首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   4264篇
  免费   165篇
  国内免费   37篇
化学   2731篇
晶体学   9篇
力学   170篇
数学   943篇
物理学   613篇
  2023年   22篇
  2022年   68篇
  2021年   97篇
  2020年   103篇
  2019年   91篇
  2018年   91篇
  2017年   66篇
  2016年   182篇
  2015年   149篇
  2014年   181篇
  2013年   279篇
  2012年   318篇
  2011年   325篇
  2010年   157篇
  2009年   199篇
  2008年   273篇
  2007年   263篇
  2006年   218篇
  2005年   201篇
  2004年   205篇
  2003年   145篇
  2002年   102篇
  2001年   47篇
  2000年   38篇
  1999年   35篇
  1998年   39篇
  1997年   37篇
  1996年   49篇
  1995年   22篇
  1994年   28篇
  1993年   16篇
  1992年   32篇
  1991年   21篇
  1990年   23篇
  1989年   21篇
  1988年   21篇
  1987年   18篇
  1986年   15篇
  1985年   34篇
  1984年   28篇
  1983年   27篇
  1982年   30篇
  1981年   15篇
  1980年   20篇
  1979年   11篇
  1978年   19篇
  1977年   13篇
  1976年   14篇
  1974年   11篇
  1967年   4篇
排序方式: 共有4466条查询结果,搜索用时 15 毫秒
951.
This paper is intended to be a first step towards the classification of finite flag-transitive geometries of rank 3 with affine planes and dual affine point-residues. We describe those of diameter 1. In the case of diameter > 1, we describe minimal quotients, assuming that the number of lines through two points is large enough.  相似文献   
952.
The first nucleophilic addition of an inorganic nucleophile (cyanide) to the activated, rigid, α‐diazomethine groups of a 1,2,5‐thiadiazole 1,1‐dioxide is reported here. An α‐amino nitrile and a bis α‐amino nitrile derivatives were obtained in good yields (62 and 98%, respectively) and characterized by spectroscopic, analytical, and single crystal X‐ray diffraction techniques. The course of the reaction, followed by cyclic voltammetry (CV), showed that cyanide adds to only one of the two C?N double bonds of the thiadiazole, forming an anion from which an N‐methyl derivative was obtained. Adequate concentrations of cyanide and methyl iodide (MeI) produced directly the bis α‐amino nitrile derivative. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   
953.
To quantify several acoustic features of the voice in patients withParkinson's disease (PD), 41 patients and 28 age and sex-matched controls were studied. PD severity was assessed with the Unified PD Rating Scale (UPDRS) and the Hoehn and Yahr staging. The Computerized Speech Lab 4300 program (Kay Elemetrics) was used. Two seconds of a sustained /a/ and a sentence were captured with a microphone and laryngograph equipment. Measures included fundamental frequency (FO), frequency perturbation (fitter), intensity perturbation (shimmer), and harmonic/noise ratio (H/N) of the vowel /a/, and frequency and intensity variability of a sentence, phonational range, dynamic range at the natural frequency, maximum phonational time and s/z ratio. All subjects underwent indirect laryngoscopy and/or laryngeal fibroscopy. When compared with controls, PD patients showed higher jitter, lower H/N ratio, lower frequency and intensity variability of the sentence, and lower phonational range and reported a higher frequency of the presence of low voice-intensity, monopitch, voice arrests, and struggle. These features seem to be unaffected by the duration and severity of the disease.  相似文献   
954.
 The split cuts of Cook, Kannan and Schrijver are general-purpose valid inequalities for integer programming which include a variety of other well-known cuts as special cases. To detect violated split cuts, one has to solve the associated separation problem. The complexity of split cut separation was recently cited as an open problem by Cornuéjols & Li CL01. In this paper we settle this question by proving strong 𝒩𝒫-completeness of separation for split cuts. As a by-product we also show 𝒩𝒫-completeness of separation for several other classes of inequalities, including the MIR-inequalities of Nemhauser and Wolsey and some new inequalities which we call balanced split cuts and binary split cuts. We also strengthen 𝒩𝒫-completeness results of Caprara & Fischetti CF96 (for -cuts) and Eisenbrand E99 (for Chvátal-Gomory cuts). To compensate for this bleak picture, we also give a positive result for the Symmetric Travelling Salesman Problem. We show how to separate in polynomial time over a class of split cuts which includes all comb inequalities with a fixed handle. Received: October 23, 2000 / Accepted: October 03, 2001 Published online: September 5, 2002 Key words. cutting planes – separation – complexity – travelling salesman problem – comb inequalities  相似文献   
955.
956.
The metabolism of oxabolone cipionate, 17-(3-cyclopentyl-1-oxopropoxy)-4-hydroxyestr-4-en-3-one, a synthetic anabolic steroid, was investigated in man, the cumulative urinary excretion and the metabolism of the compounds being studied by GC-MS in both electron impact and chemical ionization modes. After administration by injection to volunteers, five different metabolities were detected in urine. The metabolites and the parent compound were detected in urine up to a week after administration.  相似文献   
957.
Crew management is concerned with building the work schedules of crews needed to cover a planned timetable. This is a well-known problem in Operations Research and has been historically associated with airlines and mass-transit companies. More recently, railway applications have also come on the scene, especially in Europe. In practice, the overall crew management problem is decomposed into two subproblems, called crew scheduling and crew rostering. In this paper, we give an outline of different ways of modeling the two subproblems and possible solution methods. Two main solution approaches are illustrated for real-world applications. In particular we discuss in some detail the solution techniques currently adopted at the Italian railway company, Ferrovie dello Stato SpA, for solving crew scheduling and rostering problems.  相似文献   
958.
959.
960.
We prove that for closed 2-calibrated manifolds there always exist Lefschetz pencil structures. This generalizes similar results for symplectic and contact manifolds. To cite this article: A. Ibort, D. Marti´nez Torres, C. R. Acad. Sci. Paris, Ser. I 339 (2004).  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号