven disease-related biomarkers, and the Dice for healthy images is 0.922. Experiment results on two large datasets demonstrate the effectiveness of the proposed weakly supervised biomarker localization and classification framework.
Since the last publication a decade ago, accident numbers in the aviation field have been on a steady decline. Much of the previous research had focused on crew resource management (CRM) practices and team training. Since then, there has been a greater focus on looking at safety through the lens of multiteam systems, not just focusing on the pilots but also the cabin crew and ground and maintenance teams. Each of these entities play an important role in the overall goal of safety. However, teams have always existed in one capacity or another, a new focus on team self-maintenance and psychological safety of team members is beginning to be more important as future areas of focus. Whether the team members are human or not is of growing importance as the level of automation in the field increases, which introduces an entirely different context of team cohesion. As time goes on updated training and focus on other factors such as decision-making, psychological well-being, and expanded multiteam systems will all play a role in further improving the excellent safety record of the aviation industry. The purpose of the current chapter is to consider the current context of aviation and provide an update to how team dynamics might be considered in aviation training and team performance.
Fatigue had classically been defined as a decrease in performance or performance capability as a function of time on task. This definition formed the basis for FAA duty time regulations for over 70 years, until January 2012, with the introduction of Part 117 Fatigue Risk Management (FRMP) regulations (FAA 14 CFR Part 117, 2013). Efforts to manage fatigue in aviation moved to science-based approaches that consider operational factors and experience. This resulted from a vast amount of scientific work over the past half century that established that human fatigue is dynamically influenced by now well-described neurobiological regulation of sleep need and endogenous circadian rhythms, which interact nonlinearly to produce changes in human alertness and cognitive performance over time. An international standard for defining fatigue has been established by the International Civil Aviation Organization (ICAO) and is the foundation of fatigue management approaches worldwide. ICAO defines fatigue as “a physiological state of reduced mental or physical performance capability resulting from: (1) sleep loss; (2) extended wakefulness; (3) circadian phase; and/or (4) workload (mental and/or physical activity) that can impair a person’s alertness and ability to perform safety-related operational duties (ICAO, 2015).
Kitaev et al. (2019) described how to shorten universal words for permutations, to length n!+n−1−i(n−1) for any i∈[(n−2)!], by introducing incomparable elements. They conjectured that it is also possible to use incomparable elements to shorten universal cycles for permutations to length n!−i(n−1) for any i∈[(n−2)!]. In this note we prove their conjecture. The proof is constructive, and, on the way, we also show a new method for constructing universal cycles for permutations.
Analyzing the relationship between employment and toxic emissions at over 25,000 US manufacturing facilities between 1998 and 2012 demonstrates that significant reductions in toxic pollution can be achieved without causing equivalent reductions in employment. Three simulations provide a comparison of the combined effects on toxic releases and employment of management strategies targeted at major polluters versus strategies that encompass a random or median subset of facilities or industries. Targeted strategies are effective because toxic emissions are highly disproportionally distributed. A handful of facilities and industries account for the majority of toxic hazard released in the US manufacturing sector. Moreover, these highly polluting facilities and industries do not employ significantly more workers than peer, lower polluting facilities and industries. The research challenges the narrative that protecting the environment must come at a significant cost to economic activity. Rather, targeting egregious polluters can offer an important inroad for significantly reducing industrial pollution while limiting adverse effects on employment.
Technological advances associated with computing power and the prospect of artificial intelligence have renewed interest on the economic feasibility of socialism. The question of such feasibility turns on whether the problem of economic calculation has fundamentally changed. In spite of the prospect of what King and Petty (2021) refer to as “technosocialism,” we argue that technological advances in computation cannot replace the competitive discovery process that takes place within the context of the market. We do so by situating the case for technosocialism in the context of the socialist calculation debate. Understood in these terms, technosocialism represents a restatement of the case for market socialism, which incorrectly framed the “solution” to economic calculation under socialism as one of computing data, rather than the discovery of context-specific knowledge that only emerges through the exchange of property rights. Therefore, the arguments put forth by Ludwig von Mises and F.A. Hayek, and later Israel Kirzner and Don Lavoie, regarding the impossibility of economic calculation under socialism remains just as relevant today.
Existing scholarship examines the moral status of markets, identifying some markets as “noxious”—markets deemed morally objectionable due to the background conditions preceding exchange or the resulting consequences. This literature primarily focuses on market exchanges between private parties. We broaden the analysis to include government markets—markets involving exchanges where either the buyer or seller, or both, are government officials. We develop a general theoretical framework to understand the conditions under which government markets are likely to be more or less noxious. We then apply the framework to the international arms trade to demonstrate its usefulness for understanding noxious government markets.
