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3,172 results on '"Le, Hong-An"'

1. D-CAPTCHA++: A Study of Resilience of Deepfake CAPTCHA under Transferable Imperceptible Adversarial Attack

Author

Nguyen-Le, Hong-Hanh, Tran, Van-Tuan, Nguyen, Dinh-Thuc, and Le-Khac, Nhien-An

Subjects
Computer Science - Cryptography and Security, Computer Science - Machine Learning, Computer Science - Sound, Electrical Engineering and Systems Science - Audio and Speech Processing
Abstract

The advancements in generative AI have enabled the improvement of audio synthesis models, including text-to-speech and voice conversion. This raises concerns about its potential misuse in social manipulation and political interference, as synthetic speech has become indistinguishable from natural human speech. Several speech-generation programs are utilized for malicious purposes, especially impersonating individuals through phone calls. Therefore, detecting fake audio is crucial to maintain social security and safeguard the integrity of information. Recent research has proposed a D-CAPTCHA system based on the challenge-response protocol to differentiate fake phone calls from real ones. In this work, we study the resilience of this system and introduce a more robust version, D-CAPTCHA++, to defend against fake calls. Specifically, we first expose the vulnerability of the D-CAPTCHA system under transferable imperceptible adversarial attack. Secondly, we mitigate such vulnerability by improving the robustness of the system by using adversarial training in D-CAPTCHA deepfake detectors and task classifiers., Comment: 14 pages

Published
2024

12. Unital $C_\infty$-algebras and the real homotopy type of $(r-1)$-connected compact manifolds of dimension $\le \ell(r-1)+2$

Author

Fiorenza, Domenico and Lê, Hông Vân

Subjects
Mathematics - Algebraic Topology, Mathematics - Commutative Algebra, Mathematics - Differential Geometry, Mathematics - K-Theory and Homology, 55P62, 57R19, 13D03, 58A10
Abstract

We encode the real homotopy type of an $n$-dimensional $(r-1)$-connected compact manifold $M$, $ r\ge 2$ into a minimal unital $C_\infty$-structure on $H^* (M,\mathbb R)$, obtained via homotopy transfer of the unital DGCA structure of the small quotient algebra associated with a Hodge decomposition of the de Rham algebra $\mathcal A^*(M)$, which has been proposed by Fiorenza-Kawai-L\^e-Schwachh\"ofer in \cite{FKLS2021}. We prove that if $n \le \ell (r-1) +2$, with $\ell \geq 4$, the multiplication $\mu_k$ on the minimal unital $C_\infty$-algebra $H^*(M,\mathbb R)$ vanishes for all $k \ge \ell-1$. This extends the results from \cite{FKLS2021}, extending the bound on the dimension from $5r-3$ to the general bound $\ell(r-1) +2$. We also prove a variant of this result, conjectured by Zhou, stating that if $n \le \ell(r-1)+4$ and $b_r (M) =1$ then the multiplication $\mu_k$ for all $k \ge \ell-1$ vanishes. This implies two formality results by Cavalcanti \cite{Cavalcanti2006}. We show that in any dimension $n$ the Harrison cohomology class $[\mu_3]\in \mathrm {HHarr}^{3,-1}(H^* (M, \mathbb R), H^*(M, \mathbb R)) $ is a homotopy invariant of $M$ and the first obstruction to formality, and provide a detailed proof that if $n\leq 4r-1$ this is the only obstruction. Furthermore, we show that in any dimension $n$ the class $[\mu_3]$ and the Bianchi-Massey tensor invented by Crowley-Nordstr\"om in \cite{CN} define each other uniquely., Comment: v4, 31 p., Subsection 2.3 added, proving Zhou's conjecture

Published
2023

13. Prevalence of Campylobacter and antibiotic susceptibility in chickens at slaughterhouses and retail markets in the Mekong Delta

Author

Le Hong Phong, Nguyen Phuc Bao Tran, Nguyen Thi Kim Cuc, Thanh Lam Nguyen, and Duc Hien Nguyen

