University of Massachusetts Amherst

About: University of Massachusetts Amherst is a education organization based out in Amherst Center, Massachusetts, United States. It is known for research contribution in the topics: Population & Galaxy. The organization has 37274 authors who have published 83965 publications receiving 3834996 citations. The organization is also known as: UMass Amherst & Massachusetts State College.

DTN routing as a resource allocation problem

Abstract: Many DTN routing protocols use a variety of mechanisms, including discovering the meeting probabilities among nodes, packet replication, and network coding. The primary focus of these mechanisms is to increase the likelihood of finding a path with limited information, so these approaches have only an incidental effect on such routing metrics as maximum or average delivery latency. In this paper, we present RAPID, an intentional DTN routing protocol that can optimize a specific routing metric such as worst-case delivery latency or the fraction of packets that are delivered within a deadline. The key insight is to treat DTN routing as a resource allocation problem that translates the routing metric into per-packet utilities which determine how packets should be replicated in the system.We evaluate RAPID rigorously through a prototype of RAPID deployed over a vehicular DTN testbed of 40 buses and simulations based on real traces. To our knowledge, this is the first paper to report on a routing protocol deployed on a real DTN at this scale. Our results suggest that RAPID significantly outperforms existing routing protocols for several metrics. We also show empirically that for small loads RAPID is within 10% of the optimal performance.

Adsorption Mechanisms of Organic Chemicals on Carbon Nanotubes

Abstract: Carbon nanotubes (CNTs) have drawn special research attention because of their unique properties and potential applications. This review summarizes the research progress of organic chemical adsorption on CNTs, and will provide useful information for CNT application and risk assessment. Adsorption heterogeneity and hysteresis are two widely recognized features of organic chemical−CNT interactions. However, because different mechanisms may act simultaneously, mainly hydrophobic interactions, π−π bonds, electrostatic interactions, and hydrogen bonds, the prediction of organic chemical adsorption on CNTs is not straightforward. The dominant adsorption mechanism is different for different types of organic chemicals (such as polar and nonpolar), thus different models may be needed to predict organic chemical−CNT interaction. Adsorption mechanisms will be better understood by investigating the effects of properties of both CNTs and organic chemicals along with environmental conditions. Another major factor affec...

A Two Micron All Sky Survey view of the Sagittarius dwarf galaxy. I. Morphology of the Sagittarius core and tidal arms

Abstract: We present the first all-sky view of the Sagittarius (Sgr) dwarf galaxy mapped by M-giant star tracers detected in the complete Two Micron All Sky Survey (2MASS). Near-infrared photometry of Sgr's prominent M-giant population permits an unprecedentedly clear view of the center of Sgr. The main body is fitted with a King profile of limiting major-axis radius 30°—substantially larger than previously found or assumed—beyond which is a prominent break in the density profile from stars in the Sgr tidal tails; thus the Sgr radial profile resembles that of Galactic dwarf speroidal (dSph) satellites. Adopting traditional methods for analyzing dSph light profiles, we determine the brightness of the main body of Sgr to be MV = -13.27 (the brightest of the known Galactic dSph galaxies) and the total Sgr mass-to-light ratio to be 25 in solar units. However, we regard the latter result with suspicion and argue that much of the observed structure beyond the King-fit core radius (224') may be outside the actual Sgr tidal radius as the former dwarf spiral/irregular satellite undergoes catastrophic disruption during its last orbits. The M-giant distribution of Sgr exhibits a central density cusp at the same location as, but not due to, the old stars constituting the globular cluster M54. A striking trailing tidal tail is found to extend from the Sgr center and arc across the south Galactic hemisphere with approximately constant density and mean distance varying from ~20 to 40 kpc. A prominent leading debris arm extends from the Sgr center northward of the Galactic plane to an apogalacticon ~45 kpc from the Sun and then turns toward the north Galactic cap (NGC), from where it descends back toward the Galactic plane, becomes foreshortened, and, at brighter magnitudes, covers the NGC. The leading and trailing Sgr tails lie along a well-defined orbital plane about the Galactic center. The Sun lies within a kiloparsec of that plane and near the path of leading Sgr debris; thus, it is possible that former Sgr stars are near or in the solar neighborhood. We discuss the implications of this new view of the Sgr galaxy and its entrails for the character of the Sgr orbit, mass, mass-loss rate, and contribution of stars to the Milky Way halo. The minimal precession displayed by the Sgr tidal debris along its inclined orbit supports the notion of a nearly spherical Galactic potential. The number of M giants in the Sgr tails is at least 15% that contained within the King limiting radius of the main Sgr body. The fact that M giants, presumably formed within the past few gigayears in the Sgr nucleus, are nevertheless so widespread along the Sgr tidal arms not only places limits on the dynamical age of these arms but also poses a timing problem that bears on the recent binding energy of the Sgr core and that is most naturally explained by recent and catastrophic mass loss. Sgr appears to contribute more than 75% of the high-latitude, halo M giants, despite substantial reservoirs of M giants in the Magellanic Clouds. No evidence of extended M-giant tidal debris from the Magellanic Clouds is found. Generally good correspondence is found between the M-giant, all-sky map of the Sgr system and all previously published detections of potential Sgr debris, with the exception of Sgr carbon stars, which must be subluminous compared with counterparts in other Galactic satellites in order to resolve the discrepancy.

The complexity of decentralized control of Markov decision processes

Abstract: Planning for distributed agents with partial state information is considered from a decisiontheoretic perspective. We describe generalizations of both the MDP and POMDP models that allow for decentralized control. For even a small number of agents, the finite-horizon problems corresponding to both of our models are complete for nondeterministic exponential time. These complexity results illustrate a fundamental difference between centralized and decentralized control of Markov processes. In contrast to the MDP and POMDP problems, the problems we consider provably do not admit polynomialtime algorithms and most likely require doubly exponential time to solve in the worst case. We have thus provided mathematical evidence corresponding to the intuition that decentralized planning problems cannot easily be reduced to centralized problems and solved exactly using established techniques.