Journals Published by Vilnius Tech
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Approximation of analytic functions by generalized shifts of the Lerch zeta-function
Get PDFIn the paper, we approximate analytic functions by generalized shifts of the Lerch zeta-function, where g is a certain increasing to real function having a monotonic derivative. We prove that, for arbitrary parameters λ and α, there exists a closed set of analytic functions defined in the strip 1/2 < σ < 1 which functions are approximated by the above shifts. If the set of logarithms is linearly independent over the field of rational numbers, then the set coincides with the set of all analytic functions in that strip
A partially derivative-free cyclic block coordinate descent method for nonseparable composite optimization
Get PDFIn this paper, we address a composite optimization problem in which the objective function consists of two terms: the first presents a function with a gradient that satisfies a Lipschitz–Hölder composition, while the second one is a convex function. Under general settings, we propose and analyze a new coordinate descent method that can operate without the use of derivatives. The algorithm is an adaptation of the coordinate proximal gradient method, specifically designed to consider the composite form of the objective function. We perform a complete worst-case complexity analysis, assuming that the coordinates (or blocks of coordinates) are selected in a cyclic manner. In addition, we present academic numerical examples that illustrate the efficiency of our algorithm in practical problems
Optimizing airway network efficiency with the A-star algorithm: a case study of Ho Chi Minh FIR
Get PDFAirway network optimization is crucial for airspace planning, addressing high traffic density and alleviating pressure on Air Traffic Controllers. With the rapid growth of civil aviation, overloaded airways and congestion have become significant challenges. Vietnam’s air transport sector plays a vital role in economic and social development, enhancing connectivity across the Asia-Pacific region. Additionally, Vietnam has led the global recovery of domestic aviation post-COVID-19 and, as the world’s fifth fastest-growing aviation market, is expected to reach 150 million air transport passengers by 2035. This highlights Vietnam’s dynamic growth and strategic importance within global aviation. However, this rapid expansion has exposed vulnerabilities in Vietnam’s airways, particularly within the Ho Chi Minh Flight Information Region (HCM FIR), where congestion and overload are evident, especially during adverse weather. To address these challenges, this study proposes optimizing the airway network in 2D space using the A-star algorithm, tailored for the HCM FIR. This approach aims to reduce flight distances, improve operational efficiency, and ease ATC workloads, marking a critical step toward enhancing Vietnam’s airspace management system
Modal and random vibration analysis for structural design of aircraft external stores
Get PDFThis study introduces a novel methodology for the structural design of aircraft external stores, focusing on the integration of modal and random vibration analysis through the Power Spectral Density (PSD) method. The primary goal is to develop a robust design approach that enhances structural reliability and avoids resonance, a critical challenge in aerospace applications. Unlike conventional methods, this research presents a convergent strategy combining Finite Element Analysis (FEA), Computational Fluid Dynamics (CFD), and fluid-structure interaction to accurately predict stress, displacement, and frequency responses under dynamic loading. The methodology is specifically applied to external stores mounted at the fuselage station of a subsonic turboprop aircraft. A key novelty of the work is the validation of structural response through PSD-based vibration analysis, offering a high-fidelity prediction of dynamic behavior. Results indicate a close correlation between FEA and vibration outcomes, with maximum stress and deformation well within material limits and outside the aircraft’s natural frequency range, thus avoiding resonance effects. The conclusions of this study provide a roadmap for the design of aircraft stores based on modal and random vibration analysis. The proposed methodology helps to understand aeroelastic behavior, avoid resonance, resist aerodynamic forces, and prevent structural damage to aircraft external stores
Reducing CO2 emissions by improving road design: a driving simulator study
No full textIn the last decade, the causes of Greenhouse Gas (GHG) emissions were widely studied, to delete or, at least, mitigate them. In the road context, as reasonable, greater importance was assigned to the vehicles, since huge traffic flows, including high percentages of trucks, determine negative impacts on the environment. On the contrary, the role of the road infrastructure has always been considered marginal. It was thought as a functional element on which the traffic flows move, without evaluating the role of its geometrical characteristics on exhaust gas emissions. The proposed research aims to verify whether some road features, related to its horizontal geometry, influence the carbon dioxide production of vehicles or, on the contrary, if it is not sensitive to the different geometrical compositions. A driving simulator gives the opportunity to calculate the emissions from fuel consumption data, in turn, calculated through the engine mapping of an ordinary vehicle. The proposed procedure may be easily applied to any road context and may represent a further checking element for the infrastructure efficiency, in terms of environmental impacts. The results, derived from a test phase in a simulated environment and obtained using 3 different one-way ANOVAs, allowed the authors to define some interesting conclusions. The trend of the carbon dioxide function depends on curve radius and lengths and on tangent length; therefore, an opportune alignment design can effectively contribute to control emission values. The experiments confirmed that designing a consistent road is fundamental, but this cannot be deduced by traditional literature models
Parametric analysis of wing planforms to determine an optimal wing design
