Variations in spruce (Picea sp.) distribution in the Chinese Loess Plateau and surrounding areas during the Holocene

&nbsp;&nbsp;&nbsp; Based on published pollen data from 44 sites in the Loess Plateau and surrounding areas, we recalculate values of Picea abundance digitized from the original diagrams and construct six isopoll maps to reconstruct the distribution of spruce forest at 2000 yr intervals during the Holocene. Spruce forest gradually expanded from about 10 ka BP ago and reached its peak distribution around 8 ka BP. It retained its maximum extent between 8 ka BP and 6 ka BP, broadly covering the subalpine area and stretching to the valleys of the western Loess Plateau with high densities and even reaching the edges of the adjacent desert regions. After 6 ka BP, spruce forest began to retreat to higher elevations. This trend was accelerated after 4 ka BP. After 2 ka BP, it disappeared completely from the Loess Plateau and most of its surrounding areas. Precipitation strongly dominated by the Asian summer monsoon was the primary controlling factor in the distribution of spruce before 2 ka BP. After about 2 ka BP, the increasing intensity of human activity became the primary factor in the disappearance of spruce in the Loess Plateau, resulting in an enormous impact on the present landscape.</h3

Verification Theorems within the Framework of Viscosity Solutions

AbstractIn optimal control theory, the verification technique plays an important role in testing for the optimality of a given control, and in constructing optimal feedback controls. However, the existing classical verification theorem is restrictive in that it requires the associated dynamic programming equation to have smooth solutions. In this paper, some verification theorems are presented within the framework of viscosity solutions under mild assumptions. These theorems are shown to have wider applicability than the classical verification theorem. As a relevant problem, some differences and relationships between the viscosity solution and Clarke′s generalized gradient are also discussed

Sufficient stochastic maximum principle in a regime-switching diffusion model

We prove a sufficient stochastic maximum principle for the optimal control of a regime-switching diffusion model. We show the connection to dynamic programming and we apply the result to a quadratic loss minimization problem, which can be used to solve a mean-variance portfolio selection problem

LQ control without Ricatti equations: deterministic systems

We study a deterministic linear-quadratic (LQ) control problem over an infinite horizon, and develop a general apprach to the problem based on semi-definite programming (SDP)and related duality analysis. This approach allows the control cost matrix R to be non-negative (semi-definite), a case that is beyond the scope of the classical approach based on Riccati equations. We show that the complementary duality condition of the SDP is necessary and sufficient for the existence of an optimal LQ control. Moreover, when the complementary duality does hold, an optimal state feedback control is constructed explicitly in terms of the solution to the semidefinite program. On the other hand, when the complementary duality fails, the LQ problem has no attainable optimal solution, and we develop an E-approximation scheme that achieves asymptotic optimality

On Necessary and Sufficient Conditions for Near-Optimal Singular Stochastic Controls

This paper is concerned with necessary and sufficient conditions for near-optimal singular stochastic controls for systems driven by a nonlinear stochastic differential equations (SDEs in short). The proof of our result is based on Ekeland's variational principle and some delicate estimates of the state and adjoint processes. This result is a generalization of Zhou's stochastic maximum principle for near-optimality to singular control problem.Comment: 19 pages, submitted to journa

Green’s function method to the ground state properties of a two-component Bose–Einstein condensate

The elementary excitation spectrum of a two-component Bose–Einstein condensate is obtained by Green’s function method. It is found to have two branches. In the long-wave limit, the two branches of the excitation spectrum are reduced to one phonon excitation and one single-particle excitation. With the obtained excitation spectrum and the Green’s functions, the depletion of the condensate and the ground state energy have also been calculated in this paper

Measurement of Trace I-129 Concentrations in CsI Powder and Organic Liquid Scintillator with Accelerator Mass Spectrometry

