全文获取类型
收费全文 | 115093篇 |
免费 | 1420篇 |
国内免费 | 393篇 |
专业分类
化学 | 44987篇 |
晶体学 | 938篇 |
力学 | 7474篇 |
综合类 | 1篇 |
数学 | 37875篇 |
物理学 | 25631篇 |
出版年
2023年 | 207篇 |
2021年 | 313篇 |
2020年 | 496篇 |
2019年 | 343篇 |
2018年 | 10751篇 |
2017年 | 10547篇 |
2016年 | 6959篇 |
2015年 | 1659篇 |
2014年 | 1231篇 |
2013年 | 2367篇 |
2012年 | 5327篇 |
2011年 | 12239篇 |
2010年 | 6533篇 |
2009年 | 6758篇 |
2008年 | 8311篇 |
2007年 | 10402篇 |
2006年 | 1804篇 |
2005年 | 2724篇 |
2004年 | 2650篇 |
2003年 | 2923篇 |
2002年 | 1890篇 |
2001年 | 996篇 |
2000年 | 951篇 |
1999年 | 609篇 |
1998年 | 607篇 |
1997年 | 553篇 |
1996年 | 719篇 |
1995年 | 585篇 |
1994年 | 559篇 |
1993年 | 657篇 |
1992年 | 522篇 |
1991年 | 418篇 |
1990年 | 444篇 |
1989年 | 380篇 |
1988年 | 400篇 |
1987年 | 389篇 |
1986年 | 369篇 |
1985年 | 543篇 |
1984年 | 513篇 |
1983年 | 472篇 |
1982年 | 484篇 |
1981年 | 484篇 |
1980年 | 473篇 |
1979年 | 419篇 |
1978年 | 452篇 |
1977年 | 415篇 |
1976年 | 362篇 |
1975年 | 347篇 |
1974年 | 328篇 |
1973年 | 346篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Otto Funk Lothar Schwabe Karl-Heinz Frömming 《Journal of inclusion phenomena and macrocyclic chemistry》1993,16(4):299-314
The purpose of the study was to examine the formation of inclusion compounds in the freeze-dried products obtained from aqueous solutions of nicotinic acid and -cyclodextrin (-CD), or heptakis (2,6-0-dimethyl)--cyclodextrin (DIMEB). The molar ratios used were 1:1 and 2:1. In addition two freezing temperatures (–40 and –196°C) and different secondary drying temperatures (+50 and +80°C) were used. Freeze-dried products with -CD obtained after low temperature freezing are of the same crystallographic structure as seen in a pure inclusion compound prepared by coprecipitation. Amorphous products were formed after fast freezing. The molar ratios of included nicotinic acid in the freeze-dried products vary — dependent on the preparation conditions — between 0.75:1 and 1:1. A factorial design proves that the included drug amount can be increased by enhancement of the amount of nicotinic acid used, by faster freezing, and by the combination of these two factors. The proof of inclusion formation was given by a combination of X-ray diffractography, differential scanning calorimetry, thermogravimetry and thermofractography.The freeze-dried preparations obtained with DIMEB were amorphous mixtures of the two components. No proof for inclusion of the nicotinic acid in the cyclodextrin cavity could be given. Higher (–40°C) or lower (–196°C) freezing temperatures and the running of the secondary drying process could not influence these results. The very low stability constant of the complex and steric reasons are responsible for this behavior. 相似文献
992.
Inge Söderkvist 《BIT Numerical Mathematics》1993,33(4):687-694
Given two arbitrary real matricesA andB of the same size, the orthogonal Procrustes problem is to find an orthogonal matrixM such that the Frobenius norm MA – B is minimized. This paper treats the common case when the orthogonal matrixM is required to have a positive determinant. The stability of the problem is studied and supremum results for the perturbation bounds are derived. 相似文献
993.
Ingemar Bengtsson 《Theoretical and Mathematical Physics》1993,95(2):511-513
I give a very brief review of Ashtekar's variables, with an emphasis on the non-trivial reality condition. As a toy model a formulation of spin 1/2 fields is presented, in which the Dirac equation appears as a reality condition. Then, some new ways to modify complex general relativity are mentioned. It is not known whether the new theories admit any reality condition at all.Published in Teoreticheskaya i Matematicheskaya Fizika, Vol. 95, No. 2, pp. 211–213, May, 1993. 相似文献
994.
