Mathematics

5 pages, 198 KiB

Open AccessEditorial

Mathematics in Finite Element Modeling of Computational Friction Contact Mechanics 2021–2022

by Nicolae Pop, Marin Marin and Sorin Vlase

Mathematics 2023, 11(1), 255; https://doi.org/10.3390/math11010255 - 3 Jan 2023

Cited by 3 | Viewed by 2647

In engineering practice, structures with identical components or parts are useful from several points of view: less information is needed to describe the system; designs can be conceptualized quicker and easier; components are made faster than during traditional complex assembly; and finally, the [...] Read more.

In engineering practice, structures with identical components or parts are useful from several points of view: less information is needed to describe the system; designs can be conceptualized quicker and easier; components are made faster than during traditional complex assembly; and finally, the time needed to achieve the structure and the cost involved in manufacturing decrease. Additionally, the subsequent maintenance of this system then becomes easier and cheaper. The aim of this Special Issue is to provide an opportunity for international researchers to share and review recent advances in the finite element modeling of computational friction contact mechanics. Numerical modeling in mathematics, mechanical engineering, computer science, computers, etc. presents many challenges. The finite element method applied in solid mechanics was designed by engineers to simulate numerical models in order to reduce the design costs of prototypes, tests and measurements. This method was initially validated only by measurements but gave encouraging results. After the discovery of Sobolev spaces, the abovementioned results were obtained, and today, numerous researchers are working on improving this method. Some of applications of this method in solid mechanics include mechanical engineering, machine and device design, civil engineering, aerospace and automotive engineering, robotics, etc. Frictional contact is a complex phenomenon that has led to research in mechanical engineering, computational contact mechanics, composite material design, rigid body dynamics, robotics, etc. A good simulation requires that the dynamics of contact with friction be included in the formulation of the dynamic system so that an approximation of the complex phenomena can be made. To solve these linear or nonlinear dynamic systems, which often have non-differentiable terms, or discontinuities, software that considers these high-performance numerical methods and computers with high computing power are needed. This Special Issue is dedicated to this kind of mechanical structure and to describing the properties and methods of analysis of these structures. Discrete or continuous structures in static and dynamic cases are also considered. Additionally, theoretical models, mathematical methods and numerical analysis of these systems, such as the finite element method and experimental methods, are used in these studies. Machine building, automotive, aerospace and civil engineering are the main areas in which such applications appear, but they can also be found in most other engineering fields. With this Special Issue, we want to disseminate knowledge among researchers, designers, manufacturers and users in this exciting field. Full article

(This article belongs to the Special Issue Finite Element Modeling in Computational Friction Contact Mechanics)

19 pages, 973 KiB

Open AccessArticle

Estimating the Risk of Contracting COVID-19 in Different Settings Using a Multiscale Transmission Dynamics Model

by Dramane Sam Idris Kanté, Aissam Jebrane, Anass Bouchnita and Abdelilah Hakim

Mathematics 2023, 11(1), 254; https://doi.org/10.3390/math11010254 - 3 Jan 2023

Cited by 7 | Viewed by 3577

Abstract

Airborne transmission is the dominant route of coronavirus disease 2019 (COVID-19) transmission. The chances of contracting COVID-19 in a particular situation depend on the local demographic features, the type of inter-individual interactions, and the compliance with mitigation measures. In this work, we develop [...] Read more.

Airborne transmission is the dominant route of coronavirus disease 2019 (COVID-19) transmission. The chances of contracting COVID-19 in a particular situation depend on the local demographic features, the type of inter-individual interactions, and the compliance with mitigation measures. In this work, we develop a multiscale framework to estimate the individual risk of infection with COVID-19 in different activity areas. The framework is parameterized to describe the motion characteristics of pedestrians in workplaces, schools, shopping centers and other public areas, which makes it suitable to study the risk of infection under specific scenarios. First, we show that exposure to individuals with peak viral loads increases the chances of infection by 99%. Our simulations suggest that the risk of contracting COVID-19 is especially high in workplaces and residential areas. Next, we determine the age groups that are most susceptible to infection in each location. Then, we show that if 50% of the population wears face masks, this will reduce the chances of infection by 8%, 32%, or 45%, depending on the type of the used mask. Finally, our simulations suggest that compliance with social distancing reduces the risk of infection by 19%. Our framework provides a tool that assesses the location-specific risk of infection and helps determine the most effective behavioral measures that protect vulnerable individuals. Full article

(This article belongs to the Special Issue Mathematical Modelling in Biomedicine III)

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21 pages, 927 KiB

Open AccessArticle

Equation-Based Modeling vs. Agent-Based Modeling with Applications to the Spread of COVID-19 Outbreak

by Selain K. Kasereka, Glody N. Zohinga, Vogel M. Kiketa, Ruffin-Benoît M. Ngoie, Eddy K. Mputu, Nathanaël M. Kasoro and Kyamakya Kyandoghere

Mathematics 2023, 11(1), 253; https://doi.org/10.3390/math11010253 - 3 Jan 2023

Cited by 12 | Viewed by 4092

Abstract

In this paper, we explore two modeling approaches to understanding the dynamics of infectious diseases in the population: equation-based modeling (EBM) and agent-based modeling (ABM). To achieve this, a comparative study of these approaches was conducted and we highlighted their advantages and disadvantages. [...] Read more.

In this paper, we explore two modeling approaches to understanding the dynamics of infectious diseases in the population: equation-based modeling (EBM) and agent-based modeling (ABM). To achieve this, a comparative study of these approaches was conducted and we highlighted their advantages and disadvantages. Two case studies on the spread of the COVID-19 pandemic were carried out using both approaches. The results obtained show that differential equation-based models are faster but still simplistic, while agent-based models require more machine capabilities but are more realistic and very close to biology. Based on these outputs, it seems that the coupling of both approaches could be an interesting compromise. Full article

(This article belongs to the Special Issue Mathematical Methods for Computer Science)

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17 pages, 2652 KiB

Open AccessArticle

Estimation of Coefficient of Variation Using Calibrated Estimators in Double Stratified Random Sampling

by Usman Shahzad, Ishfaq Ahmad, Amelia V. García-Luengo, Tolga Zaman, Nadia H. Al-Noor and Anoop Kumar

Mathematics 2023, 11(1), 252; https://doi.org/10.3390/math11010252 - 3 Jan 2023

Cited by 11 | Viewed by 2744

Abstract

One of the most useful indicators of relative dispersion is the coefficient of variation. The characteristics of the coefficient of variation have contributed to its widespread use in most scientific and academic disciplines, with real life applications. The traditional estimators of the coefficient [...] Read more.