Subjects
Veterinary medicine, SF600-1100
Abstract

This study aims to determine the prevalence of Campylobacter and its antibiotic susceptibility at slaughterhouses and live poultry markets in the Mekong Delta, Vietnam. A total of 177 samples, including 112 fecal samples, 28 cecal samples, and 37 rectal swab samples were collected. Thirty-nine samples tested positive for Campylobacter spp. by culture method, accounting for 22%. Isolation rates of Campylobacter in rectal swab samples were highest at 29.7% (11/37), followed by cecal samples at 25% (7/28) and fecal dropping samples at 18.8% (21/112). Species identification was performed by multiplex PCR and the result showed that the prevalence of Campylobacter coli (12.4%) was higher than Campylobacter jejuni (3.4%). Antibiotic susceptibility test showed that all isolated Campylobacter were susceptible to imipenem and meropenem (100%). Most Campylobacter were resistant to tetracycline (96.4%), ciprofloxacin (92.9%), nalidixic acid (89.3%), and ampicillin (67.9%). This study highlights the high contamination of Campylobacter and its antibiotic resistance in poultry in the Mekong Delta, Vietnam.

Published
2024
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14. Supervised learning with probabilistic morphisms and kernel mean embeddings

Author

Lê, Hông Vân

Subjects
Mathematics - Statistics Theory, Computer Science - Machine Learning, Mathematics - Category Theory, Mathematics - Functional Analysis, Mathematics - Probability, 46N30, 60B10, 62G05, 18N99
Abstract

In this paper I propose a generative model of supervised learning that unifies two approaches to supervised learning, using a concept of a correct loss function. Addressing two measurability problems, which have been ignored in statistical learning theory, I propose to use convergence in outer probability to characterize the consistency of a learning algorithm. Building upon these results, I extend a result due to Cucker-Smale, which addresses the learnability of a regression model, to the setting of a conditional probability estimation problem. Additionally, I present a variant of Vapnik-Stefanuyk's regularization method for solving stochastic ill-posed problems, and using it to prove the generalizability of overparameterized supervised learning models., Comment: V5: 51 p., minor correction, Corollary 6.13(2) added

Published
2023

20. APPLYING THE NOTE-TAKING METHOD TO ENHANCE LISTENING COMPREHENSION SKILLS FOR SECOND-YEAR ENGLISH LANGUAGE TEACHER EDUCATION STUDENTS AT VINH UNIVERSITY

Author

Le Hong Ninh, Hoang Thi Quynh Nhu, Lang Thi Mai Dung, Le Ha Trang, Le Thi Quynh Trang, Phung Nguyen Quynh Nga

Subjects
note-taking method, mixed-method intervention design, positive feedback, english pedagogical major, vinh university, Technology, Social sciences (General), H1-99
Abstract

This study evaluates the effectiveness of the note-taking method on the listening performance of second-year English language teacher education students and their feedbacks. The experimental study was conducted with a class of forty-four second-year students majoring in English language teacher education at Vinh University. The participants were divided into a pilot group and a control group. A mixed-method intervention design was applied, in which data were collected from pre-test, post-test, and semi-structured interviews. The results of statistical analysis, topic analysis, and comparison of data sets from tools showed that second-year English language teacher education students using the note-taking method performed better than those in the control group. More specifically, students in the experimental group made significant progress after applying the note-taking method. Most of the feedback was positive on the use of the note-taking method. Although research results are limited due to speaking speed and many other factors, the note-taking method should be used in teaching listening skills to English language teacher education students with some suggested notes

Published
2024
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21. The connection between vocabulary size and vocabulary learning strategies: a case study of English as foreign language learners at UD-UFLS

Author

Nguyen Anh Thi, Nguyen Le Hong Giang, and Nguyen Thi My Hang

Subjects
vocabulary learning strategies (vls), vocabulary size, second-english-language learners, Technology
Abstract

Undoubtedly, the significance of employing effective strategies for vocabulary acquisition in language learning cannot be overstated. Following this, the authors' research delves into the potential relationship between vocabulary learning strategies (VLS) and the vocabulary size of undergraduate students at the University of Danang - University of Foreign Language Studies (UD-UFLS). This study has three primary objectives: firstly, to gather and analyze the diverse strategies employed by undergraduate students for acquiring new English vocabulary; secondly, to examine the vocabulary size of these university students; and finally, to investigate the impact of VLS on the overall vocabulary size of the learners. Through this comprehensive approach, the authors' aim to gain deeper insights into the dynamic interplay between strategies and vocabulary knowledge, ultimately serving as a foundation for students at the university to make well-informed choices as they embark on their language -learning journey.

