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DSpace at MIT (76,953 recursos)
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MIT Open Access Articles

Mostrando recursos 41 - 60 de 13,579

41. Microfabrication of Optically Flat Silicon Micro-Mirrors for Fully Programmable Micro-Diffraction Gratings - Timotijevic, B.; Lockhart, R.; Stanley, R.; Luetzelschwab, M.; Zamkotsian, F.; Lanzoni, P.; Noell, W.; Tormen, M.; Canonica, Michael D.
We have fabricated and characterized a Fully Programmable Micro-Diffraction Grating (FPMDG) with 64 silicon micro-mirrors for spectral shaping in the visible and near-infrared wavelength range. The FPMDG arrays of 50 μm and 80 μm wide and 700 μm long silicon micro-mirrors have been fabricated in a process based on anodic bonding of an 8 μm-SOI wafer and a borosilicate glass wafer. The detrimental bending of the micro-mirrors during electrostatic actuation has been minimized through separation of the mechanical and optical sections of the device. Flexures incorporating serpentine structures have been used to reduce the actuation dependence on length and thickness....

42. Three Quantitative Management Problems in Public Procurement and Decision Procedures for their Analysis and Solving - Belenky, Alexander
Three management problems that a state (or a public administration acting on its behalf) faces in procuring goods and/or services are considered: a) choosing the type of a contract to be awarded and the type of a competitive bidding to determine the winning bid, b) setting the initial price for a contract being the subject of the bidding, and c) designing (or choosing) a set of rules for determining the winning bid by means of the chosen competitive bidding. Mathematical models and decision procedures for analyzing and solving these problems are discussed.

43. Radiation damage tolerant nanomaterials - Beyerlein, I.J.; Caro, A.; Mara, N.A.; Misra, A.; Uberuaga, B.P.; Demkowicz, Michael J.
Designing a material from the atomic level to achieve a tailored response in extreme conditions is a grand challenge in materials research. Nanostructured metals and composites provide a path to this goal because they contain interfaces that attract, absorb and annihilate point and line defects. These interfaces recover and control defects produced in materials subjected to extremes of displacement damage, impurity implantation, stress and temperature. Controlling radiation-induced-defects via interfaces is shown to be the key factor in reducing the damage and imparting stability in certain nanomaterials under conditions where bulk materials exhibit void swelling and/or embrittlement. We review the recovery...

44. Coal-CO[subscript 2] Slurry Feed for Pressurized Gasifiers: Slurry Preparation System Characterization and Economics - Botero, Cristina; Herzog, Howard J.; Ghoniem, Ahmed F.; Field, Randall
Gasification-based plants with coal-CO[subscript 2] slurry feed are predicted to be more efficient than those with coal-water slurry feed. This is particularly true for high moisture, low rank coal such as lignite. Nevertheless, preparation of the CO[subscript 2] slurry is challenging and the losses associated with this process have not been accounted for in previous analyses. This work introduces the Phase Inversion-based Coal-CO[subscript 2] Slurry (PHICCOS) feeding system, in which coal-CO[subscript 2] slurry is prepared at ambient temperature via coal-water slurry. Steady-state process simulation is used to estimate the performance of the proposed slurry preparation and feeding system for bituminous...

45. Search for excited leptons in pp collisions at √s = 7 TeV - Apyan, Aram; Bauer, Gerry P.; Bendavid, Joshua L.; Busza, Wit; Butz, Erik M.; Cali, Ivan Amos; Chan, M.; Dutta, V.; Gomez-Ceballos, Guillelmo; Goncharov, Maxim; Hahn, Kristian Allan; Kim, Y.; Klute, Markus; Krajczar, Krisztian F.; Luckey, P. David, Jr.; Ma, Teng; Nahn, Steven; Paus, Christoph M. E.; Ralph, Duncan Kelley; Roland, Christof E.; Roland, Gunther M.; Rudolph, M.; Stephans, George S. F.; Stockli, Fabian; Sumorok, Konstanty C.; Sung, Kevin Kai Hong; Velicanu, Dragos Alexandru; Wenger, Edward Allen; Wolf, R.; Wyslouch, Boleslaw; Yang, M.; Yilmaz, Yetkin; Yoon, A. S.; Zanetti, Marco
Results are presented of a search for compositeness in electrons and muons using a data sample of pp collisions at a center-of-mass energy √s = 7 TeV collected with the CMS detector at the LHC and corresponding to an integrated luminosity of 5.0 fb[superscript −1]. Excited leptons (ℓ[superscript ⁎]) are assumed to be produced via contact interactions in conjunction with a standard model lepton and to decay via ℓ[superscript ⁎] → ℓγ, yielding a final state with two energetic leptons and a photon. The number of events observed in data is consistent with that expected from the standard model. The...

