全文获取类型
收费全文 | 57624篇 |
免费 | 17429篇 |
国内免费 | 81篇 |
专业分类
化学 | 66493篇 |
晶体学 | 94篇 |
力学 | 2366篇 |
数学 | 4200篇 |
物理学 | 1981篇 |
出版年
2024年 | 460篇 |
2023年 | 4762篇 |
2022年 | 1613篇 |
2021年 | 2807篇 |
2020年 | 5180篇 |
2019年 | 2602篇 |
2018年 | 2571篇 |
2017年 | 698篇 |
2016年 | 6209篇 |
2015年 | 6178篇 |
2014年 | 5578篇 |
2013年 | 5892篇 |
2012年 | 3665篇 |
2011年 | 1410篇 |
2010年 | 3882篇 |
2009年 | 3831篇 |
2008年 | 1393篇 |
2007年 | 1070篇 |
2006年 | 380篇 |
2005年 | 371篇 |
2004年 | 334篇 |
2003年 | 278篇 |
2002年 | 233篇 |
1997年 | 222篇 |
1996年 | 226篇 |
1995年 | 306篇 |
1994年 | 225篇 |
1993年 | 367篇 |
1992年 | 233篇 |
1988年 | 253篇 |
1987年 | 217篇 |
1985年 | 245篇 |
1984年 | 223篇 |
1983年 | 224篇 |
1982年 | 253篇 |
1981年 | 267篇 |
1980年 | 310篇 |
1979年 | 301篇 |
1978年 | 305篇 |
1977年 | 419篇 |
1976年 | 474篇 |
1975年 | 537篇 |
1974年 | 592篇 |
1973年 | 404篇 |
1972年 | 506篇 |
1971年 | 487篇 |
1970年 | 670篇 |
1969年 | 491篇 |
1968年 | 536篇 |
1960年 | 245篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
Dr. Richard Koch 《Monatshefte für Mathematik》1977,83(2):133-146
Let ={e(u)|uI} be a one-parameter family of straight lines forming a ruledC
r-2-surface E
n (n2,r1) without singular generatorse(u) (uI). As a synopsis, a generalization and an improvement of various results already known about the strictional properties of ruled surfaces E
n (especially in the casen=3) the author demonstrates a uniform geometrical way of defining and uniquely obtaining thestriction point S(u) and theparameter of distribution d(u) of a generatore(u) under the minimal assumptions thate(u)E
n
(n2) be noncylindrical andr1. Other methods of obtainingS(u) andd(u) are discussed in comparison, and special strictional properties ofskew ruled surfaces E
n are proved.
Herrn Prof. Dr. H. R. Müller zum 65. Geburtstag 相似文献
Herrn Prof. Dr. H. R. Müller zum 65. Geburtstag 相似文献
152.
Dr. Moritz Armsen 《manuscripta mathematica》1977,20(3):245-253
The classical Gauss-Bonnet formula has the form I(gij)=2, where I(gij) represents a sum of three terms each of which depends on the metric tensor gij. It is shown that the first variation I of I(gij) with respect to the metric gij vanishes and that for the Euclidean metric ij we have I(ij)=2. From this the formula I(gij)=2 follows. In the process, explicit expressions are obtained for the first variation of each of the three terms which comprise I(gij). Furthermore, a general expression for the first variation of a multiple integral whose integrand is a scalar density depending on the metric tensor gij and its derivatives up to the second order is obtained with the aid of results of Rund [1]. 相似文献
153.
Dr. Bohuslav Diviš 《Monatshefte für Mathematik》1977,84(1):21-28
LetQ(u) be a positive definite quadratic form inr2 variables with a real symmetric coefficient matrix of determinantD. Given a real vectorb with 0b
j
<1, forx>0 letA(x) be the number of lattice points in the ellipsoidQ(u+b)x, letV(x) be the volume of this ellipsoid andP(x)=A(x)–V(x). Let
. By introduction of a parameter we shall show how the treatment of estimates onP(x) and onM(x) can be unified. 相似文献
154.
One considers m-dimensional Riemannian manifoldsM with tangent spaces Tp(M), p M that are a direct sum of a spacelike m-2 planeR
p and a 2-planeH
p. It is supposed that onM there exists a connection whose space-like components are parallel conformal flat (pkf). These components are generated by a vector field X. Assuming that X belongs to a pair X,Y of reciprocal quasi-cocircular vector fields and that the Pfaffian of this pair is the 1-form associated with the connection, the following results are derived: 1. X and Y are of equal constant length (This is true for all Riemannian manifolds). 2. The immersion of the integral manifold ofH
p intoM is cylindrical and the normal connection is flat. 3. The immersion of any space-like submanifold intoM is cylindrical with respect to the sections inH
p and umbilical with respect to all spacelike sections. 4. If m 4, the integral manifoldP
p is flat.
Herrn Prof. Dr. WERNER BURAU zum 70.Geburtstag Klaus Buchner und Radu Rosca 相似文献
Herrn Prof. Dr. WERNER BURAU zum 70.Geburtstag Klaus Buchner und Radu Rosca 相似文献
155.