One of the most useful indicators of relative dispersion is the coefficient of variation. The characteristics of the coefficient of variation have contributed to its widespread use in most scientific and academic disciplines, with real life applications. The traditional estimators of the coefficient of variation are based on conventional moments; therefore, these are highly affected by the presence of extreme values. In this article, we develop some novel calibration-based coefficient of variation estimators for the study variable under double stratified random sampling (DSRS) using the robust features of linear (L and TL) moments, which offer appropriate coefficient of variation estimates. To evaluate the usefulness of the proposed estimators, a simulation study is performed by using three populations out of which one is based on the COVID-19 pandemic data set and the other two are based on apple fruit data sets. The relative efficiency of the proposed estimators with respect to the existing estimators has been calculated. The superiority of the suggested estimators over the existing estimators are clearly validated by using the real data sets. Full article

(This article belongs to the Special Issue Functional Data Analysis: Theory and Applications to Different Scenarios)

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15 pages, 3361 KiB

Open AccessArticle

Newton-Based Extremum Seeking for Dynamic Systems Using Kalman Filtering: Application to Anaerobic Digestion Process Control

by Yang Tian, Ning Pan, Maobo Hu, Haoping Wang, Ivan Simeonov, Lyudmila Kabaivanova and Nicolai Christov

Mathematics 2023, 11(1), 251; https://doi.org/10.3390/math11010251 - 3 Jan 2023

Cited by 3 | Viewed by 1913

Abstract

In this paper, a new Newton-based extremum-seeking control for dynamic systems is proposed using Kalman filter for gradient and Hessian estimation as well as a stochastic perturbation signal with time-varying amplitude. The obtained Kalman filter based Newton extremum-seeking control (KFNESC) makes it possible [...] Read more.

In this paper, a new Newton-based extremum-seeking control for dynamic systems is proposed using Kalman filter for gradient and Hessian estimation as well as a stochastic perturbation signal with time-varying amplitude. The obtained Kalman filter based Newton extremum-seeking control (KFNESC) makes it possible to accelerate the convergence to the extremum and attenuate the steady-state oscillations. The convergence and oscillation attenuation properties of the closed-loop system with KFNESC are considered, and the proposed control is applied to a two-stages anaerobic digestion process in order to maximize the hydrogen production rate, which has better robustness and a slower steady-state oscillation with the comparison of Newton-based ESC and sliding mode ESC. Full article

(This article belongs to the Special Issue Automatic Control and Soft Computing in Engineering)

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17 pages, 888 KiB

Open AccessEditor’s ChoiceArticle

Delayed Impulsive Control for μ-Synchronization of Nonlinear Multi-Weighted Complex Networks with Uncertain Parameter Perturbation and Unbounded Delays

by Hongguang Fan, Jiahui Tang, Kaibo Shi, Yi Zhao and Hui Wen

Mathematics 2023, 11(1), 250; https://doi.org/10.3390/math11010250 - 3 Jan 2023

Cited by 6 | Viewed by 1694

Abstract

The global

μ

-synchronization problem for nonlinear multi-weighted complex dynamical networks with uncertain parameter perturbation and mixed time-varying delays is investigated in this paper. Unlike other existing works, all delays, including sampling and internal and coupling delays, are assumed to be unbounded, making [...] Read more.

The global

μ

-synchronization problem for nonlinear multi-weighted complex dynamical networks with uncertain parameter perturbation and mixed time-varying delays is investigated in this paper. Unlike other existing works, all delays, including sampling and internal and coupling delays, are assumed to be unbounded, making the considered model more general and practical. Based on the generalized impulsive comparison principles, a time-varying impulsive controller with sampling delays is designed, and some new sufficient conditions are obtained to make drive–response multi-weighted networks reach

μ

-synchronization. In addition, the external coupling matrices do not need to meet the requirement of zero-row sum, and the limitation of time delay on pulse interval is weakened. The results obtained in this article can be seen as extensions of previous related research. Full article

(This article belongs to the Topic Engineering Mathematics)

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19 pages, 2620 KiB

Open AccessArticle

Retaliation against Ransomware in Cloud-Enabled PureOS System

by Atef Ibrahim, Usman Tariq, Tariq Ahamed Ahanger, Bilal Tariq and Fayez Gebali

Mathematics 2023, 11(1), 249; https://doi.org/10.3390/math11010249 - 3 Jan 2023

Cited by 4 | Viewed by 2309

Abstract

Ransomware is malicious software that encrypts data before demanding payment to unlock them. The majority of ransomware variants use nearly identical command and control (C&C) servers but with minor upgrades. There are numerous variations of ransomware, each of which can encrypt either the [...] Read more.

Ransomware is malicious software that encrypts data before demanding payment to unlock them. The majority of ransomware variants use nearly identical command and control (C&C) servers but with minor upgrades. There are numerous variations of ransomware, each of which can encrypt either the entire computer system or specific files. Malicious software needs to infiltrate a system before it can do any real damage. Manually inspecting all potentially malicious file types is a time-consuming and resource-intensive requirement of conventional security software. Using established metrics, this research delves into the complex issues of identifying and preventing ransomware. On the basis of real-world malware samples, we created a parameterized categorization strategy for functional classes and suggestive features. We also furnished a set of criteria that highlights the most commonly featured criteria and investigated both behavior and insights. We used a distinct operating system and specific cloud platform to facilitate remote access and collaboration on files throughout the entire operational experimental infrastructure. With the help of our proposed ransomware detection mechanism, we were able to effectively recognize and prevent both state-of-art and modified ransomware anomalies. Aggregated log revealed a consistent but satisfactory detection rate at 89%. To the best of our knowledge, no research exists that has investigated the ransomware detection and impact of ransomware for PureOS, which offers a unique platform for PC, mobile phones, and resource intensive IoT (Internet of Things) devices. Full article

(This article belongs to the Special Issue Mathematical Models in Security, Defense, Cyber Security and Cyber Defense II)

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11 pages, 790 KiB

Open AccessArticle

Product Convolution of Generalized Subexponential Distributions

by Gustas Mikutavičius and Jonas Šiaulys

Mathematics 2023, 11(1), 248; https://doi.org/10.3390/math11010248 - 3 Jan 2023

Cited by 3 | Viewed by 1350

Abstract

Assume that

ξ

and

η

are two independent random variables with distribution functions

F_{ξ}

and

F_{η}

, respectively. The distribution of a random variable

ξ η

, denoted by

F_{ξ} \otimes F_{η}

, is called the product-convolution of [...] Read more.