Global savannas are the third largest carbon sink with large human populations being highly dependent on their ecosystem services. However, savannas are changing rapidly due to climate change, fire, animal management, and intense fuelwood harvesting. In southern Africa, large trees (>5 m in height) are under threat while shrub cover (<3 m) is increasing. The collection of multi-date airborne LiDAR (ALS) data, initiated over a decade ago in the Lowveld of South Africa, provided a rare opportunity to quantify the ability of L-band SAR to track changes in savanna vegetation structure and this study is the first to do so, to our knowledge. The objective was to test the ability of ALOS PALSAR 1&2, dual-pol (HH, HV) data to quantify woody cover and volume change in savannas over 2-, 8- and 10-year periods through comparison to ALS. For each epoch (2008, 2010, 2018), multiple PALSAR images were processed to Gamma0 (γ⁰) at 15 m resolution with multi-temporal speckle filtering. ALS data were processed to fractional canopy cover and volume, and then compared to 5 × 5 aggregated (75 m) SAR mean γ⁰. The ALS cover change (∆CALS) and volume change between pairs of years were highly correlated, with (R² > 0.8), thus results for cover change applied equally to volume change. Cover change was predicted using (i) direct backscatter change or (ii) the difference between annual cover map product derived using the Bayesian Water Cloud Model (BWCM) and logarithmic models. The linear relationship between ∆γ⁰ and ∆CALS varied between year pairs but reached a maximum R² of 0.7 for 2018–2010 and a moderate R² of 0.4 for 2018–2008. Overall, 1 dB ∆γ⁰ corresponded to approximately 0.1 cover change. The three cover change models had very similar uncertainties with mean RMSE = 0.15, which is 13% of the observed cover change range (−0.6 to +0.6). The direct backscatter change approach had less underestimation of positive and negative cover change. The L-band backscatter had a higher sensitivity than suggested by previous studies, as it was able to reliably distinguish cover change at 0.25 increments. The SAR-derived cover change maps detected the loss of stands of big trees, and widespread increases in cover of 0.35–0.65 in communal rangelands due to shrub encroachment. In contrast, the maps suggest that cover generally decreased in conservation areas, forming distinct fence-line effects, potentially caused by significant increases in elephant numbers and frequent, intense wildfires in reserves.
This study uses Bureau of Economic Analysis data on state-level productivity levels and growth rates over the period 1977–2019. We find that states with relatively high productivity tend to experience somewhat lower productivity growth over time, whereas states with relatively lower productivity experience somewhat higher productivity growth over time. We find compelling evidence for significant contributions from education (in the form of a college degree) as well as the role played by higher growth rates in the state-level Hispanic population as factors contributing to increased productivity. Worker/labour productivity constitutes a good indicator of changes to wages and living standards. Empirically examining interstate differences in state-level worker productivity growth across different time intervals helps to identify factors that influence geographical differentials in productivity as well as aids in the identification of the specific factors that determine rates of productivity growth and decline.
The 311 phone number is for reporting non-emergency service requests (SRs) to authorities. This service is available through web, e-mail, and text message as well. Through this service, citizens would describe the issue and its location and the authorities would determine its category and the responsible unit and track the problem until it is resolved. The number of 311 SRs would amount to hundreds of thousands every year in some cities and determining the category of SRs manually is time-consuming, burdensome, and prone to human error. Additionally, these categories are not standardized across the states. In this paper, we standardize these categories across two cities and study the recurrent neural network’s ability in automatically determining the category of SRs based on the transcript of customer-provided descriptions. According to our results, the automatic categorization of these descriptions is not only faster and less cumbersome, but also more accurate than manual categorization. A close look at the mistakes made by the machine in labeling SRs revealed that in many cases either the SR’s description was insufficient to infer its category or the category identified by the machine was correct but the ground truth label assigned to that SR was incorrect.
We employ the ‘benefit of the doubt’ approach rooted in non-parametric techniques to evaluate the entrepreneurial ecosystem of 71 countries for the period 2016. By scrutinizing the relative efficiency of countries’ entrepreneurial ecosystems, the proposed analysis of composite indicators allows the computation of endogenous (country-specific) weights that can be used for developing more informed policy making. The results show that countries prioritize different aspects of their national system of entrepreneurship which confirms that, contrary to homogeneous prescription, tailor-made policy is necessary if the objective is to optimize the resources deployed to enhance the countries’ entrepreneurial ecosystem. The findings of the empirical application reveal significant improvements in the quality of the entrepreneurial ecosystem can be realized by targeting the policy priorities of the local entrepreneurship system identified by the ‘benefit of the doubt’ weights. By analyzing the variation in economic and entrepreneurship outcomes over the seven-year period centered on the study year (period 2013–2019), we found a significant positive correlation between quality improvements in the entrepreneurial ecosystem and venture capital investments.