Get PDFIn designing of Unmanned Aerial Vehicle (UAV), selection of an optimal wing design is a crucial part of complete UAV design process. This research explores the different aerodynamic parameters and the comparison of different wing planforms to ascertain the optimal wing design and improve the overall efficiency of an UAV. The computational analysis using XFLR5 and Open-VSP software is studied to investigate the various aerodynamic parameters of wing. The impact of aspect ratio, taper ratio, wing reference area, coefficient of lift and drag, and stall angle of attack are examined using the Analytical Hierarchy Process (AHP). The results emphasize the importance of different wing planforms and create easier selection of planform for the UAV designers. The study does not only provide the values for operating parameters but also offers practical guidance for design optimization. The semi tapered, and moderate tapered (λ = 0.5) wings are the good choice to select at the initial phase of design. The highly tapered and elliptical wings provide higher lift but are not efficient in the stalling conditions. Furthermore, the rectangular wing provides elliptical lift distribution, but it is inefficient in the lift generation
Digital economy’s role in shaping China’s outward investment in Belt and Road countries
Get PDFPrevious international investment-related literature is less likely to include host country digital economy as an influential factor in the analysis framework. Meanwhile digital economy-related literature mainly focuses on the concept and characteristics of digital economy, and few scholars explore the impact and mechanism of China’s OFDI in the context of digital economy. Based on the theory and mechanism analysis, this article first constructs a comprehensive digital economy indicator system using data from 46 B&R countries from 2004 to 2020 and then constructs extended investment gravity model, technological innovation intermediary impact and trade cost moderating effect to thoroughly investigate the effect and fundamental mechanism of digital economy on China’s OFDI. The conclusions are as follows: First, digital economy indicators’ computation reveals significant disparities among B&R countries. Second, baseline regression finds that B&R countries’ digital economy considerably boosts China’s OFDI. Third, regional heterogeneity reveals that digital economy in ASEAN countries plays a more visible role in boosting China’s OFDI. Fourth, mechanism analysis reveals that B&R countries’ digital economy can encourage China’s OFDI by improving technological innovation and reducing trade costs.
First published online 6 December 202
Can green bonds hedge against geopolitical risk? A cross-market connectedness analysis with portfolio implications
Get PDFThis study investigates whether green bonds (GBs) can hedge against geopolitical risk (GPR). This study extends the booming literature on GPR and GBs, develops a modified connectedness network model to measure the connectedness between GPR and GBs, confirms the hedging property of GBs against GPR, and becomes the first to discuss alternative hedging properties of GBs against GPR. We find evidence of market-, time-, and quantile-varying linkage between GPR and GB markets based on the time-varying Granger causality test and quantile extended joint spillover index model. We confirm via a regression model that only the GB markets in China and Japan can hedge against GPR. At the same time, GB in China remains a weak hedging and safety-haven asset simultaneously. The results remain robust for alternative proxy variables, data frequency, and model specification. Finally, the MVP approach provides superior performance while maintaining weak hedging and safety-haven properties against GPR. This study has considerable portfolio-related implications: (1) it offers an efficient hedge (i.e., GB) against GPR, (2) the heterogeneous performance of regional GB markets reminds investors to be cautious when selecting GBs assets, and (3) it encourages reasonable investment allocations on GBs to achieve a balance between profit and risk.
First published online 24 September 202
Host government digitalization and Chinese OFDI entry mode choice: green-field investment vs cross-border M&A
Get PDFBased on a theoretical framework based on the perspectives of open governance, transparent governance, and smart governance, the article comprehensively explains the potential impact of host country government digitalization on Chinese enterprises’ OFDI entry mode selection. The results indicate that host government digitalization significantly promotes the probability of Chinese enterprises investing by greenfield investment mode; After using instrumental variable to eliminate endogenous interference and undergoing a series of robustness checks, this conclusion remains robust; Host government digitalization influences the OFDI mode choices of Chinese enterprises by suppressing corruption and strengthening intellectual property protection. Based on the research conclusions, we put forward policy suggestions on “how Chinese enterprises should make OFDI entry mode decision and optimize the project management after entry based on the level of host government digitalization“.
First published online 02 April 202
Tax reform, tax shifting and enterprise innovation
Get PDFFiscal and tax policies, as important forms of government regulation and control of the economy, have a profound impact on both macroeconomics and microeconomics and have been widely studied by scholars. To deeply explore the relationship between tax reform and enterprise innovation, based on classic financial theories such as financing constraints and tax shifting, annual data on Chinese A-share listed companies from 2009 to 2022 and the time-varying difference-in-differences method, we investigated the impact mechanism and economic effects of the “replace the business tax with a value-added tax” policy (VAT reform)on enterprise innovation and examined the moderating effect of the tax shifting ability during this process. The research results indicate the following: VAT reform can lower the tax base of enterprises, reduce cash outflows, and alleviate financing constraints, which helps enterprises stimulate innovation vitality and enhance R&D investments. The negotiation leverage that a company possesses in dealings with its providers and purchasers influences its capacity to transfer fiscal burdens. The enhancement of corporate innovation resulting from the VAT reform is notably pronounced. In contrast to state-owned enterprises, non-state-owned enterprises face fierce market competition and greater survival pressure and do not undertake policy-related activities. They are more sensitive to the tax reduction effect of the VAT reform and are more willing to carry out innovative activities when pursuing long-term development. At the same time, they are also more willing to enhance their tax shifting ability to fully obtain policy dividends for enterprise innovation. This conclusion can help the government correctly judge and comprehensively evaluate the effect of the VAT reform, providing management insights into how the government can better improve tax arrangements and promote enterprise innovation to achieve balanced development and how enterprises can better obtain policy dividends to promote technological innovation