Otto Röschel 《Journal of Geometry》1993,47(1-2):131-140
In part I we have studied a map of osculating elements of an affine Cayley-Klein (CK-) plane into the Lie algebra A4(2) of the aequiform transformations A4(2) of the given plane A2(, 2). If we use the real projective space P3() over A4(2) each osculating element defines a straight line in P3(). We now give a one parameter motion in A4(2) and study second order properties and their analogon in the Lie algebra and P3(), respectively. We show that the wellknown relationship between the points of the moving frame and the osculating circles of the point paths in the fixed frame may be interpreted as part of a quadratic map of certain straight Lines of P3(). An analogous result holds for the curvature of pairs of envelopes; the mapV induced in P3() than is contained in a cubic relationship of straight lines.
Herrn Professor Oswal Giering zum 60. Geburtstag gewidmet 相似文献
Herrn Professor Oswal Giering zum 60. Geburtstag gewidmet 相似文献
995.
We prove, for the class of real locally convex spacesE that are continuously and linearly injectable into somec
0(), that every non-zero homomorphism on the algebraC
(E) ofC
-functions onE is given by a point evaluation at some point ofE. Furthermore, if every real-valuedC
-function on the weak topology of a quasi-complete locally convex spaceE is bounded on a subsetA ofE, thenA is relatively weakly compact. 相似文献
996.
Ingo Althöfer Gautam Das David Dobkin Deborah Joseph José Soares 《Discrete and Computational Geometry》1993,9(1):81-100
Given a graphG, a subgraphG' is at-spanner ofG if, for everyu,v V, the distance fromu tov inG' is at mostt times longer than the distance inG. In this paper we give a simple algorithm for constructing sparse spanners for arbitrary weighted graphs. We then apply this algorithm to obtain specific results for planar graphs and Euclidean graphs. We discuss the optimality of our results and present several nearly matching lower bounds.The work of G. Das and D. Joseph was supported by NSF PYI Grant DCR-8402375. The work of D. Dobkin was supported by NSF Grant CCR-8700917. The work of J. Soares was supported by CNPq proc 203039/87.4 (Brazil) and NSF Grant CCR-9014562. This research was accomplished while G. Das was a student at the University of Wisconsin-Madison. A preliminary version was presented at the Second Scandinavian Workshop on Algorithm Theory, Bergen, Norway, 1990, under the title Generating Sparse Spanners for Weighted Graphs, and proceedings appear in the series Lecture Notes in Computer Science, Springer-Verlag. The preliminary version also appears as Princeton University Technical Report CS-TR-261-90, and as University of Wisconsin-Madison Computer Sciences Technical Report 882. 相似文献
997.
Vidmantas Bentkus Friedrich Götze Ričardas Zitikis 《Journal of Theoretical Probability》1993,6(4):727-780
LetB be a real separable Banach space and letX,X 1,X 2,...∈B denote a sequence of independent identically distributed random variables taking values inB. DenoteS n =n ?1/2(X 1+...X n ). Let π:B→R be a polynomial. We consider (truncated) Edgeworth expansions and other asymptotic expansions for the distribution function of the r.v. π(S n ) with uniform and nonuniform bounds for the remainder terms. Expansions for the density of π(S n ) and its higher order derivatives are derived as well. As an application of the general results we get expansions in the integral and local limit theorems for ω-statistics $$\omega _n^p (q)\mathop { = n^{{p \mathord{\left/ {\vphantom {p 2}} \right. \kern-\nulldelimiterspace} 2}} }\limits^\Delta \smallint _{(0,1)} \{ F_n (x) - x\} ^p q(x)dx$$ and investigate smoothness properties of their distribution functions. Herep≥2 is an even number,q: [0, 1]→[0, ∞] is a measurable weight function, andF n denotes the empirical distribution function. Roughly speaking, we show that in order to get an asymptotic expansion with remainder termO(n ?α), α<p/2, for the distribution function of the ω-statistic, it is sufficient thatq is nontrivial, i.e., mes{t∈(0, 1):q(t)≠0}>0. Expansions of arbitrary length are available provided the weight functionq is absolutely continuous and positive on an nonempty subinterval of (0, 1). Similar results hold for the density of the distribution function and its derivatives providedq satisfies certain very mild smoothness condition and is bounded away from zero. The last condition is essential since the distribution function of the ω-statistic has no density whenq is vanishing on an nonempty subinterval of (0, 1). 相似文献
998.
999.
1000.