We investigate the magnetism of a previously unexplored distorted spin-1/2 kagome model consisting of three symmetry-inequivalent nearest-neighbor antiferromagnetic Heisenberg couplings J ⬡ , J , and $$J^{\prime}$$ J ′ , and uncover a rich ground state phase diagram even at the classical level. Using analytical arguments and numerical techniques we identify a collinear $$\overrightarrow{Q}=0$$ Q → = 0 magnetic phase, two unusual non-collinear coplanar $$\overrightarrow{Q}=(1/3,1/3)$$ Q → = ( 1 / 3 , 1 / 3 ) phases and a classical spin liquid phase with a degenerate manifold of non-coplanar ground states, resembling the jammed spin liquid phase found in the context of a bond-disordered kagome antiferromagnet. We further show with density functional theory calculations that the recently synthesized Y-kapellasite Y 3 Cu 9 (OH) 19 Cl 8 is a realization of this model and predict its ground state to lie in the region of $$\overrightarrow{Q}=(1/3,1/3)$$ Q → = ( 1 / 3 , 1 / 3 ) order, which remains stable even after the inclusion of quantum fluctuation effects within variational Monte Carlo and pseudofermion functional renormalization group. The presented model opens a new direction in the study of kagome antiferromagnets.
Geographically-explicit simulations have become crucial in understanding cities and are playing an important role in Urban Science. One such approach is that of agent-based modeling which allows us to explore how agents interact with the environment and each other (e.g., social networks), and how through such interactions aggregate patterns emerge (e.g., disease outbreaks, traffic jams). While the use of agent-based modeling has grown, one challenge remains, that of creating realistic, geographically-explicit, synthetic populations which incorporate social networks. To address this challenge, this paper presents a novel method to create a synthetic population which incorporates social networks using the New York Metro Area as a test area. To demonstrate the generalizability of our synthetic population method and data to initialize models, three different types of agent-based models are introduced to explore a variety of urban problems: traffic, disaster response, and the spread of disease. These use cases not only demonstrate how our geographically-explicit synthetic population can be easily utilized for initializing agent populations which can explore a variety of urban problems, but also show how social networks can be integrated into such populations and large-scale simulations.
The automation in the flight deck of a modern airliner has grown in functionality over the past several decades, improving safety and operational efficiency. As the sophistication of automation has increased, some traditional input devices in the cockpit, such as throttle levers, yokes, and switches, have unobtrusively changed the way they work. Instead of operating the same way at all times, the behavior of these input devices has become context-sensitive (i.e., “moded”). It is well known in human factors design that nonsalient moded input devices require increased cognitive workload and can result in a startle, surprise, and confusion. An analysis of modern airliner cockpits identified three types of moded input devices: 1) disabled, 2) alternative behavior, and 3) command override. The implications of these results on cockpit design, certification, and flight crew training are discussed.
Background Homicide is a major cause of death and contributes to health disparities in the United States. This burden overwhelmingly affects people from racial and ethnic minority populations as homicide occurs more often in neighborhoods with high proportions of racial and ethnic minority residents. Research has identified that environmental factors contribute to variation in homicide rates between neighborhoods; however, it is not clear why some neighborhoods with high concentrations of racial and ethnic minority residents have high homicide rates while neighborhoods with similar demographic compositions do not. The aim of this study was to assess whether relative socioeconomic disadvantage, (i.e., income inequality), or absolute socioeconomic disadvantage (i.e., income) measured at the ZIP code- and state-levels, is associated with high homicide rates in US ZIP codes, independent of racial and ethnic composition. Methods This ecological case–control study compared median household income and income inequality in 250 ZIP codes with the highest homicide rate in our sample in 2017 (cases) to 250 ZIP codes that did not experience any homicide deaths in 2017 (controls). Cases were matched to controls 1:1 based on demographic composition. Variables were measured at both the ZIP code- and state-levels. Results Lower median household income at the ZIP code-level contributed most substantially to the homicide rate. Income inequality at the state-level, however, was additionally significant when controlling for both ZIP code- and state-level factors. Conclusions Area-based interventions that improve absolute measures of ZIP code socioeconomic disadvantage may reduce gaps in homicide rates.
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12,545 members
• Bioinformatics and Computational Biology Program, School of Systems Biology
• Department of Communication
• Department of Computational and Data Sciences
• Department of Rehabilitation Science
• Department of Civil, Environmental and Infrastructure Engineering
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