Search

Massive Open Online Courses (MOOCs) have become a popular learning mode in recent years, especially since the outbreak of COVID-19 in late 2019, which had resulted in a significant increase in associated research. This paper presents a bibliometric review of 1078 peer-reviewed MOOC studies between 2008 and 2019. These papers are extracted from three influential databases, the Web of Science (WOS), Scopus, and the Education Resources Information Center (ERIC). The MOOC literature analysis with a bibliometric approach identified the research trends, journals, countries/regions, and institutions with high H-index, scientific collaborations, research topics, topic distributions of the prolific countries/regions and institutions, and annual topic distributions, after which the representative research and research implications were discussed. This review gives researchers a deep and comprehensive understanding of current MOOC research and identifies potential research topics and collaborative partners, which supports MOOC-related future research.

Large-amplitude current-driven plasma instabilities, which can transition to the Buneman instability, were observed in one-dimensional simulations to generate high-energy back-streaming ions. We investigate the saturation of multi-dimensional plasma instabilities and its effects on energetic ion formation. Such ions directly impact spacecraft thruster lifetimes and are associated with magnetic reconnection and cosmic ray inception. An Eulerian Vlasov–Poisson solver employing the grid-based direct kinetic method is used to study the growth and saturation of 2D2V collisionless, electrostatic current-driven instabilities spanning two dimensions each in the configuration (D) and velocity (V) spaces supporting ion and electron phase-space transport. Four stages characterise the electric potential evolution in such instabilities: linear modal growth, harmonic growth, accelerated growth via quasi-linear mechanisms alongside nonlinear fill-in and saturated turbulence. Its transition and isotropisation process bears considerable similarities to the development of hydrodynamic turbulence. While a tendency to isotropy is observed in the plasma waves, followed by electron and then ion phase spaces after several ion-acoustic periods, the formation of energetic back-streaming ions is more limited in the 2D2V than in the 1D1V simulations. Plasma waves formed by two-dimensional electrostatic kinetic instabilities can propagate in the direction perpendicular to the net electron drift. Thus, large-amplitude multi-dimensional waves generate high-energy transverse-streaming ions and eventually limit energetic backward-streaming ions along the longitudinal direction. The multi-dimensional study sheds light on interactions between longitudinal and transverse electrostatic plasma instabilities, as well as fundamental characteristics of the inception and sustenance of unmagnetised plasma turbulence. [ABSTRACT FROM AUTHOR]

Interfacial Rayleigh–Taylor mixing is crucial to describing important natural and engineering processes, such as exploding supernovae, laser micromachining, hot spots in inertial confinement fusion, and optical telecommunications. These require the characterization of the time dependence of the driving acceleration. We compare our theoretical formulation based on group theory foundations with interface-capturing numerical simulations for linear and nonlinear two-dimensional Rayleigh–Taylor instabilities in a finite-sized domain with time-varying acceleration over broad ranges of Atwood numbers and acceleration exponents. Detailed corroboration between theory and simulations is provided for this foundational case. Both demonstrate the strong interfacial nature of Rayleigh–Taylor instabilities, which suggests that practical flow fields can be reconstructed from the derived fluid potential using the proposed theory. A robust agreement is also obtained for the early and late-time evolution of the amplitudes of the bubble and spike, which demonstrate that the Rayleigh–Taylor flow can transition to the mixing regime even for a single-mode initial perturbation. Corroboration with experiments of high energy density plasmas motivated by studies of supernovae is also achieved. In addition, a long-standing puzzle in Rayleigh–Taylor dynamics on the interplay between the acceleration, the shear, and the interface morphology in the theory and simulations is resolved by accounting for finite viscosity of the fluids. The characterization of Rayleigh–Taylor instabilities as a highly interfacial phenomenon provides valuable insight into its multiscale nature, which enhances the design and understanding of numerous processes of practical interest. [ABSTRACT FROM AUTHOR]

