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991.
I-69杨竞争密度效果分析 总被引:1,自引:0,他引:1
用竞争密度效果的倒数式分析了I-69杨(Populus deltoids)的生长过程。随着时间的推移,C-D线在双对数图上向上移动。生物时间τ(τ被定义为逻辑斯蒂生长曲线中生长系数λ(t)的积分)与物理时间t的关系可以用双曲线方程表示。C-D效果倒数式(即1/w=Aρ+B,式中w和ρ分别代表平均单株材积和密度)中的系数A和B被求出。随着生物时间τ的增加,系数A急剧增另到最大值后逐渐下降。而系数B呈指数下降,倾向于接近零。随着林分的生长,生长系数λ(t)倾向于下降。 相似文献
992.
本文给出保证平方Logistic方程N′(t)=r(t)N(t)(l-bN(t-τ)-cN2(t-τ))的每一正解N(t)趋于正平衡点N*的一组充分条件.改进了 Gapalsamy,Ladas和罗交晚等人的结果. 相似文献
993.
Uwe T. Bornscheuer Gonzalo Rodríguez Ordoez Aurelio Hidalgo Anita Gollin Jonny Lyon Timothy S. Hitchman David P. Weiner 《Journal of Molecular Catalysis .B, Enzymatic》2005,36(1-6):8-13
The selectivity of 28 lipases and esterases in the hydrolysis of butanoates of o-, m- or p-substituted phenols was investigated in a microtiterplate format. The phenols released during enzyme-catalyzed hydrolysis were converted in situ with Gibbs’ reagent to form a blue indophenol complex, which was quantified spectrophotometrically at 600 nm. Substantial differences in rates were found, which exhibits that the type and position of the substituent at the alkyl group has a strong influence on the selectivity of the enzymes. For various enzymes, the p-nitro derivative was the best substrate, whereas for other enzymes the m-Cl-derivative was preferentially hydrolyzed. Analysis of the data using the Hammett equation showed that sometimes the observed changes followed a predictable trend, but in several cases the result is very unexpected. 相似文献
994.
995.
996.
用数学模型分析非典型肺炎预防和隔离措施的有效性 总被引:6,自引:3,他引:3
利用时滞常微分方程建立数学模型,研究在无任何预防和隔离措施的假想情况下非典型肺炎传染和发展的终极状态.通过对模型的讨论发现,在无任何预防和隔离措施的情况下,当非典疫情自生自灭以后,感染非典型肺炎的总人数占总人口的比例z主要取决于基本传染数R,即每个“非典”患者在其整个病程中的平均传染人数.根据有关报道[3,4],非典型肺炎的基本传染数R在2.2至3.6之间.根据我们的模型分析,当R=2.2时,z值可达85%左右,而当R=3.6时,z值可达97%左右.而事实上由于采取了预防和隔离措施,以北京市为例,感染非典型肺炎的总人数只有几千人,不到其总人口的千分之一.这充分说明了有关非典的预防和隔离措施的有效性. 相似文献
997.
We develop diagnostic measures for assessing the influence ofindividual observations when using empirical likelihood withgeneral estimating equations, and we use these measures to constructgoodness-of-fit statistics for testing possible misspecificationin the estimating equations. Our diagnostics include case-deletionmeasures, local influence measures and pseudo-residuals. Ourgoodness-of-fit statistics include the sum of local influencemeasures and the processes of pseudo-residuals. Simulation studiesare conducted to evaluate our methods, and real datasets areanalyzed to illustrate the use of our diagnostic measures andgoodness-of-fit statistics. 相似文献
998.
Single-cell-level behaviors of cells are typically inferred from ensemble measurements. However, such inferences implicitly assume a biological version of ergodicity: the percentage of cells in a state is identical to the probability to find a cell in that state. While the ergodicity does not always hold, it has been rarely tested. Here, we reveal that the ergodicity does not necessarily hold even for simple toggle switches and that apparent stabilities of the switches are due to a balance between single-cell-level biased stabilities and growth rates differences. Therefore, verification of the ergodicity and reconstructing single-cell-level behaviors are crucial for understanding intracellular systems. 相似文献
999.
Kevin M. Boswell Michael D. Kaller James H. CowanJr. Charles A. Wilson 《Hydrobiologia》2008,610(1):113-123
In the Gulf of Mexico (GOM), fish biomass estimates are necessary for the evaluation of habitat use and function following
the mandate for ecosystem-based fisheries management in the recently reauthorized Sustainable Fisheries Act of 2007. Acoustic
surveys have emerged as a potential tool to estimate fish biomass in shallow-water estuaries, however, the transformation
of acoustic data into an index of fish biomass is not straightforward. In this article, we examine the consequences of equation
selection for target strength (TS) to fish length relationships on potential error generation in hydroacoustic fish biomass
estimates. We applied structural equation models (SEMs) to evaluate how our choice of an acoustic TS–fish length equation
affected our biomass estimates, and how error occurred and propagated during this process. To demonstrate the magnitude of
the error when applied to field data, we used SEMs on normally distributed simulated data to better understand the sources
of error involved with converting acoustic data to fish biomass. As such, we describe where, and to what magnitude, error
propagates when estimating fish biomass. Estimates of fish lengths were affected by measurement errors of TS, and from inexact
relationships between fish length and TS. Differences in parameter estimates resulted in significant differences in fish biomass
estimates and led to the conclusion that in the absence of known TS–fish length relationships, Love’s (J Acoust Soc Am 46:746–752,
1969) lateral-aspect equation may be an acceptable substitute for an ecosystem-specific TS–fish length relationship. Based upon
SEMs applied to simulated data, perhaps the most important, yet most variable, component is the mean volume backscattering
strength, which significantly inflated biomass errors in approximately 10% of the cases.
Handling editor: M. Power 相似文献
1000.
Browsing by ungulates has become a hotly debated issue in many European mountain forests in the past century. Acer pseudoplatanus L. and Fraxinus excelsior L. are broadleaved tree species that are preferentially browsed by roe deer (Capreolus capreolus) in central Europe. We analyzed growth data from shaded saplings of both tree species to quantify the extent to which height
growth after game browsing is reduced in subsequent, unbrowsed years in forest stands.
Sixty saplings of F. excelsior and A. pseudoplatanus from forest stands at Albisriederberg (Switzerland) were available to us that had been dissected into pieces that then were
split in the middle for counting tree rings and assessing ungulate damage. We fitted the von Bertalanffy growth equation to
these height growth data and included a reduction factor for winter browsing.
Both tree species showed significantly reduced height growth in unbrowsed years after one to several browsing events in winter,
and this effect increased with the number of browsing events. Saplings with a high growth rate showed a higher growth reduction.
After winter browsing, height growth of A. pseudoplatanus saplings was less affected in unbrowsed years than that of F. excelsior saplings.
We conclude that browsed saplings of these species in forest stands are not able to compensate browsing-induced height loss,
but that height differences between browsed and unbrowsed saplings probably increase over time. A comparison between our analysis
and the parameters estimated using equations published by Eiberle for predicting age at 130 cm height suggests that our parameter
values are rather conservative estimates of the growth reduction effect after winter browsing. Neither F. excelsior nor A. pseudoplatanus show a distinct pattern in browsing-induced growth reduction with respect to soil moisture, nutrient level, and altitude.
We thus conclude that our results are likely to be valid for a wide range of forested sites. 相似文献