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Barman S.,California Institute of TechnologyCA | Chawla S.,University of Wisconsin - Madison | Umboh S.,University of Wisconsin - Madison
Leibniz International Proceedings in Informatics, LIPIcs | Year: 2014

We study network design with a cost structure motivated by redundancy in data traffic. We are given a graph, g groups of terminals, and a universe of data packets. Each group of terminals desires a subset of the packets from its respective source. The cost of routing traffic on any edge in the network is proportional to the total size of the distinct packets that the edge carries. Our goal is to find a minimum cost routing. We focus on two settings. In the first, the collection of packet sets desired by source-sink pairs is laminar. For this setting, we present a primaldual based 2-approximation, improving upon a logarithmic approximation due to Barman and Chawla (2012) [7]. In the second setting, packet sets can have non-trivial intersection. We focus on the case where each packet is desired by either a single terminal group or by all of the groups. This setting does not admit an O(log1/4-γ g)-approximation for any constant γ under a standard assumption; we present an O(log g)-approximation when the graph is unweighted. Our approximation for the second setting is based on a novel spanner-type construction in unweighted graphs that, given a collection of g vertex subsets, finds a subgraph of cost only a constant factor more than the minimum spanning tree of the graph, such that every subset in the collection has a Steiner tree in the subgraph of cost at most O(log g) that of its minimum Steiner tree in the original graph. We call such a subgraph a group spanner. © Siddharth Barman, Shuchi Chawla, and Seeun Umboh.

Cherukuri A.,University of California at San Diego | Mallada E.,California Institute of TechnologyCA | Cortes J.,University of California at San Diego
SIAM Conference on Control and Its Applications 2015 | Year: 2015

This paper characterizes the asymptotic convergence properties of the primal-dual dynamics to the solutions of a constrained concave optimization problem using classical notions from stability analysis. We motivate our study by providing an example which rules out the possibility of employing the invariance principle for hybrid automata to analyze the asymptotic convergence. We understand the solutions of the primal-dual dynamics in the Caratheodory sense and establish their existence, uniqueness, and continuity with respect to the initial conditions. We employ the invariance principle for Caratheodory solutions of a discontinuous dynamical system to show that the primal-dual optimizers are globally asymptotically stable under the primal-dual dynamics and that each solution of the dynamics converges to an optimizer.

Yamini S.A.,University of Wollongong | Mitchell D.R.G.,University of Wollongong | Gibbs Z.M.,California Institute of Technology | Santos R.,University of Wollongong | And 5 more authors.
Advanced Energy Materials | Year: 2015

Despite the effectiveness of sodium as a p-type dopant for lead chalcogenides, its solubility is shown to be very limited in these hosts. Here, a high thermoelectric efficiency of ≈2 over a wide temperature range is reported in multiphase quaternary (PbTe)0.65(PbS)0.25(PbSe)0.1 compounds that are doped with sodium at concentrations greater than the solubility limits of the matrix. Although these compounds present room temperature thermoelectric efficiencies similar to sodium doped PbTe, a dramatically enhanced Hall carrier mobility at temperatures above 600 K for heavily doped compounds results in significantly enhanced thermoelectric efficiencies at elevated temperatures. This is achieved through the composition modulation doping mechanism resulting from heterogeneous distribution of the sodium dopant between precipitates and the matrix at elevated temperatures. These results can lead to further advances in designing high performance multiphase thermoelectric materials with intrinsically heterogeneous dopant distributions. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Carbunaru C.,National University of Singapore | Teo Y.M.,National University of Singapore | Leong B.,National University of Singapore | Ho T.,California Institute of TechnologyCA
IEEE Transactions on Parallel and Distributed Systems | Year: 2014

Given the growing popularity of peer-to-peer file distribution in commercial applications, it is important to understand the challenges of using p2p file-sharing protocols for file distribution, and how extreme conditions such as flash crowds affect the efficiency of file distribution. In this light, there is a need to understand the impact of the utilization of available bandwidth on the performance of peer-assisted file distribution systems. With a simple measurement study on PlanetLab, we identified distinct phases in peer bandwidth utilization over the download duration. Based on the evolution of the utilization of available peer bandwidth over time, we formulated an analytical model for flash crowds in homogeneous and heterogeneous bandwidth swarms. The model estimates the instantaneous download rate and the average file download time with 10 percent error for swarms up to 160 peers. Our model can be used to predict the scalability of the system when the number of peers increases, and to provision for flash crowds by estimating the server bandwidth to achieve a minimum quality of service. Lastly, we demonstrate how our model is applied to new p2p protocols to understand their design and performance problems. © 2013 IEEE.

Noriega-Crespo A.,California Institute of TechnologyCA | Noriega-Crespo A.,US Space Telescope Science Institute | Raga A.C.,National Autonomous University of Mexico | Moro-Martin A.,US Space Telescope Science Institute | And 3 more authors.
New Journal of Physics | Year: 2014

We have used multiple mid-infrared observations at 4.5 μm obtained with the infrared array camera, of the compact (∼ 1.4′young stellar bipolar outflow Cep E to measure the proper motion of its brightest condensations. The images span a period of ∼6 yr and have been reprocessed to achieve a higher angular resolution (∼ 0.8″than their normal beam (∼2″). We found that for a distance of 730 pc, the tangential velocities of the north and south outflow lobes are and respectively, and moving away from the central source roughly along the major axis of the flow. A simple 3D hydrodynamical simulation of the H2 gas in a precessing outflow supports this idea. Observations and models confirm that the molecular hydrogen gas, traced by the pure rotational transitions, moves at highly supersonic velocities without being dissociated. This suggests either a very efficient mechanism to reform H2 molecules along these shocks or the presence of some other mechanism (e.g. strong magnetic field) that shields the H2 gas. © 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

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