关于Semigroups of Linear Operators and Applications to Partial Differential Equations的两个注解

Over the past two decades, a large amount of research effort has been devoted to developing algorithms that generate recommendations. The resulting research progress has established the importance of the user-item (U-I) matrix, which encodes the individual preferences of users for items in a collection, for recommender systems. The U-I matrix provides the basis for collaborative filtering (CF) techniques, the dominant framework for recommender systems. Currently, new recommendation scenarios are emerging that offer promising new information that goes beyond the U-I matrix. This information can be divided into two categories related to its source: rich side information concerning users and items, and interaction information associated with the interplay of users and items. In this survey, we summarize and analyze recommendation scenarios involving information sources and the CF algorithms that have been recently developed to address them. We provide a comprehensive introduction to a large body of research, more than 200 key references, with the aim of supporting the further development of recommender systems exploiting information beyond the U-I matrix. On the basis of this material, we identify and discuss what we see as the central challenges lying ahead for recommender system technology, both in terms of extensions of existing techniques as well as of the integration of techniques and technologies drawn from other research areas.

Collaborative Filtering beyond the User-Item Matrix: A Survey of the State of the Art and Future Challenges

While applications for mobile devices have become extremely important in the last few years, little public information exists on mobile application usage behavior. We describe a large-scale deployment-based research study that logged detailed application usage information from over 4,100 users of Android-powered mobile devices. We present two types of results from analyzing this data: basic descriptive statistics and contextual descriptive statistics. In the case of the former, we find that the average session with an application lasts less than a minute, even though users spend almost an hour a day using their phones. Our contextual findings include those related to time of day and location. For instance, we show that news applications are most popular in the morning and games are at night, but communication applications dominate through most of the day. We also find that despite the variety of apps available, communication applications are almost always the first used upon a device's waking from sleep. In addition, we discuss the notion of a virtual application sensor, which we used to collect the data.

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Falling asleep with Angry Birds, Facebook and Kindle: a large scale study on mobile application usage

The usefulness of mobile devices has increased greatly in recent years allowing users to perform more tasks in a mobile context. This increase in usefulness has come at the expense of the usability of these devices in some contexts. We conducted a small review of mobile usability models and found that usability is usually measured in terms of three attributes; effectiveness, efficiency and satisfaction. Other attributes, such as cognitive load, tend to be overlooked in the usability models that are most prominent despite their likely impact on the success or failure of an application. To remedy this we introduces the PACMAD (People At the Centre of Mobile Application Development) usability model which was designed to address the limitations of existing usability models when applied to mobile devices. PACMAD brings together significant attributes from different usability models in order to create a more comprehensive model. None of the attributes that it includes are new, but the existing prominent usability models ignore one or more of them. This could lead to an incomplete usability evaluation. We performed a literature search to compile a collection of studies that evaluate mobile applications and then evaluated the studies using our model.

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Usability of mobile applications: literature review and rationale for a new usability model

Systematic exploration of Android apps is an enabler for a variety of app analysis and testing tasks. Performing the exploration while apps run on actual phones is essential for exploring the full range of app capabilities. However, exploring real-world apps on real phones is challenging due to non-determinism, non-standard control flow, scalability and overhead constraints. Relying on end-users to conduct the exploration might not be very effective: we performed a 7-use study on popular Android apps, and found that the combined 7-use coverage was 30.08% of the app screens and 6.46% of the app methods. Prior approaches for automated exploration of Android apps have run apps in an emulator or focused on small apps whose source code was available. To address these problems, we present A3E, an approach and tool that allows substantial Android apps to be explored systematically while running on actual phones, yet without requiring access to the app's source code. The key insight of our approach is to use a static, taint-style, dataflow analysis on the app bytecode in a novel way, to construct a high-level control flow graph that captures legal transitions among activities (app screens). We then use this graph to develop an exploration strategy named Targeted Exploration that permits fast, direct exploration of activities, including activities that would be difficult to reach during normal use. We also developed a strategy named Depth-first Exploration that mimics user actions for exploring activities and their constituents in a slower, but more systematic way. To measure the effectiveness of our techniques, we use two metrics: activity coverage (number of screens explored) and method coverage. Experiments with using our approach on 25 popular Android apps including BBC News, Gas Buddy, Amazon Mobile, YouTube, Shazam Encore, and CNN, show that our exploration techniques achieve 59.39--64.11% activity coverage and 29.53--36.46% method coverage.

