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991.
Vegetation patterning in water-limited and other resource-limited ecosystems highlights spatial self-organization processes as potentially key drivers of community assembly. These processes provide insight into predictable landscape-level relationships between organisms and their abiotic environment in the form of regular and irregular patterns of biota and resources. However, two aspects have largely been overlooked; the roles played by plant – soil-biota feedbacks and allelopathy in spatial self-organization, and their potential contribution, along with plant-resource feedbacks, to community assembly through spatial self-organization. Here, we expand the drivers of spatial self-organization from a focus on plant-resource feedbacks to include plant – soil-biota feedbacks and allelopathy, and integrate concepts of nonlinear physics and community ecology to generate a new hypothesis. According to this hypothesis, below-ground processes can affect community assemblages through two types of spatial self-organization, global and local. The former occurs simultaneously across whole ecosystems, leading to self-organized patterns of biota, allelochemicals and resources, and niche partitioning. The latter occurs locally in ecotones, and determines ecotone structure and motion, invasion dynamics, and species coexistence. Studies of the two forms of spatial self-organization are important for understanding the organization of plant communities in drier climates which are likely to involve spatial patterning or re-patterning. Such studies are also important for developing new practices of ecosystem management, based on local manipulations at ecotones, to slow invasion dynamics or induce transitions from transitive to intransitive networks of interspecific interactions which increase species diversity. 相似文献
992.
Mangilal Choudhary Roman Bergert Sandra Moritz Slobodan Mitic Markus H. Thoma 《等离子体物理论文集》2021,61(1):e202000110
The collective dynamics of an annulus dusty plasma formed between a co-centric conducting (non-conducting) disk and ring configuration is studied in a strongly magnetized radiofrequency (rf) discharge. A superconducting electromagnet is used to introduce a homogeneous magnetic field to the dusty plasma medium. In the absence of the magnetic field, the dust grains exhibit thermal motion around their equilibrium position. The dust grains start to rotate in the anticlockwise direction with increasing magnetic field (B > 0.02 T ), and the constant value of the angular frequency at various strengths of the magnetic field confirms the rigid body rotation. The angular frequency of dust grains linearly increases up to a threshold magnetic field (B > 0.6 T ) and after that its value remains nearly constant in a certain range of magnetic field. Further increase in magnetic field (B > 1 T ) lowers the angular frequency. Low value of the angular frequency is expected by reducing the width of the annulus dusty plasma or the input rf power. The azimuthal ion drag force due to the magnetic field is assumed to be the energy source which drives the rotational motion. The resultant radial electric field in the presence of a magnetic field determines the direction of rotation. The variation of floating (plasma) potential across the annular region at given magnetic field explains the rotational properties of the annulus dusty plasma in the presence of a magnetic field. 相似文献
993.
Qingguo Wang Yiding Shen Hetang Wang Chaohang Xu 《Journal of Dispersion Science and Technology》2017,38(12):1726-1731
Foamability, liquid holdup, and foam appearance are key factors that determine dust control efficiencies. As the foam production method of the FoamScan instrument is similar to foaming device used for dust control, and its measurement means can satisfy the requirements of precise measuring, the FoamScan technology is adopted to explore the influence of xanthan gum (XG) and partial hydrolytic polyacrylamide (HPAM) on dust-related foam properties of sodium dodecyl benzene sulfonate (SDBS). It was found that with the increase of the polymer mass fraction, the liquid volume in the foam gradually increased. Additionally, the foaming time t200 of the foaming agent decreased at first, then remained almost constant for both polymers, which indicated that the foamability and liquid holdup were enhanced by the addition of polymers into SDBS. In addition, the efficiencies of XG are higher than that of HPAM. The image analysis using the CSA software revealed that the mean radius formed by XG was smaller than that by HPAM and the number of bubbles was larger with XG than with HPAM. Thus, the XG foam has more area to contact with dust and could control dust better. The highly branched molecular structure and hydrogen bonds formed by XG played important roles in dust-related foam properties. 相似文献
994.
Lori Adams John P. Cronin 《International journal of environmental analytical chemistry》2017,97(3):264-275
Exposures to beryllium (Be), even at extremely low levels, can cause severe health effects in a percentage of those exposed; consequently, occupational exposure limits (OELs) promulgated for this element are the lowest established for any element. This work describes the advantages of using highly alkaline dye solutions for determination of Be in occupational hygiene and environmental samples by means of an optical molecular fluorescence technique after sample extraction in 1–3% (w?w?1) aqueous ammonium bifluoride (NH4HF2). Improved attributes include the ability to further enhance the detection limits of Be in extraction solutions of high acidity with minimal dilution, which is particularly beneficial when NH4HF2 solutions of higher concentration are used for extraction of Be from soil samples. Significant improvements in Be method detection limits (MDLs) are obtained at levels manyfold below those reported previously for this methodology. Notably, MDLs for Be of <0.01 ng L?1 /0.1 ng per sample have been attained, which are superior to MDLs routinely reported for this element by means of the most widely used ultra-trace elemental measurement technique, inductively coupled plasma mass spectrometry (ICP-MS). Very low MDLs for Be are essential in consideration of reductions in OELs for this element in workplace air by health organisations and regulatory agencies in the USA and internationally. Applications of enhanced Be measurements to air filter samples, surface wipe samples, soils and newly designed occupational air sampler inserts are illustrated. 相似文献
995.
996.