Published
2024

22. High-Fiber Crackers Supplemented with Asparagus Hard-Stem: Impacts of Supplementation Ratios and Water Amounts in Cracker Recipe on the Product Quality

Author

Thi Thu Tra Tran, Le Hong Ngoc Ngo, Nu Minh Nguyet Ton, Thi Hong Nhung Le, Thi Thuy Le, and Van Viet Man Le

Subjects
antioxidant capacity, asparagus officinalis l, color parameters, consumer acceptability, dietary fiber, hardness, Nutrition. Foods and food supply, TX341-641
Abstract

Spears or cladophylls are edible parts of asparagus ( Asparagus officinalis L.) implemented in a human diet while its hard-stem by-products have been used for animal feeding. In this study, the asparagus hard-stem was proved to be rich in dietary fiber and total phenolics with high antioxidant capacity. Wheat flour was partially replaced by asparagus hard-stem powder (AHP) in the cracker recipe and the AHP ratios were 0 (control), 5, 10, 15, 20% of the blend weight; the nutritional constituents, antioxidant capacities, physical attributes, and sensory overall acceptability of crackers were then evaluated. As the AHP ratio increased from 0 to 20%, the dietary fiber and total phenolic contents of the fortified crackers were improved by 5.0 times and 3.2 times, respectively, while their ferric reducing antioxidant power and DPPH scavenging capacity were enhanced by 6.1 and 1.4 times, respectively. Besides, the elevated ratio of AHP also increased the product hardness and reduced its overall acceptability. The impacts of water amount used in the dough kneading on the high-fiber cracker quality were then investigated. At 20% AHP level, the appropriate water amount was 55 g/100 g of the flour blend to reduce hardness and improve overall acceptability of the fortified crackers. The study results show that AHP is a potential dietary fiber and antioxidant ingredient for high-fiber cracker making.

Published
2024
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23. Semisimple elements and the little Weyl group of real semisimple $Z_m$-graded Lie algebras

Author

de Graaf, Willem and Lê, Hông Vân

Subjects
Mathematics - Representation Theory, Mathematics - Differential Geometry, Mathematics - Group Theory, Mathematics - Rings and Algebras, 11E72, 20G05, 20G20
Abstract

We consider the semisimple orbits of a Vinberg $\theta$-representation. First we take the complex numbers as base field. By a case by case analysis we show a technical result stating the equality of two sets of hyperplanes, one corresponding to the restricted roots of a Cartan subspace, the other corresponding to the complex reflections in the (little) Weyl group. The semisimple orbits have representatives in a finite number of sets that correspond to reflection subgroups of the (little) Weyl group. One of the consequences of our technical result is that the elements in a fixed such set all have the same stabilizer in the acting group. Secondly we study what happens when the base field is the real numbers. We look at Cartan subspaces and show that the real Cartan subspaces can be classified by the first Galois cohomology set of the normalizer of a fixed real Cartan subspace. In the real case the orbits can be classified using Galois cohomology. However, in order for that to work we need to know which orbits have a real representative. We show a theorem that characterizes the orbits of homogeneous semisimple elements that do have such a real representative. This closely follows and generalizes a theorem from \cite{bgl}., Comment: Revised version, 18 p

Published
2022
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24. A Robust and Low Complexity Deep Learning Model for Remote Sensing Image Classification

Author

Le, Cam, Pham, Lam, NVN, Nghia, Nguyen, Truong, and Trang, Le Hong

Subjects
Computer Science - Computer Vision and Pattern Recognition, Computer Science - Machine Learning, Electrical Engineering and Systems Science - Image and Video Processing
Abstract