Klaus Potthoff 《Archive for Mathematical Logic》1977,18(1):67-71
We prove a tree lemma on perfect trees to obtain an ultrafilter of definable subsets of , which is Ramsey but not conservative (see [Phi 2]). 相似文献
156.
Dr. habil. F. A. Behringer 《Mathematical Methods of Operations Research》1977,21(3):103-116
Summary Letf
i
:A R ben real-valued objective functions on a convex setA -K
m
,K:=R orC, n, mN. Letg: A R
n
be defined by
, where for eachxA, (i
1
(x), ..., i
n
(x)) is a permutation of (1, ...,n) such that
. In this paper we treat the problem of findingx
*A such that
, wherel-max denotes the lexicographic maximum. If the fi's are strongly quasiconcave we can reduce the problem stepwise until finally it is in the form of a scalar programming problem. Further, we consider conditions for the existence and uniqueness of a solution and discuss the relationship of the problem to the vector maximum (i.e. Pareto) and maxmin (i.e. Chebychev) problems.
Zusammenfassung f i :AR seienn reellwertige Zielfunktionen über einer konvexen MengeA-K m ,K:=R oderC, n, mN. g:AR n sei definiert durch , wobei für jedesxA (i 1 (x), ... i n (x)) eine Permutation von (1, ...,n) derart ist, daß Wir betrachten das Problem, einx *A so zu finden, daß , wobeil-max das lexikographische Maximum bedeute. Falls dief i stark quasikonkav sind, läßt sich das Problem stufenweise reduzieren, bis es schließlich die Gestalt eines skalaren Optimierungsproblems annimmt. Wir geben Existenz- und Eindeutigkeitsbedingungen an und besprechen Zusammenhänge mit dem Vektormaximumproblem (d.h. Pareto-Optimierung) und dem Maxmin-Problem (d.h. Tschebyscheff-Optimierung).相似文献
157.
The Mitsunobu reaction is a widely used and versatile method for the dehydrative oxidation–reduction condensation of an acid/pronucleophile usually with a primary or secondary alcohol that requires the combination of a reducing phosphine reagent together with an oxidizing azo reagent. The utility of this reaction stems from the fact that it is generally highly stereoselective and occurs with inversion of the stereochemical configuration of the alcohol starting material. Furthermore, as carboxylic acids, phenols, imides, sulfonamides, and other compounds can be used as the acid/pronucleophile, this reaction is useful for the preparation of a wide variety of functional groups. This Focus Review of the Mitsunobu reaction summarizes its origins, the current understanding of its mechanism, and recent improvements and applications. 相似文献
158.
Zusammenfassung Es werden Hückel-MO-Rechnungen am Borazol und den B-Trihalogenoborazolen durchgeführt und die entsprechenden Parameter an den teils bekannten und teils gemessenen UV-Spektren geeicht. Die für diese Verbindungen bestimmten chemischen Verschiebungen der 11B- und 14N-Kernresonanz lassen sich mit den berechneten -Elektronendichten erklären.
Herrn Professor Dr. H. Hartmann danken wir für sein stetes Interesse und die wohlwollende Förderung dieser Arbeit. Für die Hilfe bei der Aufnahme des Vakuum-UV-Spektrums von Fluoroborazol danken wir Herrn Diplom-Physiker E. Mohler. 相似文献
Hückel-MO calculations are performed for Borazine and the B-Trihalogenoborazines; the parameters used are calibrated with the UV-spectra, which had either been measured here or obtained from the literature. The calculated -electron densities compare favorably well with the 11B- and 14N-nuclear magnetic resonances determined for the above mentioned compounds.
Résumé Le borazole et les B-trihalogenoborazoles sont calculés par la méthode de Hückel en utilisant des paramètres calibrés sur les spectres U.V. mesurés ou obtenus dans la littérature. Les densités électroniques sont en bon accord avec le déplacement chimique de N.M.R. de 11B et 14N déterminé pour les composés cidessus.
Herrn Professor Dr. H. Hartmann danken wir für sein stetes Interesse und die wohlwollende Förderung dieser Arbeit. Für die Hilfe bei der Aufnahme des Vakuum-UV-Spektrums von Fluoroborazol danken wir Herrn Diplom-Physiker E. Mohler. 相似文献
159.
Peter Graf Ekkehard Kaempf Klaus Reichlmeier Karl Bernhard 《Helvetica chimica acta》1977,60(7):2151-2154
Metabolism of Acetylenic Monocarboxylic and Dicarboxylic Acids Feeding of acetylenic monoacids with chain length of 11 to 18 C-atoms to rats led to excretion of dicarboxylic acids with retained triple bonds. 10-Octadecynoic acid led to the formation of metabolites with even and odd number of C-atoms, suggesting in addition to established ω- and β-oxidation an α-oxidative pathway. 相似文献
160.
[reaction: see text] A novel one-pot procedure for the three-component coupling reaction of isocyanates, aldehydes, and dienophiles (IAD reaction) has been developed. Condensation of isocyanates and aldehydes and subsequent Diels-Alder reactions with electron-deficient dienophiles furnishes endo-selective amino-substituted cyclohexenes in good yield. 相似文献