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Targeted and depth-first exploration for systematic testing of android apps

We have studied the low-field Meissner effect of polycrystalline Bi high-temperature superconductors using a special superconducting-quantum-interference-device magnetometer. In certain samples a surprising feature was observed: Instead of the usual diamagnetic moment a paramagnetic moment develops in the field cooling mode below ${\mathit{T}}_{\mathit{c}}$ for fields H1 Oe. The data are consistent with orbital paramagnetic moments due to spontaneous currents. Such currents may originate in so-called \ensuremath{\pi} contacts in the weak-link network of polycrystalline material. In some of these samples also an anomaly in the low-field microwave absorption was observed, which is obviously correlated with the existence of spontaneous currents.

Paramagnetic Meissner effect in high-temperature superconductors

Current technology development in mobile computing and upcoming application stores enable an easy development and distribution of mobile applications. This leads to an increasing number of available applications and to the user’s problem of content discovery. Recommender systems aim at guiding users to relevant items. Currently, recommender systems that suggest mobile applications neglect that the usage of mobile devices is characterized by perpetual changes of a user’s context. In this paper, we give rise to the contextaware recommendation of mobile applications. We explore the design space of recommender systems for mobile applications and describe the different dimensions and techniques for capturing the users, the items, the contexts and the corresponding relevances. For proof of concept we present the prototype of a recommender system that combines the design options in a hitherto unexplored way.

/pdf/exploring-the-design-space-of-context-aware-recommender-1czcn3wkpj.pdf

Exploring the Design Space of Context-aware Recommender Systems that Suggest Mobile Applications

The transient decay of large excess-carrier densities was investigated in ${\mathrm{Hg}}_{1\ensuremath{-}x}{\mathrm{Cd}}_{x}\mathrm{Te} (x=0.205)$ at 77 K. Excess carriers were generated by bulk impact ionization in strong electric fields. A strong variation of the carrier lifetime with excess-carrier density was observed in the transient excess-carrier decay. The comparison with calculated values shows that the carrier decay is determined by the Auger recombination process. To our knowledge this is the first observed Auger-dominated transient carrier decay in a semiconductor. A typical Auger-dominated lifetime for carriers at excess-carrier densities not far from thermal equilibrium is 460 nsec for ${n}_{0}=9\ifmmode\times\else\texttimes\fi{}{10}^{14}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ and ${\ensuremath{\mu}}_{H}=2.26\ifmmode\times\else\texttimes\fi{}{10}^{5}$ ${\mathrm{cm}}^{2}$/V sec at 77 K. This lifetime decreased to 90 nsec at an excess-carrier density equal to ${n}_{0}$. Photoconductivity experiments indicate that the majority-carrier lifetime can be enlarged due to minority-carrier trapping near contacts. This was established in photoresponse measurements with ${\mathrm{CO}}_{2}$ laser pulses. In addition to this, the Hall mobility was measured and analyzed between 4.2 and 200 K. Calculations for combined ionized-impurity and polar-optical-phonon scattering indicate that above 77 K the polar-optical-phonon scattering limits the mobility. This electron-phonon interaction was calculated taking into account the two-mode behavior of ${\mathrm{Hg}}_{1\ensuremath{-}x}{\mathrm{Cd}}_{x}\mathrm{Te}$. The high-field measurements have shown that in electric fields above 150 V/cm impact ionization occurs. Values of the carrier generation rate of ${\mathrm{Hg}}_{0.8}$${\mathrm{Cd}}_{0.2}$ Te were deduced from the time dependent $j(E)$ characteristics.

Transient carrier decay and transport properties in Hg 1-x Cd x Te

We prove global existence and uniqueness of classical solutions for the Maxwell-Lorentz system of a nonrotating rigid charge distribution, i.e.the relativistic dynamics of a nonrotating extended electron, which is subject to its own electromagnetic fields and an external potential. Local existence and uniqueness is achieved via the contraction mapping principle. Suitable a-priori-bounds yield global existence. We show that in case of a negative bare mass and an attracting external potential the stationary solution is unstable. We believe that this result clarifies the origin of the so-called "runaway"-solutions, which appear when the limit to a point charge is taken, formally described by the so called Lorentz-Dirac equation for the radiating electron.

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Gernot Bauer

Papers

Falling asleep with Angry Birds, Facebook and Kindle: a large scale study on mobile application usage

Paramagnetic Meissner effect in high-temperature superconductors

Exploring the Design Space of Context-aware Recommender Systems that Suggest Mobile Applications

Transient carrier decay and transport properties in Hg 1-x Cd x Te

The Maxwell-Lorentz System of a Rigid Charge