In this paper, we present a robust and low complexity deep learning model for Remote Sensing Image Classification (RSIC), the task of identifying the scene of a remote sensing image. In particular, we firstly evaluate different low complexity and benchmark deep neural networks: MobileNetV1, MobileNetV2, NASNetMobile, and EfficientNetB0, which present the number of trainable parameters lower than 5 Million (M). After indicating best network architecture, we further improve the network performance by applying attention schemes to multiple feature maps extracted from middle layers of the network. To deal with the issue of increasing the model footprint as using attention schemes, we apply the quantization technique to satisfy the maximum of 20 MB memory occupation. By conducting extensive experiments on the benchmark datasets NWPU-RESISC45, we achieve a robust and low-complexity model, which is very competitive to the state-of-the-art systems and potential for real-life applications on edge devices., Comment: 8 pages

Published
2022

26. Enhancing Resilience Strategies for Non-traditional Security Threats in Global Logistics and Supply Chains Within the Vietnamese Agriculture Sector

Author

Nguyen, Phi-Hung, Le, Hong-Quan, Kacprzyk, Janusz, Series Editor, Gomide, Fernando, Advisory Editor, Kaynak, Okyay, Advisory Editor, Liu, Derong, Advisory Editor, Pedrycz, Witold, Advisory Editor, Polycarpou, Marios M., Advisory Editor, Rudas, Imre J., Advisory Editor, Wang, Jun, Advisory Editor, Kahraman, Cengiz, editor, Cevik Onar, Sezi, editor, Cebi, Selcuk, editor, Oztaysi, Basar, editor, Tolga, A. Cagrı, editor, and Ucal Sari, Irem, editor

Published
2024
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27. A Study on Multi-objective Optimization of Dressing Process in Surface Grinding Hardox 500

Author

Thanh, Dinh Van, Ky, Le Hong, Dang, Nguyen, Danh, Tran Huu, Anh, Nguyen Duc, Hung, Le Xuan, Pi, Vu Ngoc, Tung, Luu Anh, Kacprzyk, Janusz, Series Editor, Gomide, Fernando, Advisory Editor, Kaynak, Okyay, Advisory Editor, Liu, Derong, Advisory Editor, Pedrycz, Witold, Advisory Editor, Polycarpou, Marios M., Advisory Editor, Rudas, Imre J., Advisory Editor, Wang, Jun, Advisory Editor, Nguyen, Duy Cuong, editor, Hai, Do Trung, editor, Vu, Ngoc Pi, editor, Long, Banh Tien, editor, Puta, Horst, editor, and Sattler, Kai-Uwe, editor

Published
2024
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28. A Study on the Influence of Plasma Nitriding Technology Parameters on the Working Surface Roughness of Hypoid Gears

Author

Hiep, Nguyen Hoang, Ky, Le Hong, Van, Nguyen Thai, Ranh, Ta Van, Kacprzyk, Janusz, Series Editor, Gomide, Fernando, Advisory Editor, Kaynak, Okyay, Advisory Editor, Liu, Derong, Advisory Editor, Pedrycz, Witold, Advisory Editor, Polycarpou, Marios M., Advisory Editor, Rudas, Imre J., Advisory Editor, Wang, Jun, Advisory Editor, Nguyen, Duy Cuong, editor, Hai, Do Trung, editor, Vu, Ngoc Pi, editor, Long, Banh Tien, editor, Puta, Horst, editor, and Sattler, Kai-Uwe, editor

Published
2024
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29. Investigation of the Particle Velocity of the Amorphous Powder

Author

Duong, Vu, Ky, Le Hong, Kacprzyk, Janusz, Series Editor, Gomide, Fernando, Advisory Editor, Kaynak, Okyay, Advisory Editor, Liu, Derong, Advisory Editor, Pedrycz, Witold, Advisory Editor, Polycarpou, Marios M., Advisory Editor, Rudas, Imre J., Advisory Editor, Wang, Jun, Advisory Editor, Nguyen, Duy Cuong, editor, Hai, Do Trung, editor, Vu, Ngoc Pi, editor, Long, Banh Tien, editor, Puta, Horst, editor, and Sattler, Kai-Uwe, editor