Many efficient open-source suffix sorters using the induced sorting (IS) method to build the fundamental data structure suffix array (SA) for compressing and indexing data have been proposed. To avoid potential faults caused by possible implementation bugs, checking the output SA from any IS sorter without engineering warranty for correctness is a de-facto process. The existing SA checkers commonly perform checking after an SA is built completely, with significant time and space complexities compared with that of builders. This article proposes an efficient solution for building and checking SA simultaneously by enhancing the original IS method with a checking scheme using hash computations to on-the-fly verify the results produced by the last induction phase of IS method. Given an input of constant alphabet, this checking scheme requires linear time and constant RAM space when running on external memory, and its time and space overheads are negligible compared with that for building SA. In our experiments on real-world data, the proposed methods take advantages over the counterparts of existing SA checkers by running faster with less space. This work can help provide a value-added bonus feature for open-source IS sorters to guarantee the correctness of a built SA, and such a feature should be desirable for applications using these sorters. [ABSTRACT FROM AUTHOR]

Three-dimensional Rayleigh–Taylor instability (RTI) with the time-varying acceleration in a finite domain is investigated in a systematic framework. The acceleration magnitude follows a power law in time with an exponent greater than −2. Applying the group theory, the instabilities are demonstrated considering the irreducible representations for observable periodic structures with a square symmetry in the plane normal to the acceleration. We derive the dynamical system and illustrate the universal form of the solutions in the linear and nonlinear regimes. The scale-dependent dynamics are shown to be single scale and multiscale in the two regimes, respectively. For the nonlinear regime solutions, fundamental scales are derived bridging the solutions in the finite- and infinite-sized domains. Special solutions for bubbles and spikes are identified from a one-parameter family of solutions. The effect of domain confinement is that the velocity and curvature decreases and shear increases as the domain size is reduced. The theory provides predictions for the flow field and demonstrates the interfacial behavior of RTI. Our results are in good agreement with the prior studies and also provide new benchmarks for experiments and simulations. [ABSTRACT FROM AUTHOR]

Breaking waves generate a distribution of bubble sizes that evolves over time. Knowledge of how this distribution evolves is of practical importance for maritime and climate studies. The analytical framework developed in Part 1 (Chan, Johnson & Moin, J. Fluid Mech., vol. 912, 2021, A42) examined how this evolution is governed by the bubble-mass flux from large- to small-bubble sizes which depends on the rate of break-up events and the distribution of child bubble sizes. These statistics are measured in Part 2 as ensemble-averaged functions of time by simulating ensembles of breaking waves, and identifying and tracking individual bubbles and their break-up events. The large-scale break-up dynamics is seen to be statistically unsteady, and two intervals with distinct characteristics were identified. In the first interval, the dissipation rate and bubble-mass flux are quasi-steady, and the theoretical analysis of Part 1 is supported by all observed statistics, including the expected $-10/3$ power-law exponent for the super-Hinze-scale size distribution. Strong locality is observed in the corresponding bubble-mass flux, supporting the presence of a super-Hinze-scale break-up cascade. In the second interval, the dissipation rate decays, and the bubble-mass flux increases as small- and intermediate-sized bubbles become more populous. This flux remains strongly local with cascade-like behaviour, but the dominant power-law exponent for the size distribution increases to $-8/3$ as small bubbles are also depleted more quickly. This suggests the emergence of different physical mechanisms during different phases of the breaking-wave evolution, although size-local break-up remains a dominant theme. Parts 1 and 2 present an analytical toolkit for population balance analysis in two-phase flows. [ABSTRACT FROM AUTHOR]

