强非线性受迫振动的多尺度分析

考虑形式为的强非线性振动响应,f(u)是奇非线性的,ε不需要是小的,且m=1,0,或-1。可用两步多尺度逼近获得近似解,与通常的多尺度法不同的是以下两个步骤:(1)失谐采用激励频率的平方Ω~2,并将它作为对所谓“主干曲线”的偏差,(2)定义新的展开参数α=α(ε)。     

阿尔泰山林区云杉和落叶松生物量分配格局研究

【目的】为了分析阿尔泰山地针叶主要树种生物量分配格局,【方法】基于2012、2013年研究区西伯利亚云杉(Picea obovata)和西伯利亚落叶松(Larix sibirica)实测样木,从年龄、径阶、林分郁闭度等角度,分析树木各组分器官生物量比例变化规律。【结果】两树种树干占整株树木地上生物量比例分别为48.1%、61.9%; 云杉枝、叶占地上生物量比例均高于西伯利亚落叶松; 两树种地上总生物量占整株生物量比例分别为79.8%和79.5%,平均根冠比为0.268和0.270。西伯利亚云杉树干生物量随径阶增大而提高,树叶和树根生物量随径阶增大而降低,树枝生物量随径阶变化幅度较小。西伯利亚落叶松树干生物量随径阶增大而提高,但在30 cm以上径阶比例稍有降低,树枝、树叶生物量与树干生物量呈相反变化趋势。两树种树干生物量均随年龄增加而提高,树枝、树叶生物量随年龄增加而降低。西伯利亚云杉树根生物量随年龄增加而降低,西伯利亚落叶松树根生物量随年龄增加呈先降后增的趋势。西伯利亚云杉树干生物量随郁闭度增加而提高,树枝、树叶和树根生物量随郁闭度增加而降低。西伯利亚落叶松总体上随郁闭度增加树干生物量呈先降后升趋势,树枝和树根生物量变化趋势与树干相反,树叶生物量随郁闭度增加而降低,但变化幅度很小。【结论】西伯利亚云杉和落叶松各组织器官生物量比例均呈现树干＞树根＞树枝＞树叶的趋势。随径阶、年龄、郁闭度增大,西伯利亚云杉树干生物量所占比例逐渐提高,树叶比例则逐渐降低,树枝与树根生物量此消彼长。西伯利亚落叶松各器官生物量所占比例在不同径阶、年龄和郁闭度条件下虽有变化,但幅度不大,总体趋势不变。     

Future lab-on-a-chip technologies for interrogating individual molecules

Advances in technology have allowed chemical sampling with high spatial resolution and the manipulation and measurement of individual molecules. Adaptation of these approaches to lab-on-a-chip formats is providing a new class of research tools for the investigation of biochemistry and life processes.     

戒指

这是一个暖洋洋的下午,我闲坐在宽大的阳台上打发时光。门铃响了,来人说,他是监察员斯特尔皮克,希望他的拜访没有打搅我。我镇静地说:“非常荣幸,我一直在等着一位监察员的访问呢。”他惊讶地把细眉弯成弓形,从我手中接过酒杯。“你的案子让我迷惑。”他陷入沉默,好像在沉思。我爽快地说:“请开始吧,我不会隐瞒的。”他冷静地说:“我敢说,直到去年4月20     

The medaka draft genome and insights into vertebrate genome evolution

Teleosts comprise more than half of all vertebrate species and have adapted to a variety of marine and freshwater habitats. Their genome evolution and diversification are important subjects for the understanding of vertebrate evolution. Although draft genome sequences of two pufferfishes have been published, analysis of more fish genomes is desirable. Here we report a high-quality draft genome sequence of a small egg-laying freshwater teleost, medaka (Oryzias latipes). Medaka is native to East Asia and an excellent model system for a wide range of biology, including ecotoxicology, carcinogenesis, sex determination and developmental genetics. In the assembled medaka genome (700 megabases), which is less than half of the zebrafish genome, we predicted 20,141 genes, including approximately 2,900 new genes, using 5'-end serial analysis of gene expression tag information. We found single nucleotide polymorphisms (SNPs) at an average rate of 3.42% between the two inbred strains derived from two regional populations; this is the highest SNP rate seen in any vertebrate species. Analyses based on the dense SNP information show a strict genetic separation of 4 million years (Myr) between the two populations, and suggest that differential selective pressures acted on specific gene categories. Four-way comparisons with the human, pufferfish (Tetraodon), zebrafish and medaka genomes revealed that eight major interchromosomal rearrangements took place in a remarkably short period of approximately 50 Myr after the whole-genome duplication event in the teleost ancestor and afterwards, intriguingly, the medaka genome preserved its ancestral karyotype for more than 300 Myr.     