Assume that

ξ

and

η

are two independent random variables with distribution functions

F_{ξ}

and

F_{η}

, respectively. The distribution of a random variable

ξ η

, denoted by

F_{ξ} \otimes F_{η}

, is called the product-convolution of

F_{ξ}

and

F_{η}

. It is proved that

F_{ξ} \otimes F_{η}

is a generalized subexponential distribution if

F_{ξ}

belongs to the class of generalized subexponential distributions and

η

is nonnegative and not degenerated at zero. Full article

(This article belongs to the Special Issue Probabilistic Models in Insurance and Finance)

23 pages, 1850 KiB

Open AccessArticle

Constrained Nonsingular Terminal Sliding Mode Attitude Control for Spacecraft: A Funnel Control Approach

by Nguyen Xuan-Mung, Mehdi Golestani and Sung Kyung Hong

Mathematics 2023, 11(1), 247; https://doi.org/10.3390/math11010247 - 3 Jan 2023

Cited by 8 | Viewed by 1974

Abstract

This paper presents an adaptive constrained attitude control for uncertain spacecraft. Inspired by the concept of nonsingular terminal sliding mode control and funnel control for nonlinear systems, a novel adaptive attitude control is introduced which contains a time-varying gain to handle the constraints [...] Read more.

This paper presents an adaptive constrained attitude control for uncertain spacecraft. Inspired by the concept of nonsingular terminal sliding mode control and funnel control for nonlinear systems, a novel adaptive attitude control is introduced which contains a time-varying gain to handle the constraints imposed on the spacecraft attitude. Indeed, when the attitude trajectory approaches the boundary of the constraint set, the control effort as well as the time-varying gain will increase in order to preclude the trajectory from intersecting the boundary. Then, it is analytically proved that the system trajectories converge to an arbitrary small region around the origin within a fixed time where the smallest upper bound of the convergence time is determined as an independent parameter in the controller. Further, the proposed control scheme is nonsingular without having to use any piecewise continuous function which simplifies stability analysis. These properties distinguish the proposed control scheme from the existing finite/fixed-time attitude controls. Finally, several simulation results confirm the robustness and performance of the proposed control framework. Full article

(This article belongs to the Topic Dynamical Systems: Theory and Applications)

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26 pages, 562 KiB

Open AccessArticle

Non-Parametric Non-Inferiority Assessment in a Three-Arm Trial with Non-Ignorable Missing Data

by Wei Li, Yunqi Zhang and Niansheng Tang

Mathematics 2023, 11(1), 246; https://doi.org/10.3390/math11010246 - 3 Jan 2023

Viewed by 1889

Abstract

A three-arm non-inferiority trial including a placebo is usually utilized to assess the non-inferiority of an experimental treatment to a reference treatment. Existing methods for assessing non-inferiority mainly focus on the fully observed endpoints. However, in some clinical trials, treatment endpoints may be [...] Read more.

A three-arm non-inferiority trial including a placebo is usually utilized to assess the non-inferiority of an experimental treatment to a reference treatment. Existing methods for assessing non-inferiority mainly focus on the fully observed endpoints. However, in some clinical trials, treatment endpoints may be subject to missingness for various reasons, such as the refusal of subjects or their migration. To address this issue, this paper aims to develop a non-parametric approach to assess the non-inferiority of an experimental treatment to a reference treatment in a three-arm trial with non-ignorable missing endpoints. A logistic regression is adopted to specify a non-ignorable missingness data mechanism. A semi-parametric imputation method is proposed to estimate parameters in the considered logistic regression. Inverse probability weighting, augmented inverse probability weighting and non-parametric methods are developed to estimate treatment efficacy for known and unknown parameters in the considered logistic regression. Under some regularity conditions, we show asymptotic normality of the constructed estimators for treatment efficacy. A bootstrap resampling method is presented to estimate asymptotic variances of the estimated treatment efficacy. Three Wald-type statistics are constructed to test the non-inferiority based on the asymptotic properties of the estimated treatment efficacy. Empirical studies show that the proposed Wald-type test procedure is robust to the misspecified missingness data mechanism, and behaves better than the complete-case method in the sense that the type I error rates for the former are closer to the pre-given significance level than those for the latter. Full article

(This article belongs to the Special Issue Statistical Methods in Data Science and Applications)

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12 pages, 1642 KiB

Open AccessArticle

CNN-Based Temporal Video Segmentation Using a Nonlinear Hyperbolic PDE-Based Multi-Scale Analysis

by Tudor Barbu

Mathematics 2023, 11(1), 245; https://doi.org/10.3390/math11010245 - 3 Jan 2023

Cited by 3 | Viewed by 1690

Abstract

An automatic temporal video segmentation framework is introduced in this article. The proposed cut detection technique performs a high-level feature extraction on the video frames, by applying a multi-scale image analysis approach combining nonlinear partial differential equations (PDE) to convolutional neural networks (CNN). [...] Read more.

An automatic temporal video segmentation framework is introduced in this article. The proposed cut detection technique performs a high-level feature extraction on the video frames, by applying a multi-scale image analysis approach combining nonlinear partial differential equations (PDE) to convolutional neural networks (CNN). A nonlinear second-order hyperbolic PDE model is proposed and its well-posedness is then investigated rigorously here. Its weak and unique solution is determined numerically applying a finite difference method-based numerical approximation algorithm that quickly converges to it. A scale-space representation is then created using that iterative discretization scheme. A CNN-based feature extraction is performed at each scale and the feature vectors obtained at multiple scales are concatenated into a final frame descriptor. The feature vector distance values between any two successive frames are then determined and the video transitions are identified next, by applying an automatic clustering scheme on these values. The proposed PDE model, its mathematical investigation and discretization, and the multi-scale analysis based on it represent the major contributions of this work. Some temporal segmentation experiments and method comparisons that illustrate the effectiveness of the proposed framework are finally described in this research paper. Full article

(This article belongs to the Special Issue Advances in PDE-Based Models, Artificial Intelligence and Deep Learning)

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14 pages, 17667 KiB

Open AccessArticle

Multiaxial Strength Criterion Model of Concrete Based on Random Forest

by Xingqiao Chen, Dongjian Zheng, Yongtao Liu, Xin Wu, Haifeng Jiang and Jianchun Qiu

Mathematics 2023, 11(1), 244; https://doi.org/10.3390/math11010244 - 3 Jan 2023

Cited by 3 | Viewed by 1640

Abstract

The concrete strength criterion is the basis of strength analysis and evaluation under a complex stress state. In this paper, a large number of multiaxial strength tests were carried out, and many mathematical expressions of strength criteria were proposed based on the geometric [...] Read more.