46. Agent-based Housing Market Microsimulation for Integrated Land Use, Transportation, Environment Model System - Rosenfield, Adam; Chingcuanco, Franco; Miller, Eric J.
The Housing Market Evolutionary System (HoMES) is the updated housing market module for the Integrated Land Use, Transportation, Environment (ILUTE) model system. HoMES is a disaggregate, agent-based microsimulation of the owner-occupied housing market, with models for households’ residential mobility decisions, location choices and valuations, the endogenous supply of housing by type and location, and the endogenous determination of sale prices and rents. The new model offers significant improvements over previous attempts by including a reformulated market clearing mechanism, market dependency on macro-economic conditions, and improved computational performance. A 100% synthesized population is validated against historical data for the Greater Toronto-Hamilton...

47. An Electrochemically-mediated Gas Separation Process for Carbon Abatement - Stern, Michael C.; Simeon, Fritz; Hatton, T. Alan; Herzog, Howard J.
This work describes a promising alternative to conventional thermal processes for absorber/desorber processing of for removal of CO[subscript 2] from flue gas streams at fossil fuel fired power plants. Our electrochemically-mediated amine regeneration (EMAR) process offers the advantages of an electrical system coupled with the desirable high output purities typical of amine sorbents that are difficult to achieve with most electric systems such as pressure-swing sorption, membrane separation, and oxy-fuel combustion. Preliminary experimental results are presented that demonstrate the feasibility of using ethylenediamine as the CO[subscript 2] sorbent and copper electro-cycling to isothermally modulate the amine affinity for CO[subscript 2]....

48. Smart subtitles for vocabulary learning - Kovacs, Geza; Miller, Robert C.
Language learners often use subtitled videos to help them learn. However, standard subtitles are geared more towards comprehension than vocabulary learning, as translations are nonliteral and are provided only for phrases, not vocabulary. This paper presents Smart Subtitles, which are interactive subtitles tailored towards vocabulary learning. Smart Subtitles can be automatically generated from common video sources such as subtitled DVDs. They provide features such as vocabulary definitions on hover, and dialog-based video navigation. In our pilot study with intermediate learners studying Chinese, participants correctly defined over twice as many new words in a post-viewing vocabulary test when they used Smart...

49. Crowdsourcing step-by-step information extraction to enhance existing how-to videos - Nguyen, Phu Tran; Weir, Sarah; Guo, Philip J.; Miller, Robert C.; Gajos, Krzysztof Z.; Kim, Ju Ho
Millions of learners today use how-to videos to master new skills in a variety of domains. But browsing such videos is often tedious and inefficient because video player interfaces are not optimized for the unique step-by-step structure of such videos. This research aims to improve the learning experience of existing how-to videos with step-by-step annotations. We first performed a formative study to verify that annotations are actually useful to learners. We created ToolScape, an interactive video player that displays step descriptions and intermediate result thumbnails in the video timeline. Learners in our study performed better and gained more self-efficacy using ToolScape...

50. Understanding in-video dropouts and interaction peaks in online lecture videos - Guo, Philip J.; Seaton, Daniel T.; Mitros, Piotr; Gajos, Krzysztof Z.; Miller, Robert C.; Kim, Ju Ho
With thousands of learners watching the same online lecture videos, analyzing video watching patterns provides a unique opportunity to understand how students learn with videos. This paper reports a large-scale analysis of in-video dropout and peaks in viewership and student activity, using second-by-second user interaction data from 862 videos in four Massive Open Online Courses (MOOCs) on edX. We find higher dropout rates in longer videos, re-watching sessions (vs first-time), and tutorials (vs lectures). Peaks in re-watching sessions and play events indicate points of interest and confusion. Results show that tutorials (vs lectures) and re-watching sessions (vs first-time) lead to...

51. Feature engineering for clustering student solutions - Glassman, Elena L.; Singh, Rishabh; Miller, Robert C.
Open-ended homework problems such as coding assignments give students a broad range of freedom for the design of solutions. We aim to use the diversity in correct solutions to enhance student learning by automatically suggesting alternate solutions. Our approach is to perform a two-level hierarchical clustering of student solutions to first partition them based on the choice of algorithm and then partition solutions implementing the same algorithm based on low-level implementation details. Our initial investigations in domains of introductory programming and computer architecture demonstrate that we need two different classes of features to perform effective clustering at the two levels,...

52. Frenzy: Collaborative data organization for creating conference sessions - Chilton, Lydia B.; Cordeiro, Felicia; Landay, James A.; Weld, Daniel S.; Dow, Steven P.; Miller, Robert C.; Zhang, Haoqi; Kim, Ju Ho; Andre, Paul
Organizing conference sessions around themes improves the experience for attendees. However, the session creation process can be difficult and time-consuming due to the amount of expertise and effort required to consider alternative paper groupings. We present a collaborative web application called Frenzy to draw on the efforts and knowledge of an entire program committee. Frenzy comprises (a) interfaces to support large numbers of experts working collectively to create sessions, and (b) a two-stage process that decomposes the session-creation problem into meta-data elicitation and global constraint satisfaction. Meta-data elicitation involves a large group of experts working simultaneously, while global constraint satisfaction...