Published
2024
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30. Physical Mechanism and Numerical Simulation of Landslide Dam Formation

Author

Van Tien, Pham, Nhan, Tran Thanh, Luong, Le Hong, Cuong, Tran Quoc, Sassa, Kyoji, Series Editor, Konagai, Kazuo, Series Editor, Sassa, Shinji, Series Editor, Abolmasov, Biljana, editor, Alcántara-Ayala, Irasema, editor, Arbanas, Željko, editor, Huntley, David, editor, Mihalić Arbanas, Snježana, editor, Mikoš, Matjaž, editor, Ramesh, Maneesha V., editor, Tang, Huiming, editor, and Tiwari, Binod, editor

Published
2024
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31. Design an Intelligent Braking System Using Ultrasonic Sensors and IR Sensors

Author

Quan, Vu Hai, Ngoc, Nguyen Anh, Quan, Le Hong, Khoa, Nguyen Xuan, Khanh, Nguyen Quoc, Tao, Ngo Quang, Chaari, Fakher, Series Editor, Gherardini, Francesco, Series Editor, Ivanov, Vitalii, Series Editor, Haddar, Mohamed, Series Editor, Cavas-Martínez, Francisco, Editorial Board Member, di Mare, Francesca, Editorial Board Member, Kwon, Young W., Editorial Board Member, Tolio, Tullio A. M., Editorial Board Member, Trojanowska, Justyna, Editorial Board Member, Schmitt, Robert, Editorial Board Member, Xu, Jinyang, Editorial Board Member, Long, Banh Tien, editor, Ishizaki, Kozo, editor, Kim, Hyung Sun, editor, Kim, Yun-Hae, editor, Toan, Nguyen Duc, editor, Minh, Nguyen Thi Hong, editor, and Duc An, Pham, editor

Published
2024
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33. Image Restoration under Semantic Communications

Author

Van Chien, Trinh, Phong, Le Hong, Phuc, Dao Xuan, and Hoa, Nguyen Tien

Subjects
Computer Science - Information Theory
Abstract

Semantic communication has emerged as the breakthrough beyond the Shannon theorem by transmitting and receiving semantic information instead of data bits or symbols regardless of its content. This paper proposes a two-stage reconstruction process to boost the system's performance. In the first phase, the image information is first decoded from the noisy received data by exploiting the channel knowledge. The decoded image is enhanced by a post-filter and image statistics. Different metrics are exploited to evaluate the image restoration quality of our considered model. Numerical results are obtained using natural images that verify the superior improvements of the proposed two-stage reconstruction process over the traditional decoded data. Moreover, the different metrics assessing the system performance based on their criteria can be conflicted with each other., Comment: 6 pages, 5 figures. Accepted by the IEEE ATC 2022

Published
2022

34. Natural differentiable structures on statistical models and the Fisher metric

Author

Lê, Hông Vân

Subjects
Mathematics - Differential Geometry, Mathematics - Probability, Mathematics - Statistics Theory, 53B12, 6202, 62B11
Abstract

In this paper I discuss the relation between the concept of the Fisher metric and the concept of differentiability of a family of probability measures. I compare the concepts of smooth statistical manifolds, differentiable families of measures, $k$-integrable parameterized measure models, diffeological statistical models, differentiable measures, which arise in Information Geometry, mathematical statistics and measure theory, and discuss some related problems., Comment: 22p. submitted version to Information Geometry

Published
2022
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37. Potential of a deep eutectic solvent in silver nanoparticle fabrication for antibiotic residue detection

Author

Le Hong Tho, Bui Xuan Khuyen, Ngoc Xuan Dat Mai, and Nhu Hoa Thi Tran

Subjects
ag nps, antibiotic residue, deep eutectic solvents, potential, sers, Technology, Chemical technology, TP1-1185, Science, Physics, QC1-999
Abstract