本文研究了几类图的距离模式识别性.利用构造法,求出了它们的距离模式识别集和距离模式识别数,提出距离模式识别率的概念,推广了距离模式识别数的概念. 相似文献
997.
Sergey Kitaev 《Journal of Graph Theory》2017,85(3):661-668
Recently, Jones et al. (Electron J Comb 22(2) (2015), #P2.53) introduced the study of u‐representable graphs, where u is a word over containing at least one 1. The notion of a u‐representable graph is a far‐reaching generalization of the notion of a word‐representable graph studied in the literature in a series of papers. Jones et al. have shown that any graph is 11???1‐representable assuming that the number of 1s is at least three, while the class of 12‐representable graphs is properly contained in the class of comparability graphs, which, in turn, is properly contained in the class of word‐representable graphs corresponding to 11‐representable graphs. Further studies in this direction were conducted by Nabawanda (M.Sc. thesis, 2015), who has shown, in particular, that the class of 112‐representable graphs is not included in the class of word‐representable graphs. Jones et al. raised a question on classification of u‐representable graphs at least for small values of u . In this article, we show that if u is of length at least 3 then any graph is u‐representable. This rather unexpected result shows that from existence of representation point of view there are only two interesting nonequivalent cases in the theory of u‐representable graphs, namely, those of and . 相似文献
998.
A residual based sparse approximate inverse preconditioning procedure for large sparse linear systems 下载免费PDF全文
We propose a residual based sparse approximate inverse (RSAI) preconditioning procedure, for the large sparse linear system A x =b . Different from the SParse Approximate Inverse (SPAI) algorithm proposed by Grote and Huckle (SIAM Journal on Scientific Computing, 18 (1997), pp. 838–853.), RSAI uses only the dominant other than all the information on the current residual and augments sparsity patterns adaptively during loops. In order to control the sparsity of M , we develop two practical algorithms RSAI(f i x ) and RSAI(t o l ). RSAI(f i x ) retains the prescribed number of large nonzero entries and adjusts their positions in each column of M among all available ones, in which the number of large entries is increased by a fixed number at each loop. In contrast, the existing indices of M by SPAI are untouched in subsequent loops and a few most profitable indices are added to each column of M from the new candidates in the next loop. RSAI(t o l ) is a tolerance based dropping algorithm and retains all large entries by dynamically dropping small ones below some tolerances, and it better suits for the problem where the numbers of large entries in the columns of A ?1 differ greatly. When the two preconditioners M have almost the same or comparable numbers of nonzero entries, the numerical experiments on real‐world problems demonstrate that RSAI(f i x ) is highly competitive with SPAI and can outperform the latter for some problems. We also make comparisons of RSAI(f i x ), RSAI(t o l ), and power sparse approximate inverse(t o l ) proposed Jia and Zhu (Numerical Linear Algebra with Applications, 16 (2009), pp.259–299.) and incomplete LU factorization type methods and draw some general conclusions. 相似文献
999.
Surface pattern formation on soft polymer substrate through photo‐initiated graft polymerization 下载免费PDF全文
Techniques for large‐area pattern formation on polymeric substrates are important for fabricating a large variety of functional devices, such as flexible electronics, tunable optical devices, adhesives, and so on. The present study demonstrates a method for pattern formation on poly(dimethylsiloxane) that involves grafting methacrylate polymers through photo‐initiated polymerization. The influence of substrate stiffness and monomers type on pattern formation was investigated. Firstly, the stiffness of the substrate was found to affect the topology of the patterns produced. The gap width of convex regions of the pattern was enlarged with decreasing stiffness. It was found that the gap width trended in a manner that was consistent with previous reports, but in this study, relatively large gap widths were observed compared with those from previous studies. Secondly, it was revealed that the solubility of the monomer in the poly(dimethylsiloxane) precursor was the dominant factor in determining whether or not pattern formation occurred. When using insoluble monomers (glycidyl methacrylate and benzyl methacrylate), characteristic patterns were observed. It is speculated that intermolecular attractive forces between the grafted polymers induce lateral aggregation on the substrate, resulting in buckling instability of the grafted polymer layer caused by a mismatch in the equilibrium between the grafted polymer layer and the substrate. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
1000.
Three compounds, [Cu(Pic)2(H2O)] (1a), [M(Pic)2(H2O)2]·2H2O] (M=Co (2a), Zn (3a), Pic = 2-picolinic acid) were obtained by solvent-free synthesis through grinding of metal acetate salt with 2-picolinic acid. Favorable comparison of solvent-free with solution based method of 1b, 2b and 3b was observed. Good resemblance of identity of compounds obtained through the two methods was confirmed by elemental analysis, spectroscopic techniques (UV-Vis and FTIR), TGA and PXRD. The single crystal diffraction data for [Co(Pic)2(H2O)2]·2H2O obtained from the Cambridge structure database (CSD), its PXRD simulated patterns closely matched that of complex 2a by solvent-free synthesis. Vapochromic behavior of this complex was studied using colour change, FT-IR, TGA, PXRD and solid state UV-visible spectroscopies. This complex generated specific colour which is also evident in the shifting of the vibrational frequencies (νO-H and ν C=O bands). The resulting inclusion compounds have different colours depending on the solvent used. In addition, exposure of the resultant inclusion compounds to ambient environment or heating for a few minutes regenerate the original material without degradation even after exposure/heating cycles as evident from TGA/DTG thermograms. 相似文献