全文获取类型
收费全文 | 79804篇 |
免费 | 313篇 |
国内免费 | 568篇 |
专业分类
系统科学 | 1297篇 |
丛书文集 | 1015篇 |
教育与普及 | 298篇 |
理论与方法论 | 559篇 |
现状及发展 | 38475篇 |
研究方法 | 1894篇 |
综合类 | 34954篇 |
自然研究 | 2193篇 |
出版年
2013年 | 827篇 |
2012年 | 932篇 |
2011年 | 3121篇 |
2008年 | 1184篇 |
2007年 | 1443篇 |
2006年 | 1436篇 |
2005年 | 1645篇 |
2004年 | 2788篇 |
2003年 | 2372篇 |
2002年 | 2088篇 |
2001年 | 2281篇 |
2000年 | 1800篇 |
1999年 | 1450篇 |
1997年 | 787篇 |
1994年 | 706篇 |
1993年 | 706篇 |
1992年 | 1374篇 |
1991年 | 1139篇 |
1990年 | 1267篇 |
1989年 | 1052篇 |
1988年 | 1004篇 |
1987年 | 1048篇 |
1986年 | 1071篇 |
1985年 | 1342篇 |
1984年 | 1075篇 |
1983年 | 946篇 |
1982年 | 952篇 |
1981年 | 972篇 |
1980年 | 1117篇 |
1979年 | 2154篇 |
1978年 | 1793篇 |
1977年 | 1730篇 |
1976年 | 1484篇 |
1975年 | 1538篇 |
1974年 | 1775篇 |
1973年 | 1774篇 |
1972年 | 1856篇 |
1971年 | 2054篇 |
1970年 | 2552篇 |
1969年 | 2074篇 |
1968年 | 1956篇 |
1967年 | 1905篇 |
1966年 | 1702篇 |
1965年 | 1313篇 |
1959年 | 758篇 |
1958年 | 1201篇 |
1957年 | 896篇 |
1956年 | 756篇 |
1955年 | 688篇 |
1954年 | 731篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
对一台机器加工时间可控时的延误工件数问题,如何构造延误工件数和加工时间减少量之间的产工线,文献「1」给出了一些理论结果,但未给出具体算法。该文在wj恒为1的条件下,分析了文献「1」所获得结果,证明了曲线上有效值的个数,并从考虑延误工件数出发在文献「1」基础上给出了构造上述曲线的具体算法。 相似文献
942.
k consisting of k clusters, with k > 2. Bottom-up agglomerative approaches are also commonly used to construct partitions,
and we discuss these in terms of worst-case performance for metric data sets. Our main contribution derives from a new restricted
partition formulation that requires each cluster to be an interval of a given ordering of the objects being clustered. Dynamic programming can optimally split such an ordering into a partition Pk for a large class of objectives that includes min-diameter. We explore a variety of ordering heuristics and show that our
algorithm, when combined with an appropriate ordering heuristic, outperforms traditional algorithms on both random and non-random
data sets. 相似文献
943.
944.
945.
T. Matsuoka T. Nishizaki Y. Ikeuchi Y. Okada K. Sumino 《Cellular and molecular life sciences : CMLS》1997,53(3):233-236
Effects of serotonin (5-HT) on cerebral cortical neurons were examined by patch clamp techniques. 5-HT produced a variety
of responses such as outward (19/73 patches/neurons), slow inward (15/73 patches/neurons), fast inward (8/73 patches/neurons),
and mixed currents (initially fast inward deflection followed by an outward response: 2/73 patches/neurons), with a latency
of 12 sec, 15 sec, 0 sec, and 0 sec respectively, at a holding potential of −60 mV in whole-cell patches. The fast inward
currents were again evoked by a selective 5-HT3 receptor agonist, 1-(m-chlorophenyl)-biguanide hydrochloride (CPBG). In the
cell-attached patch clamp configuration, 5-HT inside the patch pipette elicited single channel currents with slope conductances
of 42 pS and 132 pS (4/42 patches/neurons). CPBG inside the patch pipette evoked inward single channel currents with a lower
slope conductance of 41 pS (3/23 patches/neurons). In contrast, application of 5-HT or a 5-HT2 receptor agonist, α-methyl-5-hydroxytryptamine-maleate, outside the patch pipette induced outward single channel currents with a major slope
conductance of 140 pS (8/30 patches/neurons) or 135 pS (6/20 patches/neurons), respectively. These results indicate that the
outward and fast inward currents may be mediated respectively by the 5-HT2 receptor, which is coupled to a G-protein, and by the 5-HT3 receptor, which contains the non-selective cation channel, and that the mixed type may be caused by both the 5-HT2 and 5-HT3 receptors.
