全文获取类型
收费全文 | 22492篇 |
免费 | 57篇 |
国内免费 | 53篇 |
专业分类
系统科学 | 226篇 |
丛书文集 | 493篇 |
教育与普及 | 45篇 |
理论与方法论 | 60篇 |
现状及发展 | 9512篇 |
研究方法 | 885篇 |
综合类 | 11009篇 |
自然研究 | 372篇 |
出版年
2013年 | 129篇 |
2012年 | 278篇 |
2011年 | 668篇 |
2010年 | 101篇 |
2008年 | 331篇 |
2007年 | 378篇 |
2006年 | 401篇 |
2005年 | 393篇 |
2004年 | 377篇 |
2003年 | 384篇 |
2002年 | 309篇 |
2001年 | 709篇 |
2000年 | 706篇 |
1999年 | 415篇 |
1992年 | 410篇 |
1991年 | 363篇 |
1990年 | 387篇 |
1989年 | 327篇 |
1988年 | 368篇 |
1987年 | 366篇 |
1986年 | 342篇 |
1985年 | 485篇 |
1984年 | 363篇 |
1983年 | 305篇 |
1982年 | 238篇 |
1981年 | 256篇 |
1980年 | 343篇 |
1979年 | 689篇 |
1978年 | 580篇 |
1977年 | 551篇 |
1976年 | 472篇 |
1975年 | 531篇 |
1974年 | 663篇 |
1973年 | 579篇 |
1972年 | 588篇 |
1971年 | 703篇 |
1970年 | 926篇 |
1969年 | 726篇 |
1968年 | 624篇 |
1967年 | 653篇 |
1966年 | 596篇 |
1965年 | 436篇 |
1964年 | 116篇 |
1959年 | 264篇 |
1958年 | 400篇 |
1957年 | 303篇 |
1956年 | 269篇 |
1955年 | 238篇 |
1954年 | 263篇 |
1948年 | 176篇 |
排序方式: 共有10000条查询结果,搜索用时 4 毫秒
11.
Astacin, a digestive zinc-endopeptidase from the crayfish Astacus astacus L., is the prototype for the 'astacin family', which includes mammalian metallo-endopeptidases and developmentally regulated proteins of man, fruitfly, frog and sea urchin. Here we report the X-ray crystal structure of astacin, which reveals a deep active-site cleft, with the zinc at its bottom ligated by three histidines, a water molecule and a more remote tyrosine. The third histidine (His 102) forms part of a consensus sequence, shared not only by the members of the astacin family, but also by otherwise sequentially unrelated proteinases, such as vertebrate collagenases. It may therefore represent the elusive 'third' zinc ligand in these enzymes. The amino terminus of astacin is buried forming an internal salt-bridge with Glu 103, adjacent to His 102. Astacin pro-forms extended at the N terminus, as observed for some 'latent' mammalian astacin homologues, did not exhibit this 'active' conformation, indicating an activation mechanism reminiscent of trypsin-like serine proteinases. 相似文献
12.
13.
14.
15.
R. Poggioli A. V. Vergoni A. Bertolini 《Cellular and molecular life sciences : CMLS》1985,41(2):265-266
Summary Hydrochlorothiazide, acutely injected in rats, has a weak analgesic activity per se and potentiates and prolongs the antinociceptive effect of morphine.This work was supported in part by grants from Consiglio Nazionale delle Ricerche, and by Ministero della Pubblica Istruzione, Roma. 相似文献
16.
H R Brenner 《Nature》1985,317(6038):572-573
17.
18.
In econometrics, as a rule, the same data set is used to select the model and, conditional on the selected model, to forecast. However, one typically reports the properties of the (conditional) forecast, ignoring the fact that its properties are affected by the model selection (pretesting). This is wrong, and in this paper we show that the error can be substantial. We obtain explicit expressions for this error. To illustrate the theory we consider a regression approach to stock market forecasting, and show that the standard predictions ignoring pretesting are much less robust than naive econometrics might suggest. We also propose a forecast procedure based on the ‘neutral Laplace estimator’, which leads to an improvement over standard model selection procedures. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
19.
Urinary excretion of glycated albumin was quantitated in genetically hyperglycemic mice (C57BL-Ks-J, db/db mice), a model for non-insulin-dependent diabetes mellitus, and compared with their non-diabetic littermates. The data indicated a preferential excretion of glycated albumin in non-diabetic mice. This phenomenon of 'editing' of glycated albumin is decreased significantly in diabetic mice. Quantitative measurements of overall excretion of glycated albumin suggested that the loss of editing in diabetic mice is due to the dilution of glycated albumin by the unmodified albumin which is excreted in large amounts in diabetic mice. Therefore, the loss of editing observed in this model resembled the one we characterized in insulin-dependent diabetic humans and a streptozotocin-diabetic rat model. 相似文献
20.
Nicholas R. Farnum 《Journal of forecasting》1992,11(1):47-56
When using simple exponential smoothing on a given time series the nature of the relationship between the optimal smoothing constant and the autocorrelation structure of the series remains an unresolved question. Although numerical search routines can easily be used to find optimal values of the smoothing constant, they offer little insight into the nature of the relationship between the estimated smoothing constant and the structure of the underlying time series. We suggest that renewed investigations of the ex-post sum of squares function may prove helpful in this pursuit. Results are presented that illustrate how the optimal smoothing constant depends upon the value used to initialize the smoothing and upon the sample autocorrelation coefficients of the observed series. These results are based on a new formula for the derivative of the ex-post sum of squares function. In particular, the derivative is examined near 0 and 1, where great simplifications occur in its form, thereby facilitating investigations near these points. A necessary and sufficient condition is stated for when the ex-post sum of squares must have a positive derivative at 0 and the autocorrelation coefficients of the differenced series are shown to affect the sign of the derivative near 1. Based on these results, a general algorithm is presented as an alternative to grid search routines for minimizing the ex-post sum of squares. 相似文献