Breaking waves entrain gas beneath the surface. The wave-breaking process energizes turbulent fluctuations that break bubbles in quick succession to generate a wide range of bubble sizes. Understanding this generation mechanism paves the way towards the development of predictive models for large-scale maritime and climate simulations. Garrett et al. (J. Phys. Oceanogr., vol. 30, 2000, pp. 2163–2171) suggested that super-Hinze-scale turbulent break-up transfers entrained gas from large- to small-bubble sizes in the manner of a cascade. We provide a theoretical basis for this bubble-mass cascade by appealing to how energy is transferred from large to small scales in the energy cascade central to single-phase turbulence theories. A bubble break-up cascade requires that break-up events predominantly transfer bubble mass from a certain bubble size to a slightly smaller size on average. This property is called locality. In this paper, we analytically quantify locality by extending the population balance equation in conservative form to derive the bubble-mass-transfer rate from large to small sizes. Using our proposed measures of locality, we show that scalings relevant to turbulent bubbly flows, including those postulated by Garrett et al. (J. Phys. Oceanogr., vol. 30, 2000, pp. 2163–2171) and observed in breaking-wave experiments and simulations, are consistent with a strongly local transfer rate, where the influence of non-local contributions decays in a power-law fashion. These theoretical predictions are confirmed using numerical simulations in Part 2 (Chan et al., J. Fluid. Mech. vol. 912, 2021, A43), revealing key physical aspects of the bubble break-up cascade phenomenology. Locality supports the universality of turbulent small-bubble break-up, which simplifies the development of cascade-based subgrid-scale models to predict oceanic small-bubble statistics of practical importance. [ABSTRACT FROM AUTHOR]

With the rapid development of the Internet, an increasing number of users enjoy to shop online and express their reviews on the products and services. Analysis of these online reviews can not only help potential users make rational decisions when purchasing but also improves the quality of products and services. Hence, sentiment analysis for online reviews has become an important and meaningful research domain. [ABSTRACT FROM AUTHOR]

Sorting all suffixes of an input string $X$ will produce the suffix array that is a fundamental data structure for full-text search on $X$ . To utilize the parallel computing power of a multicore machine with shared memory, this article designs a fast linear-time and in-place parallel algorithm called pSACAK, for sorting the suffixes of an input string with a constant alphabet. This algorithm is a parallel variant of the sequential suffix sorting algorithm SACAK which improved the linear-time SAIS to be in-place for constant alphabets, and hence requires only a workspace of $\mathcal {O}(K)$ for alphabet size $K$ . While our recent work has successfully designed the parallel variant of SAIS on a multicore machine, it remains a challenge to parallelize SACAK due to the strong data dependencies caused by the in-place constraint. A number of new techniques are proposed here to overcome the difficulties for designing pSACAK from the sequential SACAK. An experimental study is conducted to evaluate the performance of pSACAK versus other existing parallel suffix sorting algorithms. Our experimental results show that pSACAK is the most time and space efficient among all in comparison. To the best of our knowledge, pSACAK is the only linear-time and in-place parallel suffix sorting algorithm for constant alphabets reported so far. [ABSTRACT FROM AUTHOR]

Semiquantum key distribution (SQKD) allows two parties to share a common string when one of them is quantum and the other has rather limited quantum capability. Almost all existing SQKD protocols have been proved to be robust in theory, namely that if an eavesdropper tries to gain information, he will inevitably induce some detectable errors. However, ideal devices do not exist in reality and their imperfection may result in side-channel attacks, which can be used by an adversary to get some information on the secret key string. In this paper, we design a measurement-device-independent SQKD protocol for the first time, which can remove the threat of all detector side-channel attacks and show that it is also robust. In addition, we discuss the possible use of the proposed protocol in real-world applications and in QKD networks. [ABSTRACT FROM AUTHOR]

For full-text indexing of massive data, the suffix and LCP (longest common prefix) arrays have been recognized as fundamental data structures, and there are at least two needs in practice for checking their correctness, i.e., program debugging and verifying the arrays constructed by probabilistic algorithms. Two probabilistic methods are proposed to check the suffix and LCP arrays of constant or integer alphabets in external memory using a Karp-Rabin fingerprinting technique, where the checking is wrong only with a negligible error probability. The first method checks the lexicographical order and the LCP-value of two suffixes by computing and comparing the fingerprints of their LCPs. This method is general in terms of that it can verify any full or sparse suffix/LCP array of any order. The second method uses less space, it first employs the fingerprinting technique to verify a subset of the given suffix and LCP arrays, from which two new suffix and LCP arrays are induced and compared with the given arrays for verification, where the induced suffix and LCP arrays can be removed for constant alphabets to save space. [ABSTRACT FROM AUTHOR]