The concrete strength criterion is the basis of strength analysis and evaluation under a complex stress state. In this paper, a large number of multiaxial strength tests were carried out, and many mathematical expressions of strength criteria were proposed based on the geometric characteristics and the assumption of a convex function. However, the rationality of the assumption of a convex function limits the use of these strength criteria. In particular, misjudgment will occur near the failure curve surface. Therefore, this paper does not assume the shape function of the criterion in advance. By collecting experimental data and using a machine learning method, it proposes a method of hidden function of failure curve surface. Based on 777 groups of experimental data, the random forest (RF), the back propagation neural network (BP) and the radial basis neural network (RBF) models were used to analyze and verify the feasibility and effectiveness of the method. Subsequently, the results were compared with the Ottosen strength criterion, the Guo Wang strength criterion and the Drucker–Prager (DP) strength criterion. The results show that the consistency between the strength criterion model established by the machine learning algorithm (especially random forest) and the experimental data is higher than the convex function multiaxis strength criterion of the preset failure surface shape. Moreover, the physical significance is clearer, the deficiency of the convex function failure surface hypothesis is avoided and the established multiaxial strength criterion of concrete is more universal. Full article

(This article belongs to the Special Issue Mathematical Modeling and Numerical Analysis for Applied Sciences)

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26 pages, 10354 KiB

Open AccessArticle

Effect of Overburden Depth and Stress Anisotropy on a Ground Reaction Caused by Advancing Excavation of a Circular Tunnel

by Yu-Lin Lee, Ming-Long Zhu, Chi-Huang Ma, Chih-Sheng Chen and Chi-Min Lee

Mathematics 2023, 11(1), 243; https://doi.org/10.3390/math11010243 - 3 Jan 2023

Cited by 1 | Viewed by 1948

Abstract

The assumption of the Convergence–Confinement Method (CCM) is the analysis of the interaction behavior of the support and ground of a deep circular tunnel under an isotropic stress field. Aiming to improve this method, this paper proposes a discussion on the influence of [...] Read more.

The assumption of the Convergence–Confinement Method (CCM) is the analysis of the interaction behavior of the support and ground of a deep circular tunnel under an isotropic stress field. Aiming to improve this method, this paper proposes a discussion on the influence of the overburden depth and stress anisotropy. To consider the influence of the overburden effect, the ground reaction in different depths due to tunnel advancing excavation is investigated. Under anisotropic stress conditions, the analytical solutions of the stress/displacement in the plastic and elastic regions of this ground reaction can also be suitable for theoretical analysis in a consistent manner. The key factor in this study is the use of confinement loss, which can not only describe the simulation of tunnel advancing effects but also become a superimposed value of the incremental procedure. In addition, the calculation spreadsheets can be used to estimate and implement the theoretical analytical solutions into executable computational solutions. To check the validity of the analytical solution, finite element analysis is used to examine the distribution of stress/displacement around the tunnel, especially the distribution along the overburden pressure line in the circular tunnel cross-section. Comparing the analytical solution calculated by the incremental procedure with the result of the numerical analysis shows a consistent trend. Full article

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16 pages, 604 KiB

Open AccessArticle

Adaptive Hyperparameter Fine-Tuning for Boosting the Robustness and Quality of the Particle Swarm Optimization Algorithm for Non-Linear RBF Neural Network Modelling and Its Applications

by Zohaib Ahmad, Jianqiang Li and Tariq Mahmood

Mathematics 2023, 11(1), 242; https://doi.org/10.3390/math11010242 - 3 Jan 2023

Cited by 14 | Viewed by 2407

Abstract

A method is proposed for recognizing and predicting non-linear systems employing a radial basis function neural network (RBFNN) and robust hybrid particle swarm optimization (HPSO) approach. A PSO is coupled with a spiral-shaped mechanism (HPSO-SSM) to optimize the PSO performance by mitigating its [...] Read more.

A method is proposed for recognizing and predicting non-linear systems employing a radial basis function neural network (RBFNN) and robust hybrid particle swarm optimization (HPSO) approach. A PSO is coupled with a spiral-shaped mechanism (HPSO-SSM) to optimize the PSO performance by mitigating its constraints, such as sluggish convergence and the local minimum dilemma. Three advancements are incorporated into the hypothesized HPSO-SSM algorithms to achieve remarkable results. First, the diversity of the search process is promoted to update the inertial weight

ω

based on the logistic map sequence. Then, two distinct parameters are trained in the original position update algorithm to enhance the work efficiency of the successive generation. Finally, the proposed approach employs a spiral-shaped mechanism as a local search operator inside the optimum solution space. Moreover, the HPSO-SSM method concurrently improves the RBFNN parameters and network size, building a model with a compact configuration and higher precision. Two non-linear benchmark functions and the total phosphorus (TP) modelling issue in a waste water treatment process (WWTP) are utilized to assess the overall efficacy of the creative technique. The results of testing indicate that the projected HPSO-SSM-RBFNN algorithm performed very effectively. Full article

(This article belongs to the Special Issue Mathematical Methods for Nonlinear Dynamics)

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21 pages, 360 KiB

Open AccessArticle

Efficiency and Competitiveness of the Equatorial Guinean Financial Sector

by Tito Ondo Ela-Medja and Pilar Alberca

Mathematics 2023, 11(1), 241; https://doi.org/10.3390/math11010241 - 3 Jan 2023

Viewed by 1771

Abstract

The private sector, in order to function properly, needs financing from the national financial sector, and so the efficiency and competitiveness of said financial sector arouse the interest of many researchers, who perform analyses in order to provide authorities and decision makers with [...] Read more.

