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   1504篇
  免费   77篇
  国内免费   9篇
化学   1214篇
晶体学   16篇
力学   22篇
数学   156篇
物理学   182篇
  2023年   19篇
  2022年   23篇
  2021年   47篇
  2020年   45篇
  2019年   50篇
  2018年   30篇
  2017年   24篇
  2016年   73篇
  2015年   46篇
  2014年   50篇
  2013年   67篇
  2012年   140篇
  2011年   171篇
  2010年   70篇
  2009年   62篇
  2008年   113篇
  2007年   91篇
  2006年   111篇
  2005年   92篇
  2004年   58篇
  2003年   44篇
  2002年   41篇
  2001年   10篇
  2000年   11篇
  1999年   10篇
  1998年   8篇
  1997年   5篇
  1996年   6篇
  1995年   8篇
  1994年   4篇
  1993年   5篇
  1992年   2篇
  1991年   6篇
  1990年   3篇
  1985年   3篇
  1984年   3篇
  1983年   4篇
  1982年   5篇
  1981年   2篇
  1980年   2篇
  1979年   2篇
  1978年   4篇
  1977年   3篇
  1975年   3篇
  1969年   1篇
  1968年   2篇
  1967年   1篇
  1934年   1篇
  1889年   1篇
  1876年   1篇
排序方式: 共有1590条查询结果,搜索用时 15 毫秒
991.
We consider an n-person economy in which efficiency is independent of distribution but the cardinal properties of the agents’ utility functions may preclude transferable utility (a property we call “Almost TU”). Holding the disagreement point fixed, we show that Almost TU is a necessary and sufficient condition for all agents to either benefit jointly or suffer jointly with any change in production possibilities under well-behaved generalized utilitarian bargaining solutions (of which the Nash bargaining and the utilitarian solutions are special cases). We apply the result to policy analysis and to incentive compatibility.  相似文献   
992.
We prove a new, tight upper bound on the number of incidences between points and hyperplanes in Euclidean d-space. Given n points, of which k are colored red, there are O d (m 2/3 k 2/3 n (d−2)/3+kn d−2+m) incidences between the k red points and m hyperplanes spanned by all n points provided that m=Ω(n d−2). For the monochromatic case k=n, this was proved by Agarwal and Aronov (Discrete Comput. Geom. 7(4):359–369, 1992).  相似文献   
993.
We develop a model of firm behavior in the presence of risk, resource constraints, and a cash flow constraint. Given imperfect capital markets, the producer confronts an uncertain cash flow. Utilizing chance constrained programming, we show that an increase in aversion to liquidity risk can cause an increased allocation to high-risk production alternatives. With a binding cash flow constraint, risk-averse firms appear to demonstrate risk-seeking behavior over losses and risk-averse behavior over gains.  相似文献   
994.
Novel two-step solution phase protocols for the synthesis of dihydroquinazolines and fused dihydroquinazoline-benzodiazepine tetracycles are reported. The methodology employs the Ugi reaction to assemble the desired diversity and acid treatment enables ring-closing transformations. The protocols are further facilitated by the use of microwave irradiation and n-butyl isocyanide to control the rate of each ring-forming transformation.  相似文献   
995.
Statistical correlation of mass spectrum peak broadening with product dispersity in protein conjugation reactions allows more detailed characterization of putative therapeutic conjugates.  相似文献   
996.
Insulin transport across the epithelial cell layer in the small intestine was studied using insulin/transferrin conjugates with and without the presence of P(MAA‐g‐EG) microparticles in contact with a co‐culture of Caco‐2/HT29‐MTX cells. The insulin/transferrin conjugate was shown to increase transport relative to pure insulin by a factor of 7, achieving an apparent permeability of 37 × 109 cm · s?1. The presence of P(MAA‐g‐EG) microparticles increased conjugate transport by a factor of 14 times relative to insulin, achieving an apparent permeability of 72.8 × 109 cm · s?1. The presence of the microparticles in solution was found to improve conjugate transport by nearly 100% with little to no change in cell monolayer integrity.

  相似文献   

997.
We demonstrate conical pores etched in tracked glass chips for fabricating patterns at the micrometer scale. Highly fluorescent patterns based on photopolymerization of diacetylene films were formed by irradiating UV light through conical pores called "photo-pens". The properties of photopens were investigated through experiments, finite-difference-time-domain (FDTD) simulations and numerical calculations based on Fresnel equations. We show that the pattern dimensions are easily controlled by adjusting the exposure time. Thus, patterns with a range of dimensions can be fabricated without any need of changes in the pore diameter. Parallel patterning was also demonstrated by simultaneously exposing the films to photons through multiple pores in the chip. Our method provides an inexpensive, versatile, and efficient way for patterning without the use of sophisticated masks.  相似文献   
998.
A protocol for the rapid determination of the absolute configuration and enantiomeric excess (ee) of α‐chiral primary amines with potential applications in asymmetric reaction discovery has been developed. The protocol requires derivatization of α‐chiral primary amines through condensation with pyridine carboxaldehyde to quantitatively yield the corresponding imine. The CuI complex with 2,2′‐bis (diphenylphosphino)‐1,1′‐dinaphthyl (BINAP? CuI) with the imine yields a metal‐to‐ligand charge‐transfer (MLCT) band in the visible region of the circular dichroism (CD) spectrum upon binding. Diastereomeric host–guest complexes give CD signals of the same signs but different amplitudes, allowing for differentiation of enantiomers. Processing the primary optical data from the CD spectrum with linear discriminant analysis (LDA) allows for the determination of the absolute configuration and identification of the amines, and processing with a supervised multilayer perceptron artificial neural network (MLP‐ANN) allows for the simultaneous determination of the ee and concentration. The primary optical data necessary to determine the ee of unknown samples is obtained in two minutes per sample. To demonstrate the utility of the protocol in asymmetric reaction discovery, the ee values and concentrations for an asymmetric metal‐catalyzed reaction are determined. The potential of the application of this protocol in high‐throughput screening (HTS) of ee is discussed.  相似文献   
999.
Anodic oxidation of cyclooctatetraene (COT) was carried out in acetonitrile containing excess allyltrimethylsilane. Under these conditions COT is oxidized preferentially to afford the COT cation radical, which then undergoes nucleophilic attack by the silane followed by further anodic oxidation. The process results in a substance in which the original eight carbons of COT are highly differentiated by the addition of three allyl groups and the formation of two new rings.  相似文献   
1000.
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