Computing the Lempel-Ziv factorization (LZ77) of a string is a key step in many applications. However, at the same time, it constitutes a bottleneck of the entire computation. The investigation of time and space efficient computation of the LZ77 has become an important topic. In this paper, we present a lightweight linear-time algorithm called LZone for computing the LZ77, which is designed by improvements on the existing linear-time space efficient LZ77 algorithm BGone for speed acceleration. For an input string T[1.. n] over a constant alphabet size of O(1), LZone requires only n words of workspace in addition to the input string and the output factorization, ⌈log n⌉ bits per word. This is the same space requirement for the algorithm BGone. LZone has two versions, LZoneT and LZoneSA, corresponding to BGoneT and BGoneSA, respectively. Our experimental results show that for computing the LZ77 from an input string T, LZoneT and LZoneSA run at around 26 % and 57 %, respectively, faster than their counterparts in BGone. Moreover, for computing the LZ77 from the suffix array of T, the speed of LZoneSA is on average twice that of BGoneSA. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The dynamic and unpredictable nature of energy harvesting sources available for wireless sensor networks, and the time variation in network statistics like packet transmission rates and link qualities, necessitate the use of adaptive duty cycling techniques. Such adaptive control allows sensor nodes to achieve long-run energy neutrality, where energy supply and demand are balanced in a dynamic environment such that the nodes function continuously. In this paper, we develop a new framework enabling an adaptive duty cycling scheme for sensor networks that takes into account the node battery level, ambient energy that can be harvested, and application-level QoS requirements. We model the system as a Markov decision process (MDP) that modifies its state transition policy using reinforcement learning. The MDP uses continuous time Markov chains (CTMCs) to model the network state of a node to obtain key QoS metrics like latency, loss probability, and power consumption, as well as to model the node battery level taking into account physically feasible rates of change. We show that with an appropriate choice of the reward function for the MDP, as well as a suitable learning rate, exploitation probability, and discount factor, the need to maintain minimum QoS levels for optimal network performance can be balanced with the need to promote the maintenance of a finite battery level to ensure node operability. Extensive simulation results show the benefit of our algorithm for different reward functions and parameters. [ABSTRACT FROM PUBLISHER]

Abstract: In this paper, we study the strong game colouring number of directed graphs. It is proved that every oriented partial 2-tree has strong game colouring number at most 7 and every oriented planar graph has strong game colouring number at most 15. [Copyright &y& Elsevier]

We present, in this paper, two efficient algorithms for linear time suffix array construction. These two algorithms achieve their linear time complexities, using the techniques of divide-and-conquer, and recursion. What distinguish the proposed algorithms from other linear time suffix array construction algorithms (SACAs) are the variable-length leftmost S-type (LMS) substrings and the fixed-length d-critical substrings sampled for problem reduction, and the simple algorithms for sorting these sampled substrings: the induced sorting algorithm for the variable-length LMS substrings and the radix sorting algorithm for the fixed-length d-critical substrings. The very simple sorting mechanisms render our algorithms an elegant design framework, and, in turn, the surprisingly succinct implementations. The fully functional sample implementations of our proposed algorithms require only around 100 lines of C code for each, which is only 1/10 of the implementation of the KA [CHECK END OF SENTENCE] algorithm and comparable to that of the KS [CHECK END OF SENTENCE] algorithm. The experimental results demonstrate that these two newly proposed algorithms yield the best time and space efficiencies among all the existing linear time SACAs. [ABSTRACT FROM AUTHOR]