The private sector, in order to function properly, needs financing from the national financial sector, and so the efficiency and competitiveness of said financial sector arouse the interest of many researchers, who perform analyses in order to provide authorities and decision makers with relevant information for the decision-making process and the design of their financial policies. This study contributes to this line of research, analyzing both technical and economic efficiency (allocative and cost efficiency) in the financial sector, focusing on banks, using a sample of Equatorial Guinean firms during the period of 2013–2019. Furthermore, the competitiveness of the financial sector is also analyzed. Knowing how efficient and competitive the financial sector is could answer many of the questions that arise when regulating the national business sector. To carry out this analysis, parametric approaches such as stochastic frontiers and non-parametric techniques such as data envelopment analysis are used, as well as different competitiveness indicators (Boone, Panzar–Rosse). During the research, it is found that the banking sector, which represents the financial sector of the country, operates with low levels of technical efficiency: the Cobb–Douglas production function and the trans-logarithmic production function showed similar average efficiency results. Regarding competitiveness, the financial sector operates under monopolistic competition. Therefore, much remains to be achieved to improve the efficiency and competitiveness of the financial sector for the development of Equatorial Guinea. It is the responsibility of economic agents to provide a good business climate in the country and guarantee perfect competition in the financial market to promote national development. Full article

(This article belongs to the Special Issue New Advances in Economic Analysis and Statistics Modeling with Applications to Social Sciences)

18 pages, 319 KiB

Open AccessArticle

Geometry of Tangent Poisson–Lie Groups

by Ibrahim Al-Dayel, Foued Aloui and Sharief Deshmukh

Mathematics 2023, 11(1), 240; https://doi.org/10.3390/math11010240 - 3 Jan 2023

Viewed by 1485

Abstract

Let G be a Poisson–Lie group equipped with a left invariant contravariant pseudo-Riemannian metric. There are many ways to lift the Poisson structure on G to the tangent bundle

T G

of

G .

In this paper, we induce a left invariant contravariant [...] Read more.

Let G be a Poisson–Lie group equipped with a left invariant contravariant pseudo-Riemannian metric. There are many ways to lift the Poisson structure on G to the tangent bundle

T G

of

G .

In this paper, we induce a left invariant contravariant pseudo-Riemannian metric on the tangent bundle

T G

, and we express in different cases the contravariant Levi-Civita connection and curvature of

T G

in terms of the contravariant Levi-Civita connection and the curvature of G. We prove that the space of differential forms

Ω^{*} (G)

on G is a differential graded Poisson algebra if, and only if,

Ω^{*} (T G)

is a differential graded Poisson algebra. Moreover, we show that G is a pseudo-Riemannian Poisson–Lie group if, and only if, the Sanchez de Alvarez tangent Poisson–Lie group

T G

is also a pseudo-Riemannian Poisson–Lie group. Finally, some examples of pseudo-Riemannian tangent Poisson–Lie groups are given. Full article

(This article belongs to the Special Issue Geometry of Manifolds and Applications)

17 pages, 888 KiB

Open AccessArticle

The ISM Method to Analyze the Relationship between Blockchain Adoption Criteria in University: An Indonesian Case

by Vincent F. Yu, Achmad Bahauddin, Putro F. Ferdinant, Agustina Fatmawati and Shih-Wei Lin

Mathematics 2023, 11(1), 239; https://doi.org/10.3390/math11010239 - 3 Jan 2023

Cited by 6 | Viewed by 1851

Abstract

Referring to the widespread problem of diploma forgery in Indonesian educational institutions as the impetus for UNTIRTA’s latest vision as an “Integrated, Smart, and Green University,” UNTIRTA intends to use blockchain technology to prevent diploma forgery and overcome issues related to existing platforms [...] Read more.

Referring to the widespread problem of diploma forgery in Indonesian educational institutions as the impetus for UNTIRTA’s latest vision as an “Integrated, Smart, and Green University,” UNTIRTA intends to use blockchain technology to prevent diploma forgery and overcome issues related to existing platforms at UNTIRTA, such as frequent connection interruptions when accessed by a large number of users simultaneously. Before using blockchain technology, UNTIRTA must evaluate several readiness issues. This study presented the interpretative structural modeling (ISM) method to assess the primary preparedness elements for adopting blockchain technology in universities and sought to provide pertinent strategy ideas for UNTIRTA’s blockchain technology application. The results reveal sixteen major parameters that influence the adoption readiness of blockchain technology at UNTIRTA. The primary variables impacting the adoption and deployment of blockchain technology at UNTIRTA are management and employee support and a grasp of the technology. To realize UNTIRTA’s mission as an “Integrated, Smart, and Green University”, the proposed method entails determining an initial agreement in which all stakeholders have a shared understanding and commitment to Blockchain technology implementation at UNTIRTA. The objective of the tactical proposal is to establish each unit’s mission in the blockchain implementation program. The objective of the technical proposal is to construct a planning document that will serve as a coordination tool between the chairman and members, as well as all parties interested in the adoption of Blockchain technology at UNTIRTA. Full article

(This article belongs to the Section Engineering Mathematics)

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18 pages, 320 KiB

Open AccessArticle

Perov Fixed-Point Results on F-Contraction Mappings Equipped with Binary Relation

by Fahim Ud Din, Muhammad Din, Umar Ishtiaq and Salvatore Sessa

Mathematics 2023, 11(1), 238; https://doi.org/10.3390/math11010238 - 3 Jan 2023

Cited by 4 | Viewed by 1540

Abstract

The purpose of this article is to discuss some new aspects of the vector-valued metric space. The idea of an arbitrary binary relation along with the well-known F contraction is used to demonstrate the existence of fixed points in the context of a [...] Read more.

The purpose of this article is to discuss some new aspects of the vector-valued metric space. The idea of an arbitrary binary relation along with the well-known F contraction is used to demonstrate the existence of fixed points in the context of a complete vector-valued metric space for both single- and multi-valued mappings. Utilizing the idea of binary relation, and with the help of F contraction, this work extends and complements some of the very recently established Perov-type fixed-point results in the literature. Furthermore, this work includes examples to justify the validity of the given results. During the discussion, it was found that some of the renowned metrical results proven by several authors using different binary relations, such as partial order, pre-order, transitive relation, tolerance, strict order and symmetric closure, can be weakened by using an arbitrary binary relation. Full article

(This article belongs to the Section Mathematics and Computer Science)

13 pages, 1804 KiB

Open AccessArticle

Mapping between Spin-Glass Three-Dimensional (3D) Ising Model and Boolean Satisfiability Problem

by Zhidong Zhang

Mathematics 2023, 11(1), 237; https://doi.org/10.3390/math11010237 - 3 Jan 2023

Cited by 10 | Viewed by 4732

Abstract

The common feature for a nontrivial hard problem is the existence of nontrivial topological structures, non-planarity graphs, nonlocalities, or long-range spin entanglements in a model system with randomness. For instance, the Boolean satisfiability (K-SAT) problems for K ≥ 3 [...] Read more.