Deep eutectic solvents (DESs) have recently emerged as an alternative solvent for nanoparticle synthesis. There have been numerous advancements in the fabrication of silver nanoparticles (Ag NPs), but the potential of DESs in Ag NP synthesis was neither considered nor studied carefully. In this study, we present a novel strategy to fabricate Ag NPs in a DES (Ag NPs-DES). The DES composed of ᴅ-glucose, urea, and glycerol does not contain any anions to precipitate with Ag+ cations. Our Ag NPs-DES sample is used in a surface-enhanced Raman scattering (SERS) sensor. The two analytes for SERS quantitation are nitrofurantoin (NFT) and sulfadiazine (SDZ) whose residues can be traced down to 10−8 M. The highest enhancement factors (EFs) are competitive at 6.29 × 107 and 1.69 × 107 for NFT and SDZ, respectively. Besides, the linearity coefficients are extremely close to 1 in the range of 10−8 to 10−3 M of concentration, and the SERS substrate shows remarkable uniformity along with great selectivity. This powerful SERS performance indicates that DESs have tremendous potential in the synthesis of nanomaterials for biosensor substrate construction.

Published
2024
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38. Mechanical and electrical properties of self-healable rubber blends under influence of imidazole mixture and selective wetting of hybrid filler

Author

Le Hong Hai, Hoang Xuan Tung, Yashwanth Sai Anjaneya Varma Kosuri, Subhradeep Mandal, Kedar Nath Dhakal, Rameshwar Adhikari, Beate Langer, and Sven Wiessner

Subjects
crosslinking, filler, mixing, nanocomposites, elastomer, Materials of engineering and construction. Mechanics of materials, TA401-492, Chemical technology, TP1-1185
Abstract

Flexible rubber composites based on blends of bromobutyl rubber (BIIR)/epoxidized natural rubber (ENR) filled with a hybrid filler of layered double hydroxides (LDH)/carbon nanotubes (CNT) were prepared, which may be of interest for use in tires and inner liner. An imidazole mixture of butyl-imidazole and 1H-imidazole was proposed to achieve reversible non-covalent networks consisting of ionic clusters and hydrogen bonding that led to the self-healing properties of each blend phase. The objective of the present work was to characterize the effects of selective modification of the rubber blends by imidazole and selective wetting of the hybrid filler on the mechanical and electrical properties and the self-healing effectivity. The wetting concept used to characterize the selective wetting of a single filler in a rubber blend was further developed for a hybrid filler in BIIR/ENR blends. During the one-step mixing and the masterbatch mixing process, re-localization of the hybrid filler between the blend phases under thermodynamic driving forces was observed. Within the specified mixing time, the thermodynamic equilibrium state of filler localization could not be reached. The nearly uniform distribution of LDH in both blend phases and the preferential localization of CNTs in the ENR matrix were found to be essential in giving the BIIR/ENR blends the best combination of self-healing properties and electrical conductivity. The high electrical conductivity of the composites can be exploited to generate a high temperature locally at the damage site inside the rubber samples when an electrical voltage is applied, resulting in an acceleration of the self-healing process.

Published
2024
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39. Hardware System Implementation for Human Detection using HOG and SVM Algorithm

Author

Nguyen, Van-Cam, Le, Hong-Tuan-Dinh, and Huynh, Huu-Thuan

Subjects
Computer Science - Hardware Architecture, Computer Science - Computer Vision and Pattern Recognition
Abstract

Human detection is a popular issue and has been widely used in many applications. However, including complexities in computation, leading to the human detection system implemented hardly in real-time applications. This paper presents the architecture of hardware, a human detection system that was simulated in the ModelSim tool. As a co-processor, this system was built to off-load to Central Processor Unit (CPU) and speed up the computation timing. The 130x66 RGB pixels of static input image attracted features and classify by using the Histogram of Oriented Gradient (HOG) algorithm and Support Vector Machine (SVM) algorithm, respectively. As a result, the accuracy rate of this system reaches 84.35 percent. And the timing for detection decreases to 0.757 ms at 50MHz frequency (54 times faster when this system was implemented in software by using the Matlab tool).