Soil fertility limits carbon sequestration by forest ecosystems in a CO2-enriched atmosphere

Northern mid-latitude forests are a large terrestrial carbon sink. Ignoring nutrient limitations, large increases in carbon sequestration from carbon dioxide (CO2) fertilization are expected in these forests. Yet, forests are usually relegated to sites of moderate to poor fertility, where tree growth is often limited by nutrient supply, in particular nitrogen. Here we present evidence that estimates of increases in carbon sequestration of forests, which is expected to partially compensate for increasing CO2 in the atmosphere, are unduly optimistic. In two forest experiments on maturing pines exposed to elevated atmospheric CO2, the CO2-induced biomass carbon increment without added nutrients was undetectable at a nutritionally poor site, and the stimulation at a nutritionally moderate site was transient, stabilizing at a marginal gain after three years. However, a large synergistic gain from higher CO2 and nutrients was detected with nutrients added. This gain was even larger at the poor site (threefold higher than the expected additive effect) than at the moderate site (twofold higher). Thus, fertility can restrain the response of wood carbon sequestration to increased atmospheric CO2. Assessment of future carbon sequestration should consider the limitations imposed by soil fertility, as well as interactions with nitrogen deposition.     

Sub-laser-cycle electron pulses for probing molecular dynamics

Experience shows that the ability to make measurements in any new time regime opens new areas of science. Currently, experimental probes for the attosecond time regime (10(-18) 10(-15) s) are being established. The leading approach is the generation of attosecond optical pulses by ionizing atoms with intense laser pulses. This nonlinear process leads to the production of high harmonics during collisions between electrons and the ionized atoms. The underlying mechanism implies control of energetic electrons with attosecond precision. We propose that the electrons themselves can be exploited for ultrafast measurements. We use a 'molecular clock', based on a vibrational wave packet in H(2)(+) to show that distinct bunches of electrons appear during electron ion collisions with high current densities, and durations of about 1 femtosecond (10(-15) s). Furthermore, we use the molecular clock to study the dynamics of non-sequential double ionization.     

线性系统中随机共振现象的研究

在高斯噪声同时作用系统阻尼系数和激励信号背景下,研究了欠阻尼二阶线性系统中的随机共振现象.研究结果表明,欠阻尼二阶线性系统中存在随机共振现象,该系统的平均输出幅度增益呈现非单调变化,不仅在一定条件下大于无噪声时的增益,而且调节适当的系统参数和噪声强度能够提高幅度增益.最后,进行了计算机模拟仿真,证实了结论.     

An improved branch-and-bound algorithm to minimize the weighted flowtime on identical parallel machines with family setup times

This article investigates identical parallel machines scheduling with family setup times. The objective function being the weighted sum of completion times, the problem is known to be strongly NP-hard. We propose a constructive heuristic algorithm and three complementary lower bounds. Two of these bounds proceed by elimination of setup times or by distributing each of them to jobs of the corresponding family, while the third one is based on a lagrangian relaxation. The bounds and the heuristic are incorporated into a branch-and-bound algorithm. Experimental results obtained outperform those of the methods presented in previous works, in term of size of solved problems.     

Using administrative data sources in the estimation of emigration

It is important that migration is measured accurately, for example to inform population estimates and projections. However, current sources of information present challenges in producing robust estimates of emigration from Great Britain. This article reports on work carried out by the Office for National Statistics to investigate the potential for using administrative data sources to contribute to the measurement of emigration.