The common feature for a nontrivial hard problem is the existence of nontrivial topological structures, non-planarity graphs, nonlocalities, or long-range spin entanglements in a model system with randomness. For instance, the Boolean satisfiability (K-SAT) problems for K ≥ 3

M_{SAT}^{K \geq 3}

are nontrivial, due to the existence of non-planarity graphs, nonlocalities, and the randomness. In this work, the relation between a spin-glass three-dimensional (3D) Ising model

M_{SGI}^{3 D}

with the lattice size N = mnl and the K-SAT problems is investigated in detail. With the Clifford algebra representation, it is easy to reveal the existence of the long-range entanglements between Ising spins in the spin-glass 3D Ising lattice. The internal factors in the transfer matrices of the spin-glass 3D Ising model lead to the nontrivial topological structures and the nonlocalities. At first, we prove that the absolute minimum core (AMC) model

M_{AMC}^{3 D}

exists in the spin-glass 3D Ising model, which is defined as a spin-glass 2D Ising model interacting with its nearest neighboring plane. Any algorithms, which use any approximations and/or break the long-range spin entanglements of the AMC model, cannot result in the exact solution of the spin-glass 3D Ising model. Second, we prove that the dual transformation between the spin-glass 3D Ising model and the spin-glass 3D Z₂ lattice gauge model shows that it can be mapped to a K-SAT problem for K ≥ 4 also in the consideration of random interactions and frustrations. Third, we prove that the AMC model is equivalent to the K-SAT problem for K = 3. Because the lower bound of the computational complexity of the spin-glass 3D Ising model

C_{L} (M_{SGI}^{3 D})

is the computational complexity by brute force search of the AMC model

C^{U} (M_{AMC}^{3 D})

, the lower bound of the computational complexity of the K-SAT problem for K ≥ 4

C_{L} (M_{SAT}^{K \geq 4})

is the computational complexity by brute force search of the K-SAT problem for K = 3

C^{U} (M_{SAT}^{K = 3})

. Namely,

C_{L} (M_{SAT}^{K \geq 4}) = C_{L} (M_{SGI}^{3 D}) \geq C^{U} (M_{AMC}^{3 D}) = C^{U} (M_{SAT}^{K = 3})

. All of them are in subexponential and superpolynomial. Therefore, the computational complexity of the K-SAT problem for K ≥ 4 cannot be reduced to that of the K-SAT problem for K < 3. Full article

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19 pages, 1839 KiB

Open AccessArticle

Machine-Learning Methods on Noisy and Sparse Data

by Konstantinos Poulinakis, Dimitris Drikakis, Ioannis W. Kokkinakis and Stephen Michael Spottswood

Mathematics 2023, 11(1), 236; https://doi.org/10.3390/math11010236 - 3 Jan 2023

Cited by 34 | Viewed by 6054

Abstract

Experimental and computational data and field data obtained from measurements are often sparse and noisy. Consequently, interpolating unknown functions under these restrictions to provide accurate predictions is very challenging. This study compares machine-learning methods and cubic splines on the sparsity of training data [...] Read more.

Experimental and computational data and field data obtained from measurements are often sparse and noisy. Consequently, interpolating unknown functions under these restrictions to provide accurate predictions is very challenging. This study compares machine-learning methods and cubic splines on the sparsity of training data they can handle, especially when training samples are noisy. We compare deviation from a true function f using the mean square error, signal-to-noise ratio and the Pearson

R^{2}

coefficient. We show that, given very sparse data, cubic splines constitute a more precise interpolation method than deep neural networks and multivariate adaptive regression splines. In contrast, machine-learning models are robust to noise and can outperform splines after a training data threshold is met. Our study aims to provide a general framework for interpolating one-dimensional signals, often the result of complex scientific simulations or laboratory experiments. Full article

(This article belongs to the Special Issue Mathematical Modeling, Optimization and Machine Learning)

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12 pages, 1315 KiB

Open AccessArticle

Mathematical Modeling: Cisplatin Binding to Deoxyribonucleic Acid

by Mansoor H. Alshehri

Mathematics 2023, 11(1), 235; https://doi.org/10.3390/math11010235 - 3 Jan 2023

Viewed by 1477

Abstract

The discovery of the cisplatin drug attracted considerable research attention as scientists strove to understand the drug’s mechanism in the human body that is responsible for destroying cancer cells, particularly the coordination between the cisplatin drug and deoxyribonucleic acid. Here, the binding energies [...] Read more.

The discovery of the cisplatin drug attracted considerable research attention as scientists strove to understand the drug’s mechanism in the human body that is responsible for destroying cancer cells, particularly the coordination between the cisplatin drug and deoxyribonucleic acid. Here, the binding energies of a cisplatin molecule relative to double-stranded deoxyribonucleic acid are obtained. The interactions of the system are determined by performing double integrals, and the analytical expressions are derived from the Lennard–Jones function and the continuum approximation; here, it is assumed that a discrete atomic structure might be replaced by surfaces with a constant average atomic density. The results observed that the cisplatin molecule is binding to the double-stranded deoxyribonucleic acid at either the minor or major grooves. By minimizing the interaction energies between the cisplatin molecule and the minor and major grooves, for arbitrary distances

λ

and arbitrary tilt angles

φ

from the axis of the helix of the double-stranded deoxyribonucleic acid, the binding energies are determined, and their values are ≈

- 6

and ≈

- 12.5

(kcal/mol), respectively. Thus, we may deduce that the major groove in double-stranded deoxyribonucleic acid is the most preferred groove for linking with the cisplatin molecule. The current analysis might help in the equivalent continuum modeling of deoxyribonucleic acids and nanocomposites. Full article

(This article belongs to the Special Issue Mathematical Biology: Modeling, Analysis, and Simulations, 2nd Edition)

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14 pages, 5332 KiB

Open AccessArticle

Spatial Computing in Modular Spiking Neural Networks with a Robotic Embodiment

by Sergey A. Lobov, Alexey N. Mikhaylov, Ekaterina S. Berdnikova, Valeri A. Makarov and Victor B. Kazantsev

Mathematics 2023, 11(1), 234; https://doi.org/10.3390/math11010234 - 3 Jan 2023

Cited by 4 | Viewed by 2382

Abstract

One of the challenges in modern neuroscience is creating a brain-on-a-chip. Such a semiartificial device based on neural networks grown in vitro should interact with the environment when embodied in a robot. A crucial point in this endeavor is developing a neural network [...] Read more.