Published
2022

41. A Bibliometric Analysis of Research on Education 4.0 during the 2017-2021 Period

Author

Dao, Lai Thai, Tran, Trung, Van Le, Hong, Nguyen, Giang Ngoc, and Trinh, Thao Phuong Thi

Abstract

A bibliometric analysis of 309 publications related to Education 4.0 published in both Scopus and Web of Science (WoS) databases has been conducted to analyze the development of this research field since the first articles published in 2017. Although being an emerging field, the number of publications increased quickly. Scholars from developing countries (i.e., Malaysia, Indonesia, Mexico, Brazil, and Philippines) are among the most productive authors, but internal and cross-country collaborations between them are very limited. Quality of the publication collection is not high as within the top ten most popular journals, five journals are not qualified to the Scopus database since 2020, and only three journals belong to the WoS core collections. Research topics related to Education 4.0 can be classified into five main clusters, including application of modern technologies on Education 4.0, effects of Industry 4.0 on teaching and learning activities, Education 4.0 in engineering education, education innovations in Education 4.0, and empirical research and quantitative analyses using questionnaires on different aspects of Education 4.0.

Published
2023
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43. CR-twistor spaces over manifolds with $G_2$- and $Spin(7)$-structures

Author

Fiorenza, Domenico and Lê, Hông Vân

Subjects
Mathematics - Differential Geometry, Mathematics - Representation Theory, 53C28, 53C10, 22E45
Abstract

In 1984 LeBrun constructed a CR-twistor space over an arbitrary conformal Riemannian 3-manifold and proved that the CR-structure is formally integrable. This twistor construction has been generalized by Rossi in 1985 for $m$-dimensional Riemannian manifolds endowed with a $(m-1)$-fold vector cross product (VCP). In 2011 Verbitsky generalized LeBrun's construction of twistor-spaces to $7$-manifolds endowed with a $G_2$-structure. In this paper we unify and generalize LeBrun's, Rossi's and Verbitsky's construction of a CR-twistor space to the case where a Riemannian manifold $(M, g)$ has a VCP structure. We show that the formal integrability of the CR-structure is expressed in terms of a torsion tensor on the twistor space, which is a Grassmanian bundle over $(M, g)$. If the VCP structure on $(M,g)$ is generated by a $G_2$- or $Spin(7)$-structure, then the vertical component of the torsion tensor vanishes if and only if $(M, g)$ has constant curvature, and the horizontal component vanishes if and only if $(M,g)$ is a torsion-free $G_2$ or $Spin(7)$-manifold. Finally we discuss some open problems., Comment: Version 3: 35 p. the main theorem is strengthened, a few straightforward computations are omitted, the exposition is improved

Published
2022
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47. Lựa chọn giải pháp đấu nối và phân tích ảnh hưởng của các nhà máy điện gió ngoài khơi đến ổn định hệ thống điện

Author

Le Hong Lam, Le Hoang Viet, Le Cao Quyen, and Tran Tan Vinh

Subjects
nhà máy điện gió ngoài khơi, vsc, lcc, hvdc, hệ thống điện, Technology
Abstract

Bài báo nghiên cứu lựa chọn các giải pháp đấu nối nhà máy điện gió ngoài khơi vào hệ thống điện Việt Nam, với hai giải pháp đấu nối hiện nay là truyền tải điện xoay chiều (HVAC) và truyền tải điện một chiều (HVDC). Bài báo đã thực hiện tính toán phân tích ảnh hưởng của nhà máy điện gió ngoài khơi đến hệ thống điện dựa vào các tiêu chuẩn kỹ thuật yêu cầu đảm bảo khi thực hiện đấu nối với hai giải pháp truyền tải trên. Ngoài các tính toán trào lưu công suất và phân tích ảnh hưởng đến hệ thống điện bằng hai giải pháp đấu nối, bài báo đã thực hiện phân tích kinh tế tổng mức đầu tư cho hai giải pháp đấu nối cho nhà máy Điện Gió Ngoài Khơi (ĐGNK) Trung Nam công suất 800MW đang đề xuất bổ sung Quy hoạch nhằm lựa chọn giải pháp đấu nối cho nhà máy điện gió ngoài khơi đảm bảo về mặt kỹ thuật và kinh tế cho dự án. Kết quả cho thấy, phương án đấu nối bằng HDVC đảm bảo được các yếu tố về kỹ thuật và kinh tế.