Levels of trace radiopurity in active detector materials is a subject of major concern in low-background experiments. Procedures were devised to measure trace concentrations of I-129 in the inorganic salt CsI as well as in organic liquid scintillator with Accelerator Mass Spectrometry (AMS) which leads to improvement in sensitivities by several orders of magnitude over other methods. No evidence of their existence in these materials were observed. Limits of < 6 X 10^{-13} g/g and < 2.6 X 10^{-17} g/g on the contaminations of I-129 in CsI and liquid scintillator, respectively, were derived.These are the first results in a research program whose goals are to develop techniques to measure trace radioactivity in detector materials by AMS.Comment: Proceedings of 10th International Conference on Accelerator Mass Spectrometr

Fatigue Strength and Life Prediction of a MAR-M247 Nickel-Base Superalloy Gas Turbine Blade with Multiple Carbide Inclusions

Casting of a gas turbine blade from MAR-M247 nickel-base superalloy is followed by multiple carbide precipitates as a result of solidification. The microstructure of carbide was established with scanning electron microscopy and energy spectrum analysis. A finite element model simulating the blade with a random carbide precipitate is constructed using ANSYS software, and stress-strain evaluation is performed. The blade operation conditions are realized in a vibratory-tensile combined test setup with stress measurements using strain gauges arranged on different parts of the blade. The respective von Mises equivalent stresses obtained in simulation and test measurements are found to be comparable and can be used to assess the blade fatigue life based on the available fatigue curves and Palmgren–Miner rule of damage accumulation. Simulation and experimental results are the basis for plotting the relationship between the volumetric ratio of precipitates in the blade and its fatigue life. The results obtained at room temperature can; be extrapolated to the elevated ones to provide more reliable prediction of the superalloy blade fatigue and creep-fatigue life values.Литье лопатки газовой турбины из суперсплава на никелевой основе MAR-M247 сопровождается образованием множественных карбидных выделений в результате затвердевания. Микроструктуру карбида изучали с помощью сканирующей электронной микроскопии и энергетического спектрального анализа. Построена конечноэлементная модель лопатки со случайным карбидным выделением на основе программного обеспечения ANSYS, выполнена оценка напряженно-деформированного состояния. Условия эксплуатации лопатки реализовали на экспериментальной установке для комплексного испытания на вибрацию и растяжение, напряжения измеряли с помощью тензодатчиков, размещаемых на различных участках лопатки. Проведено сравнение эквивалентных напряжений по Мизесу, полученных при моделировании и экспериментально. Установлено, что их можно использовать для оценки усталостной долговечности лопатки на основании полученных кривых усталости и закона накопления повреждений Палмгрена Майлера. Они служат также основой построения зависимости между объемным соотношением включений в лопатке и ее усталостной долговечностью. Данные, полученные при комнатной температуре, могут быть экстраполированы на повышенные, что обеспечивает более достоверный прогноз усталости и усталостной долговечности лопатки из суперсплава при ползучести.Лиття лопатки газової турбіни з суперсплавів на нікелевої основі MAR-M247 супроводжується утворенням множинних карбідних виділень в результаті затвердіння. Мікроструктуру карбіду вивчали за допомогою скануючої електронної мікроскопії та енергетичного спектрального аналізу. Побудована скінченноелементна модель лопатки з випадковим карбідним виділенням на основі програмного забезпечення ANSYS, виконана оцінка напружено-деформованого стану. Умови експлуатації лопатки реалізували на експериментальній установці для комплексного випробування на вібрацію і розтягнення, напруги вимірювали за допомогою тензодатчиків, що розміщуються на різних ділянках лопатки. Проведено порівняння еквівалентних напружень по Мізесу, отриманих при моделюванні і експериментально. Встановлено, що їх можна використовувати для оцінки втомної довговічності лопатки на підставі отриманих кривих втоми і закону накопичення ушкоджень Палмгрена-Майлера. Вони служать також основою побудови залежності між об'ємним співвідношенням включень в лопатці і її втомної довговічністю. Дані, отримані при кімнатній температурі, можуть бути екстрапольовані на підвищені, що забезпечує більш достовірний прогноз втоми і втомної довговічності лопатки з суперсплавів при повзучості

Dissociative chemisorption of methane on Ni(111) using a chemically accurate fifteen dimensional potential energy surface

Theoretical Chemistr