Received 27 September 1996; received after revision 4 November 1996; accepted 7 November 1996 相似文献
946.
Regulation of cyclooxygenase enzymes by nitric oxide 总被引:12,自引:0,他引:12
D. Salvemini 《Cellular and molecular life sciences : CMLS》1997,53(7):576-582
947.
J. W. M. Lagerberg J. VanSteveninck T. M. A. R. Dubbelman 《Cellular and molecular life sciences : CMLS》1997,53(3):257-262
The fluorescent dye Merocyanine 540 (MC540) is often used as a probe to monitor the molecular packing of phospholipids in
the outer leaflet of biomembranes. In a previous study we showed that the increased staining of erythrocytes with a perturbed
membrane structure was mainly due to an increase in the fluorescence yield of cell-bound MC540, rather than to an increase
of the number of bound molecules. Erythrocytes and ghosts exposed to continuous fluxes of H2O2 exhibited pronounced lipid peroxidation. Further, red blood cells subjected to this form of oxidative stress also showed
increased staining with MC540. It appeared that this was caused by a strong increase in binding of MC540, together with a
slight red shift of the fluorescence emission maximum and a small increase in the fluorescence yield of bound MC540. The changed
MC540 binding characteristics were not observed when lipid peroxidation was suppressed by the presence of the antioxidant
BHT in the incubation medium. However, open ghosts exposed to H2O2 showed no increase of MC540 binding, excluding a direct involvement of lipid peroxidation. Measurement of fluorescence emission
spectra and gel filtration studies showed that MC540 can bind to H2O2-exposed hemoglobin. Experiments with erythrocytes lysed in hypotonic medium after exposure to H2O2 revealed that peroxidation of lipids with H2O2 induced a non-specific permeabilization of the plasma membrane to MC540, thereby allowing MC540 to bind to the oxidatively
denatured, more hydrophobic hemoglobin. These results indicate that conclusions about packing of phospholipids in the outer
leaflet of the membrane based on increased MC540-staining should be drawn with care.
Received 27 September 1996; received after revision 5 November 1996; accepted 27 November 1996 相似文献
948.
949.
利用专门设计的厚度方向缺口四点弯曲试样,结合相应的弹塑性有限元计算结果,在液氮温度下测取了一种海洋平台用钢的6种微观组织分层面上的解理断裂应力σ*f.结果显示,均匀细小的铁素体+贝氏体组织具有比铁素体+带状珠光体为高的σ*f值.对于同一终轧温度,轧后冷速愈高,其得到的微观组织之σ*f值愈高.而对于同一轧后冷却制度,则终轧于Ar3与再结晶停止温度之间者其得到的微观组织具有最高的σ*f值.讨论了各微观组织组成相的作用 相似文献
950.
对微可压缩非牛顿幂律流体在均质多孔介质中的非稳定径向渗流方程进行了回顾和对比,用有限差分方法建立了微分方程的数值解法.研究了时间步长、空间步长对数值解精度和稳定性的影响,并结合算例绘出了不同流性指数的瞬时压力曲线.该研究结果可用于非牛顿流体渗流机理研究及试井分析. 相似文献