Published
2024

48. Finding Approximately Convex Ropes in the Plane

Author

Trang, Le Hong, Le, Nguyen Thi, and An, Phan Thanh

Subjects
Mathematics - Optimization and Control, Computer Science - Computational Geometry, 52A30 (Primary), 52B55, 68Q25 (Secondary)
Abstract

The convex rope problem is to find a counterclockwise or clockwise convex rope starting at the vertex a and ending at the vertex b of a simple polygon P, where a is a vertex of the convex hull of P and b is visible from infinity. The convex rope mentioned is the shortest path joining a and b that does not enter the interior of P. In this paper, the problem is reconstructed as the one of finding such shortest path in a simple polygon and solved by the method of multiple shooting. We then show that if the collinear condition of the method holds at all shooting points, then these shooting points form the shortest path. Otherwise, the sequence of paths obtained by the update of the method converges to the shortest path. The algorithm is implemented in C++ for numerical experiments., Comment: 21 pages, 10 figures, added references for the proof of Theorem 3 Corrected some spelling errors and replace some figures

Published
2022

49. Propose an incentive methodology of renewable energy to participate in competitive electricity market

Author

Le Hong Lam, Le Quang Son, and Nguyen Tan Thanh

Subjects
renewable energy, electricity market, fixed feed-in premium, cap & floor, incentive mechanism, Technology
Abstract

Today, the rapidly increasing share of renewable energy sources requires a suitable mechanism to promote the development of renewable energy is urgently needed. To solve this problem, the paper will conduct a study to find out the incentive mechanism to promote the development of renewable energy with participation in the electricity market. The article compares different models of Fixed feed-in premium with CAP and FLOOR (month, week, day, and cycle) to point out the advantages and disadvantages of each model, thereby providing comments and assessing the impact of the models on the revenue of renewable energy plants. The result shows that the monthly average revenue of FIXED FIP with CAP and FLOOR (0.19-0.23 million VND/MWh) is less variation than half-hourly (0.16-0.23 million VND/MWh) and daily revenue (0.18-0.23 million VND/MWh). Moreover, the monthly model can mitigate the variation of spot price and cop with the payment period (monthly).

Published
2023
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50. Connectedness of Vietnamese bank stock returns under the impact of the COVID-19 pandemic

Author

Nguyen Phu Ha, Luong Tram Anh, and Le Hong Thai

Subjects
banks, connectedness, COVID-19 pandemic, stock returns, TVP-VAR, Banking, HG1501-3550
Abstract

The COVID-19 pandemic highlighted the sensitivity of connectedness among bank stock returns in Vietnam. The aim of this study is to examine the strength of this connectedness along with the effect of government lockdown policy and COVID-19 cases on the total connectedness index (TCI) of 16 listed banks on Vietnamese stock exchanges. They are assessed using the database of FiinPro on the banking sector between January 2020 and July 2022, Vietnam Center for Disease Control and Prevention (CDC), and The World Health Organization (WHO) on the COVID-19 pandemic, employing a time-varying-parameter vector autoregressive (TVP-VAR) connectedness framework and the conditional quantile regression model. The results show that at the firm level, there is strong interdependence among bank stock returns with the average TCI being as high as 90.66%. It is also revealed that medium and large-sized banks are receivers of shock, while smaller banks are transmitters. As far as the impact on TCI is concerned, the widespread of the pandemic with the increasing number of COVID-19 cases is significantly negative, whereas the tightening of lockdown is significantly positive. Besides, the degree of the impact varies according to the 95th, 75th, 50th and 25th levels of conditional quantile regression. Based on the study’s findings, individual investors are recommended to thoroughly analyze the connectedness of banks before making investment decisions, while bank regulators should strengthen controls on credit relationships with small banks. Regarding policy makers, it is proposed to apply flexible restrictions and short-term lockdown depending on the actual outbreak of the pandemic. AcknowledgmentThe paper was conducted within the scope of Project QG21.48 of Vietnam National University.

Published
2023
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