Abstract: The Laplacian spectral radius of a graph is the largest eigenvalue of the associated Laplacian matrix. In this paper, we provide structural and behavioral details of graphs with maximum Laplacian spectral radius among all bipartite connected graphs of given order and size. Using these results, we provide a unified approach to determine the graphs with maximum Laplacian spectral radii among all trees, and all bipartite unicyclic, bicyclic, tricyclic and quasi-tree graphs, respectively. [Copyright &y& Elsevier]

Abstract: A set of vertices in a graph is a paired-dominating set of if every vertex of is adjacent to some vertex in and the subgraph induced by contains a perfect matching. The minimum cardinality of a paired-dominating set of is the paired-domination number of , denoted by . In this work, we present several upper bounds on the paired-domination number in terms of the maximum degree, minimum degree, girth and order. [Copyright &y& Elsevier]

Purpose: The aim of this study was to evaluate the difference between intraocular pressure (IOP) measurements in children with the Icare tonomer (IT) and applanation (AT), pneumatic (PT), or digital tonometers (TT)., Design: A randomized prospective trial of children younger than age 16 attending the pediatric ophthalmology department of Manchester Royal Eye Hospital was conducted., Methods: Children had IOP measured twice, once with an IT and again with a TT, PT, or AT during the same clinic appointment., Results: Forty-four children (88 eyes) were included, with a mean [range (R)] age of 57 (2-144) months. Twelve eyes had anterior segment pathology (ASP), defined as aniridia, congenital glaucoma, or Peters anomaly. Regardless of the presence or absence of ASP, total mean difference (MD), R, positive bias (PB), and limits of agreement (LOA) between IT and other instruments were as follows: PT: MD = 3.2, R = 0 to 8, PB = 2.9, LOA = -1.0 to 6.9; TT: MD = 2.6, R = 0 to 6, PB = 0.9, LOA = -1.8 to 3.5; and AT: MD = 1.4, R = -3 to 5, PB = 0.6, LOA = -4.2 to 7.3. In eyes with ASP, IT comparisons with PT were as follows: MD = 3.9, R = 0 to 8, PB = 3.9, LOA = -0.9 to 8.8., Conclusions: In children with ASP, IOP measured with IT is higher than expected when compared with other tonometers, in some cases by up to 8 mm Hg. We found an overestimation of IOP in children using the IT with a positive bias of 4 mm Hg. We propose that the IT overestimates IOP measurements in children compared with standard tonometers. Further work should be carried out on a much larger cohort to establish a suitable correction factor for such children.

Background: Information about diagnostic features of tuberculosis (TB) peritonitis among patients with end-stage renal failure (ESRF), compared with those without ESRF, is limited. Here, we review our experience with TB peritonitis in patients with and without ESRF over a period of 11 years, with special reference to the clinical features of and diagnostic tools for TB peritonitis., Methods: The records of all patients with TB peritonitis who were seen at United Christian Hospital (Hong Kong) from 1996 through 2006 were reviewed. The diagnosis of TB peritonitis was based on (1) detection of mycobacteria on smear and/or Mycobacterium tuberculosis on culture of ascitic fluid and/or peritoneal biopsy specimens and/or (2) demonstration of caseating granulomata on histological assessment of peritoneal biopsy specimens., Results: During the study period, 52 patients (19 without ESRF and 33 with ESRF) had TB peritonitis. Patients with ESRF tended to have more-acute onset of symptoms and neutrophil-predominant peritoneal fluid, which mimicked bacterial peritonitis. Diagnosis was made earlier among patients without ESRF than among those with ESRF. Laparotomy or laparoscopy was the initial diagnostic tool for 12 patients (63%) without ESRF, whereas mycobacterial culture was the diagnostic method for 18 patients (55%) with ESRF., Conclusions: TB peritonitis has nonspecific and protean manifestations. It should be considered in the context of culture-negative peritonitis and in the context of culture-positive peritonitis that fails to respond to appropriate antibiotics. Laparoscopy with biopsy should be considered at an early stage when TB peritonitis is suspected.