53. Wait-learning: Leveraging conversational dead time for second language education - Guo, Philip J.; Miller, Robert C.; Cai, Carrie Jun; Glass, James R.
Second-language learners are often unable to find time for language practice due to constraints in their daily lives. In this paper, we examine how brief moments of waiting during a person's existing social conversations can be leveraged for second language practice, even if the conversation is exchanged in the first language. We present an instant messaging (IM) prototype, WaitChatter, that supports the notion of wait-learning by displaying contextually relevant foreign language vocabulary and micro-quizzes while the user awaits a response from her conversant. The foreign translations are displayed just-in-time in the context of the conversation to promote incidental learning. In...

54. Foreign manga reader: Learn grammar and pronunciation while reading comics - Kovacs, Geza; Miller, Robert C.
Foreign-language comics are potentially an enjoyable way to learn foreign languages. However, the difficulty of reading authentic material makes them inaccessible to novice learners. We present the Foreign Manga Reader, a system that helps readers comprehend foreign-language written materials and learn multiple aspects of the language. Specifically, it generates a sentence-structure visualization to help learners understand the grammar, pronounces dialogs to improve listening comprehension and pronunciation, and translates dialogs, phrases, and words to teach vocabulary. Learners can use the system to match their experience level, giving novices access to dialog-level translations and pronunciations, and more advanced learners with access to...

55. Cobi: A community-informed conference scheduling tool - Zhang, Haoqi; Chilton, Lydia B.; Mackay, Wendy; Beaudouin-Lafon, Michel; Miller, Robert C.; Dow, Steven P.; Kim, Ju Ho; Andre, Paul
Effectively planning a large multi-track conference requires an understanding of the preferences and constraints of organizers, authors, and attendees. Traditionally, the onus of scheduling the program falls on a few dedicated organizers. Resolving conflicts becomes difficult due to the size and complexity of the schedule and the lack of insight into community members' needs and desires. Cobi presents an alternative approach to conference scheduling that engages the entire community in the planning process. Cobi comprises (a) communitysourcing applications that collect preferences, constraints, and affinity data from community members, and (b) a visual scheduling interface that combines communitysourced data and constraint-solving...

56. Optimal and Efficient Parametric Auctions - Azar, Pablo; Micali, Silvio; Weinberg, S. Matthew; Daskalakis, Konstantinos
Consider a seller who seeks to provide service to a collection of interested parties, subject to feasibility constraints on which parties may be simultaneously served. Assuming that a distribution is known on the value of each party for service—arguably a strong assumption—Myerson's seminal work provides revenue optimizing auctions [12]. We show instead that, for very general feasibility constraints, only knowledge of the median of each party's value distribution, or any other quantile of these distributions, or approximations thereof, suffice for designing simple auctions that simultaneously approximate both the optimal revenue and the optimal welfare. Our results apply to all downward-closed...

57. Optimization of Lyapunov Invariants in Verification of Software Systems - Roozbehani, Mardavij; Megretski, Alexandre; Feron, Eric
The paper proposes a control-theoretic framework for verification of numerical software systems, and puts forward software verification as an important application of control and systems theory. The idea is to transfer Lyapunov functions and the associated computational techniques from control systems analysis and convex optimization to verification of various software safety and performance specifications. These include but are not limited to absence of overflow, absence of division-by-zero, termination in finite time, absence of dead-code, and certain user-specified assertions. Central to this framework are Lyapunov invariants. These are properly constructed functions of the program variables, and satisfy certain properties-analogous to those...

58. Stable nonlinear identification from noisy repeated experiments via convex optimization - Tobenkin, Mark M.; Manchester, Ian R.; Megretski, Alexandre
This paper introduces new techniques for using convex optimization to fit input-output data to a class of stable nonlinear dynamical models. We present an algorithm that guarantees consistent estimates of models in this class when a small set of repeated experiments with suitably independent measurement noise is available. Stability of the estimated models is guaranteed without any assumptions on the input-output data. We first present a convex optimization scheme for identifying stable state-space models from empirical moments. Next, we provide a method for using repeated experiments to remove the effect of noise on these moment and model estimates. The technique...

59. Explicit Solutions for Root Optimization of a Polynomial Family With One Affine Constraint - Blondel, Vincent D.; Gurbuzbalaban, Mert; Megretski, Alexandre; Overton, Michael L.
Given a family of real or complex monic polynomials of fixed degree with one affine constraint on their coefficients, consider the problem of minimizing the root radius (largest modulus of the roots) or root abscissa (largest real part of the roots). We give constructive methods for efficiently computing the globally optimal value as well as an optimal polynomial when the optimal value is attained and an approximation when it is not. An optimal polynomial can always be chosen to have at most two distinct roots in the real case and just one distinct root in the complex case. Examples are...

60. Quantum Support Vector Machine for Big Data Classification - Mohseni, Masoud; Lloyd, Seth; Rebentrost, Frank Patrick
Supervised machine learning is the classification of new data based on already classified training examples. In this work, we show that the support vector machine, an optimized binary classifier, can be implemented on a quantum computer, with complexity logarithmic in the size of the vectors and the number of training examples. In cases where classical sampling algorithms require polynomial time, an exponential speedup is obtained. At the core of this quantum big data algorithm is a nonsparse matrix exponentiation technique for efficiently performing a matrix inversion of the training data inner-product (kernel) matrix.

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