One of the challenges in modern neuroscience is creating a brain-on-a-chip. Such a semiartificial device based on neural networks grown in vitro should interact with the environment when embodied in a robot. A crucial point in this endeavor is developing a neural network architecture capable of associative learning. This work proposes a mathematical model of a midscale modular spiking neural network (SNN) to study learning mechanisms within the brain-on-a-chip context. We show that besides spike-timing-dependent plasticity (STDP), synaptic and neuronal competitions are critical factors for successful learning. Moreover, the shortest pathway rule can implement the synaptic competition responsible for processing conditional stimuli coming from the environment. This solution is ready for testing in neuronal cultures. The neuronal competition can be implemented by lateral inhibition actuating over the SNN modulus responsible for unconditional responses. Empirical testing of this approach is challenging and requires the development of a technique for growing cultures with a given ratio of excitatory and inhibitory neurons. We test the modular SNN embedded in a mobile robot and show that it can establish the association between touch (unconditional) and ultrasonic (conditional) sensors. Then, the robot can avoid obstacles without hitting them, relying on ultrasonic sensors only. Full article

(This article belongs to the Special Issue Dynamics and Bifurcations in Mathematical Neuroscience: Analysis, Modeling and Applications)

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19 pages, 2985 KiB

Open AccessArticle

Cyber Security for Detecting Distributed Denial of Service Attacks in Agriculture 4.0: Deep Learning Model

by Theyazn H. H. Aldhyani and Hasan Alkahtani

Mathematics 2023, 11(1), 233; https://doi.org/10.3390/math11010233 - 3 Jan 2023

Cited by 50 | Viewed by 6287

Abstract

Attackers are increasingly targeting Internet of Things (IoT) networks, which connect industrial devices to the Internet. To construct network intrusion detection systems (NIDSs), which can secure Agriculture 4.0 networks, powerful deep learning (DL) models have recently been deployed. An effective and adaptable intrusion [...] Read more.

Attackers are increasingly targeting Internet of Things (IoT) networks, which connect industrial devices to the Internet. To construct network intrusion detection systems (NIDSs), which can secure Agriculture 4.0 networks, powerful deep learning (DL) models have recently been deployed. An effective and adaptable intrusion detection system may be implemented by using the architectures of long short-term memory (LSTM) and convolutional neural network combined with long short-term memory (CNN–LSTM) for detecting DDoS attacks. The CIC-DDoS2019 dataset was used to design a proposal for detecting different types of DDoS attacks. The dataset was developed using the CICFlowMeter-V3 network. The standard network traffic dataset, including NetBIOS, Portmap, Syn, UDPLag, UDP, and normal benign packets, was used to test the development of deep learning approaches. Precision, recall, F1-score, and accuracy were among the measures used to assess the model’s performance. The suggested technology was able to reach a high degree of precision (100%). The CNN–LSTM has a score of 100% with respect to all the evaluation metrics. We used a deep learning method to build our model and compare it to existing systems to determine how well it performs. In addition, we believe that this proposed model has highest possible levels of protection against any cyber threat to Agriculture 4.0. Full article

(This article belongs to the Special Issue Analytical Frameworks and Methods for Cybersecurity)

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3 pages, 191 KiB

Open AccessEditorial

Preface to the Special Issue on “Mathematical Methods and Operation Research in Logistics, Project Planning, and Scheduling”

by Zsolt Tibor Kosztyán and Zoltán Kovács

Mathematics 2023, 11(1), 232; https://doi.org/10.3390/math11010232 - 3 Jan 2023

Cited by 1 | Viewed by 1681

Abstract

In the last decade, the Industrial Revolution 4.0 brought flexible supply chains and flexible design projects to the fore [...] Full article

(This article belongs to the Special Issue Mathematical Methods and Operation Research in Logistics, Project Planning, and Scheduling)

22 pages, 9137 KiB

Open AccessArticle

Image Encryption Scheme Based on Newly Designed Chaotic Map and Parallel DNA Coding

by Shenli Zhu, Xiaoheng Deng, Wendong Zhang and Congxu Zhu

Mathematics 2023, 11(1), 231; https://doi.org/10.3390/math11010231 - 2 Jan 2023

Cited by 59 | Viewed by 4023

Abstract

In this paper, a new one-dimensional fractional chaotic map is proposed and an image encryption scheme based on parallel DNA coding is designed by using the chaotic map. The mathematical model of the new chaotic system combines a sine map and a fraction [...] Read more.

In this paper, a new one-dimensional fractional chaotic map is proposed and an image encryption scheme based on parallel DNA coding is designed by using the chaotic map. The mathematical model of the new chaotic system combines a sine map and a fraction operation. Compared with some traditional one-dimensional chaotic systems, the new chaotic system has a larger range of chaotic parameters and better chaotic characteristics, which makes it more suitable for applications in information encryption. In addition, an image encryption algorithm based on parallel DNA coding is proposed, which overcomes the shortcoming of common DNA coding-based image encryption algorithms. Parallel computing significantly increases the speed of encryption and decryption algorithms. The initial key of the cryptosystem is designed to be related to the SHA-3 hash value of the plaintext image so that the algorithm can resist a chosen-plaintext attack. Simulation experiments and security analysis results show that the proposed image encryption scheme has good encryption performance and less time overhead, and has strong robustness to noise and data loss attacks, which indicates that the proposed image encryption scheme has good application potential in secure communication applications. Full article

(This article belongs to the Special Issue Chaos-Based Secure Communication and Cryptography)

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16 pages, 2493 KiB

Open AccessArticle

Optimal Agent Search Using Surrogate-Assisted Genetic Algorithms

by Seung-Soo Shin and Yong-Hyuk Kim

Mathematics 2023, 11(1), 230; https://doi.org/10.3390/math11010230 - 2 Jan 2023

Cited by 2 | Viewed by 1872

Abstract

An intelligent agent is a program that can make decisions or perform a service based on its environment, user input, and experiences. Due to the complexity of its state and action spaces, agents are approximated by deep neural networks (DNNs), and it can [...] Read more.

