1,721,360 research outputs found
On the Rational Polytopes with Chvatal Rank 1
No full textWe study the following problem: given a rational polytope with Chvatal rank 1, does it contain an integer point? Boyd and Pulleyblank observed that this problem is in the complexity class NP boolean AND\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}\end{document} co-NP, an indication that it is probably not NP-complete. It is open whether there is a polynomial time algorithm to solve the problem, and we give several special classes where this is indeed the case. We show that any compact convex set whose Chvatal closure is empty has an integer width of at most n, and we give an example showing that this bound is tight within an additive constant of 1. This determines the time complexity of a Lenstra-type algorithm. However, the promise that a polytope has Chvatal rank 1 seems hard to verify. We prove that deciding emptiness of the Chvatal closure of a rational polytope given by its linear description is NP-complete, even when the polytope is contained in the unit hypercube or is a rational simplex and it does not contain any integer point.
The Influence of Stock Liquidity on Tail Risk of Stock Market: An Empirical Research Based on POT Model
No full textIn view of the irrationality of the normal hypothesis of the value-at-risk model， the POT model based on the generalized pareto distribution is used to measure the tail risk of China's stock market， and the influence of stock liquidity on the tail risk of China's stock market is explored by grouping the sample data according to liquidity. In the research, the slope change point theory in statistical theory is introduced into the traditional threshold selection method， which effectively avoids the subjectivity problem when determining the optimal threshold. The test results show that for the measurement of tail risk of the stock market, the VaR value at the 95% confidence level is more reliable than the estimated result of the VaR value at the 90% confidence level, and the estimated value shows that tail risk and liquidity change in the opposite direction, that is， the smaller the stock liquidity is, the greater the market tail risk
Recent Progress in Electrochemical Late-Stage C(sp2)–H Functionalization
No full textAbstract Late-stage C–H functionalization has gained rapid development as a key approach in modern synthetic methodology, particularly for the transformation of structurally complex molecules. This strategy has reformed retrosynthetic logic and improved synthetic efficiency. C(sp2)–H functionalization, as a pivotal part in late-stage functionalization, has significantly promoted the development of medicinal chemistry. However, the latter often employ transition metal catalysts, in combination with stoichiometric amounts of chemical oxidants, which severely compromise their sustainability, functional group tolerance, and their extension to industrial applications. In sharp contrast, electrochemical late-stage C(sp2)–H functionalization has emerged as a more sustainable and environmentally friendly alternative, where electrons are used as clean redox agents. Therefore, this approach obviates the need for toxic chemical oxidants, enabling reactions to be operative under milder conditions, allowing for a wide scope of redox transformations. In this review, we highlight the recent progress in the electrochemical late-stage functionalization of C(sp2)–H bonds from 2019 to 2025, with particular focus on the formation of C(sp2)–C, C(sp2)–O, C(sp2)–N, and C(sp2)–P bonds
Thioether-enabled palladium-catalyzed atroposelective C–H olefination for N–C and C–C axial chirality
No full textThioethers allowed for highly atroposelective C–H olefinations by a palladium/chiral phosphoric acid catalytic system under ambient air
Recent advances in organic electrochemical functionalizations for specialty chemicals
No full texthttp://dx.doi.org/10.13039/501100001659 Deutsche Forschungsgemeinschafthttp://dx.doi.org/10.13039/501100000781 European Research Counci
Region-based colour modelling for joint crop and maize tassel segmentation
No full textAbstract not availableHao Lu, Zhiguo Cao, Yang Xiao, Yanan Li, Yanjun Zh
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Photoelectrocatalytic [4+2] Annulation for S‐Heterocycle Assembly Enabled by Proton‐Coupled Electron Transfer (PCET)
No full textCross‐dehydrogenative couplings (CDC) present an efficient strategy for the assembly of biorelevant heterocycles, but are thus far largely limited to toxic transition metals and rather harsh reaction conditions. In sharp contrast, we, herein report on a mild photoelectrocatalyzed CDC‐[4+2] annulation enabling the synthesis of functionalized isothiochromenes enabled by a proton‐coupled electron transfer (PCET) strategy. The transformative photoelectrocatalysis obviated toxic transition‐metal, high reaction temperatures, and stoichiometric chemical redox reagents. This approach was characterized by exceedingly mild conditions, ample substrate scope, and a commercially available catalyst. Gram‐scale reactions and a telescoped synthesis route reflected the unique potential in the green synthesis of important S‐heterocycles
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