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721.
The stabilizing effects of local enrichment are revisited. Diffusively coupled host-parasitoid and predator-prey metapopulations are shown to admit a stable fixed point, limit cycle or stable torus with a rich bifurcation structure. A linear toy model that yields many of the basic qualitative features of this system is presented. The further nonlinear complications are analyzed in the framework of the marginally stable Lotka-Volterra model, and the continuous time analog of the unstable, host-parasitoid Nicholson-Bailey model. The dependence of the results on the migration rate and level of spatial variations is examined, and the possibility of “nonlocal” effect of enrichment, where local enrichment induces stable oscillations at a distance, is studied. A simple method for basic estimation of the relative importance of this effect in experimental systems is presented and exemplified. 相似文献
722.
The four-electron reduction of dioxygen to water in multicopper oxidases takes place in a trinuclear copper cluster, which is linked to a mononuclear blue copper site, where the substrates are oxidized. Recently, several intermediates in the catalytic cycle have been spectroscopically characterized, and two possible structural models have been suggested for both the peroxy and native intermediates. In this study, these spectroscopic results are complemented by hybrid quantum and molecular mechanical (QM/MM) calculations, taking advantage of recently available crystal structures with a full complement of copper ions. Thereby, we obtain optimized molecular structures for all of the experimentally studied intermediates involved in the reductive cleavage of the O(2) molecule and energy profiles for individual reaction steps. This allows identification of the experimentally observed intermediates and further insight into the reaction mechanism that is probably relevant for the whole class of multicopper oxidases. We suggest that the peroxy intermediate contains an O(2)(2-) ion, in which one oxygen atom bridges the type 2 copper ion and one of the type 3 copper ions, whereas the other one coordinates to the other type 3 copper ion. One-electron reduction of this intermediate triggers the cleavage of the O-O bond, which involves the uptake of a proton. The product of this cleavage is the observed native intermediate, which we suggest to contain a O(2-) ion coordinated to all three of the copper ions in the center of the cluster. 相似文献
723.
Jean-François Cordeau Federico Pasin Marius M. Solomon 《Annals of Operations Research》2006,144(1):59-82
In this paper we introduce a new formulation of the logistics network design problem encountered in deterministic, single-country,
single-period contexts. Our formulation is flexible and integrates location and capacity choices for plants and warehouses
with supplier and transportation mode selection, product range assignment and product flows. We next describe two approaches
for solving the problem---a simplex-based branch-and-bound and a Benders decomposition approach. We then propose valid inequalities
to strengthen the LP relaxation of the model and improve both algorithms. The computational experiments we conducted on realistic
randomly generated data sets show that Benders decomposition is somewhat more advantageous on the more difficult problems.
They also highlight the considerable performance improvement that the valid inequalities produce in both solution methods.
Furthermore, when these constraints are incorporated in the Benders decomposition algorithm, this offers outstanding reoptimization
capabilities. 相似文献
724.
We consider the problem of deriving the asymptotic distribution of the three commonly used multivariate test statistics, namely likelihood ratio, Lawley-Hotelling and Bartlett-Nanda-Pillai statistics, for testing hypotheses on the various effects (main, nested or interaction) in multivariate mixed models. We derive the distributions of these statistics, both in the null as well as non-null cases, as the number of levels of one of the main effects (random or fixed) goes to infinity. The robustness of these statistics against departure from normality will be assessed.Essentially, in the asymptotic spirit of this paper, both the hypothesis and error degrees of freedom tend to infinity at a fixed rate. It is intuitively appealing to consider asymptotics of this type because, for example, in random or mixed effects models, the levels of the main random factors are assumed to be a random sample from a large population of levels.For the asymptotic results of this paper to hold, we do not require any distributional assumption on the errors. That means the results can be used in real-life applications where normality assumption is not tenable.As it happens, the asymptotic distributions of the three statistics are normal. The statistics have been found to be asymptotically null robust against the departure from normality in the balanced designs. The expressions for the asymptotic means and variances are fairly simple. That makes the results an attractive alternative to the standard asymptotic results. These statements are favorably supported by the numerical results. 相似文献
725.
726.
R. L. de Zafra M. Jaramillo J. Barrett L. K. Emmons A. Parrish P. M. Solomon 《International Journal of Infrared and Millimeter Waves》1990,11(4):463-467
We present a quasi-continuous record of measured atmospheric opacity obtained at 278 GHz (1.1 mm wavelength) from McMurdo Station, Antarctica during austral spring seasons in 1986 and 1987. McMurdo Station, at 78°S, 166°W, is easier to access than the Amundsen-Scott (South Pole) Station, although representing a warmer, sea level site with substantially higher typical opacity: the present record may be of interest to those contemplating mm-wave astronomical or atmospheric observations within the Antarctic region. Observations were made over a 256 MHz bandpass in 1986, during the period August 30 to October 30. In 1987, a 512 MHz bandpass was used, and observations were made during the period September 4 to October 13. All data are reduced to represent opacity in the zenith direction, and measurements were taken approximately every 20 minutes, except during storms or other periods of high opacity. The periods covered represent transitions from the polar winter towards summer conditions, and thus represent neither the best nor the worst that this site has to offer. 相似文献
727.
Measurements have been made which provide information concerning the rate of energy transfer between vibrationally excited HF and atomic fluorine. It was found that HF (v = 1) deactivates about seven times slower than was determined in previous experiments. The deactivation rate constant for HF (v = 2) is about six times larger than for HF (v = 1). 相似文献
728.
729.
A combustion apparatus capable of burning both solid and liquid samples is described. The samples were vaporized in helium and then burned in humid oxygen. Oxygen was then used to burn non-volatile residues within the vaporization chamber. The use of cheap, disposable vaporization chambers eliminated carryover from one analysis to the next. The application of this instrument to the determination of organochlorine-containing samples is described. The moist effluent hydrogen chloride from the combustion was condensed and the chloride content was determined by microcoulometry. Pure solutions of high-boiling inorganic chlorides did not evolve chloride but the addition of an amino acid resulted in evolution of all of the chloride present. 相似文献
730.
Fattal D Inoue K Vucković J Santori C Solomon GS Yamamoto Y 《Physical review letters》2004,92(3):037903
We report the generation of polarization-entangled photons, using a quantum dot single photon source, linear optics, and photodetectors. Two photons created independently are observed to violate Bell's inequality. The density matrix describing the polarization state of the postselected photon pairs is reconstructed and agrees well with a simple model predicting the quality of entanglement from the known parameters of the single photon source. Our scheme provides a method to create no more than one entangled photon pair per cycle after postselection, a feature useful to enhance quantum cryptography protocols based on shared entanglement. 相似文献