An intelligent agent is a program that can make decisions or perform a service based on its environment, user input, and experiences. Due to the complexity of its state and action spaces, agents are approximated by deep neural networks (DNNs), and it can be optimized using methods such as deep reinforcement learning and evolution strategies. However, these methods include simulation-based evaluations in the optimization process, and they are inefficient if the simulation cost is high. In this study, we propose surrogate-assisted genetic algorithms (SGAs), whose surrogate models are used in the fitness evaluation of genetic algorithms, and the surrogates also predict cumulative rewards for an agent’s DNN parameters. To improve the SGAs, we applied stepwise improvements that included multiple surrogates, data standardization, and sampling with dimensional reduction. We conducted experiments using the proposed SGAs in benchmark environments such as cart-pole balancing and lunar lander, and successfully found optimal solutions and significantly reduced computing time. The computing time was reduced by 38% and 95%, in the cart-pole balancing and lunar lander problems, respectively. For the lunar lander problem, an agent with approximately 4% better quality than that found by a gradient-based method was even found. Full article

(This article belongs to the Special Issue Swarm and Evolutionary Computation—Bridging Theory and Practice)

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13 pages, 1570 KiB

Open AccessArticle

A Trie Based Set Similarity Query Algorithm

by Lianyin Jia, Junzhuo Tang, Mengjuan Li, Runxin Li, Jiaman Ding and Yinong Chen

Mathematics 2023, 11(1), 229; https://doi.org/10.3390/math11010229 - 2 Jan 2023

Cited by 1 | Viewed by 1761

Abstract

Set similarity query is a primitive for many applications, such as data integration, data cleaning, and gene sequence alignment. Most of the existing algorithms are inverted index based, they usually filter unqualified sets one by one and do not have sufficient support for [...] Read more.

Set similarity query is a primitive for many applications, such as data integration, data cleaning, and gene sequence alignment. Most of the existing algorithms are inverted index based, they usually filter unqualified sets one by one and do not have sufficient support for duplicated sets, thus leading to low efficiency. To solve this problem, this paper designs T-starTrie, an efficient trie based index for set similarity query, which can naturally group sets with the same prefix into one node, and can filter all sets corresponding to the node at a time, thereby significantly improving the candidates generation efficiency. In this paper, we find that the set similarity query problem can be transformed into matching nodes of the first-layer (FMNodes) detecting problem on T-starTrie. Therefore, an efficient FLMNode detection algorithm is designed. Based on this, an efficient set similarity query algorithm, TT-SSQ, is implemented by developing a variety of filtering techniques. Experimental results show that TT-SSQ can be up to 3.10x faster than existing algorithms. Full article

(This article belongs to the Special Issue Data Mining: Analysis and Applications)

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20 pages, 1730 KiB

Open AccessArticle

Statistical Depth for Text Data: An Application to the Classification of Healthcare Data

by Sergio Bolívar, Alicia Nieto-Reyes and Heather L. Rogers

Mathematics 2023, 11(1), 228; https://doi.org/10.3390/math11010228 - 2 Jan 2023

Cited by 4 | Viewed by 2718

Abstract

This manuscript introduces a new concept of statistical depth function: the compositional D-depth. It is the first data depth developed exclusively for text data, in particular, for those data vectorized according to a frequency-based criterion, such as the tf-idf (term frequency–inverse document [...] Read more.

This manuscript introduces a new concept of statistical depth function: the compositional D-depth. It is the first data depth developed exclusively for text data, in particular, for those data vectorized according to a frequency-based criterion, such as the tf-idf (term frequency–inverse document frequency) statistic, which results in most vector entries taking a value of zero. The proposed data depth consists of considering the inverse discrete Fourier transform of the vectorized text fragments and then applying a statistical depth for functional data, D. This depth is intended to address the problem of sparsity of numerical features resulting from the transformation of qualitative text data into quantitative data, which is a common procedure in most natural language processing frameworks. Indeed, this sparsity hinders the use of traditional statistical depths and machine learning techniques for classification purposes. In order to demonstrate the potential value of this new proposal, it is applied to a real-world case study which involves mapping Consolidated Framework for Implementation and Research (CFIR) constructs to qualitative healthcare data. It is shown that the

D D^{G}

-classifier yields competitive results and outperforms all studied traditional machine learning techniques (logistic regression with LASSO regularization, artificial neural networks, decision trees, and support vector machines) when used in combination with the newly defined compositional D-depth. Full article

(This article belongs to the Special Issue Advances in Statistics: Theory, Methodology, Applications and Data Analysis)

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13 pages, 2410 KiB

Open AccessArticle

Approach to the Numerical Study of Wave Processes in a Layered and Fractured Porous Media in a Two-Dimensional Formulation

by Amir A. Gubaidullin, Olga Yu. Boldyreva and Dina N. Dudko

Mathematics 2023, 11(1), 227; https://doi.org/10.3390/math11010227 - 2 Jan 2023

Cited by 3 | Viewed by 1075

Abstract

A new approach to the numerical study of arbitrary waveform impulses in a layered porous and fractured-porous medium in a two-dimensional formulation has been developed. Layers can have different characteristics and contain fractures. A computer implementation of the mathematical model based on the [...] Read more.

A new approach to the numerical study of arbitrary waveform impulses in a layered porous and fractured-porous medium in a two-dimensional formulation has been developed. Layers can have different characteristics and contain fractures. A computer implementation of the mathematical model based on the finite-difference MacCormack method has been completed. A number of test calculations have been carried out confirming the reliability of the numerical solutions obtained. The possibility of using the proposed approach to solve problems of wave dynamics is shown. Full article

(This article belongs to the Special Issue Mathematical Models of Multiphase Flows in Porous Media)

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16 pages, 316 KiB

Open AccessArticle

Double-Sources Queuing-Inventory Systems with Finite Waiting Room and Destructible Stocks

by Agassi Melikov, Ramil Mirzayev and Janos Sztrik

Mathematics 2023, 11(1), 226; https://doi.org/10.3390/math11010226 - 2 Jan 2023

Cited by 4 | Viewed by 1398

Abstract

Models of double-source queuing-inventory systems are studied in the presence of a finite buffer for waiting in the queue of consumer customers, where instant destruction of inventory is possible. It is assumed that the lead times of orders, as well as the cost [...] Read more.

Models of double-source queuing-inventory systems are studied in the presence of a finite buffer for waiting in the queue of consumer customers, where instant destruction of inventory is possible. It is assumed that the lead times of orders, as well as the cost of delivery from various sources, differ from each other. Replenishment of stocks from various sources is carried out according to the following scheme: if the inventory level drops to the reorder point s, then a regular order for the supply of inventory to a slow source is generated; if the inventory level falls below a certain threshold value r, where r < s, then the system instantly cancels the regular order and generates an emergency order to the fast source. Models of systems that use (s, S) or (s, Q) replenishment policies are studied. Exact and approximate methods for finding the performance measures of the models under study are proposed. The problems of minimizing the total cost are solved by choosing the appropriate values of the parameters s and r when using different replenishment policies. Numerical examples demonstrated the high accuracy of an approximate method as well as compared performance measures of the system under various replenishment policies. Full article

(This article belongs to the Special Issue Stability Problems for Stochastic Models: Analytics, Asymptotics, Estimation and Applications)

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Mathematics, Volume 11, Issue 1 (January-1 2023) – 255 articles

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