全文获取类型
收费全文 | 8548篇 |
免费 | 300篇 |
国内免费 | 73篇 |
专业分类
化学 | 5935篇 |
晶体学 | 28篇 |
力学 | 224篇 |
数学 | 1283篇 |
物理学 | 1451篇 |
出版年
2023年 | 52篇 |
2022年 | 186篇 |
2021年 | 226篇 |
2020年 | 158篇 |
2019年 | 172篇 |
2018年 | 155篇 |
2017年 | 139篇 |
2016年 | 331篇 |
2015年 | 278篇 |
2014年 | 317篇 |
2013年 | 536篇 |
2012年 | 543篇 |
2011年 | 634篇 |
2010年 | 391篇 |
2009年 | 317篇 |
2008年 | 575篇 |
2007年 | 512篇 |
2006年 | 495篇 |
2005年 | 425篇 |
2004年 | 385篇 |
2003年 | 304篇 |
2002年 | 274篇 |
2001年 | 112篇 |
2000年 | 126篇 |
1999年 | 82篇 |
1998年 | 82篇 |
1997年 | 78篇 |
1996年 | 99篇 |
1995年 | 94篇 |
1994年 | 76篇 |
1993年 | 77篇 |
1992年 | 86篇 |
1991年 | 54篇 |
1990年 | 49篇 |
1989年 | 32篇 |
1988年 | 25篇 |
1987年 | 33篇 |
1986年 | 38篇 |
1985年 | 47篇 |
1984年 | 56篇 |
1983年 | 27篇 |
1982年 | 37篇 |
1981年 | 38篇 |
1980年 | 24篇 |
1979年 | 25篇 |
1978年 | 23篇 |
1977年 | 20篇 |
1975年 | 9篇 |
1974年 | 9篇 |
1973年 | 16篇 |
排序方式: 共有8921条查询结果,搜索用时 15 毫秒
991.
Ellis G Cano P Jadraque M Martín M López L Núñez T de la Peña E Marco C Garrido L 《Analytical and bioanalytical chemistry》2011,399(7):2379-2388
Flexible and biodegradable film substrates prepared by solvent casting from poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBHV) solutions in chloroform were microperforated by ultraviolet laser ablation and subsequently characterized
using infrared (IR) microspectroscopy and imaging techniques and scanning electron microscopy (SEM). Both transmission synchrotron
IR microspectroscopy and attenuated total reflectance microspectroscopy measurements demonstrate variations in the polymer
at the ablated pore rims, including evidence for changes in chemical structure and crystallinity. SEM results on microperforated
PHBHV substrates after cell culture demonstrated that the physical and chemical changes observed in the biomaterial did not
hinder cell migration through the pores. 相似文献
992.
The detection of corticosteroids and sex steroids in samples with no content indication, which are confiscated for forensic
investigation, is a challenge in doping analysis. A screening method based on the identification of androgens, estrogens,
gestagens, and their esters by means of a mass spectral library, along with a fast ultra-performance liquid chromatography–mass
spectrometry (UPLC-MS) method, was recently developed in our lab for the analysis of dietary supplements. However, for forensic
investigations, it is important to extend the scope of the method to corticosteroids in various matrices. Therefore, 36 corticosteroids
were added to the mass spectral library, and the sample preparation step was modified so that androgens, gestagens, corticosteroids,
and their esters could be analyzed with only one injection with the UPLC-MS method. A complementary tool to the existing library
identification was found in the extraction of common fragment ions out of the full scan data obtained for the library search.
The fragment ion with m/z 147 was found to be a good marker for the detection of steroids. Extra confirmation was obtained from the fragment ions with
m/z 135 (for all steroids) and 237 (specific for corticosteroids) or from the fragment ions with m/z 77, 91, and 105. The effectiveness of this approach was evaluated on some samples previously screened for forensic investigation
with thin-layer chromatography and confirmed with a targeted gas chromatography–mass spectrometry method. This study shows
that the combination of the library identification and the common fragment ions approach can be a valuable tool in the detection
of steroids without defining any target at the start of the analysis. 相似文献
993.
Derek Kitson Carlos Hernández 《Journal of Mathematical Analysis and Applications》2011,378(1):128-132
Approximately fifty percent of Weyl's theorem fails to transfer from Hilbert space operators to their tensor product. As a biproduct we find that the product of circles in the complex plane is a limaçon. 相似文献
994.
Carlos Pereira dos Santos 《Discrete Applied Mathematics》2011,159(8):675-682
Berlekamp asked the question “What is the habitat of ∗2?” (See Guy, 1996 [6].) It is possible to generalize the question and ask “For a game G, what is the largest n such that ∗n is a position of G?” This leads to the concept of the nim dimension. In Santos and Silva (2008) [8] a fractal process was proposed for analyzing the previous questions. For the same purpose, in Santos and Silva (2008) [9], an algebraic process was proposed. In this paper we implement a third idea related to embedding processes. With Alan Parr’s traffic lights, we exemplify the idea of estimating the “difficulty” of the game and proving that its nim dimension is infinite. 相似文献
995.
The feasibility pump (FP) has proved to be a successful heuristic for finding feasible solutions of mixed integer linear problems. Briefly, FP alternates between two sequences of points: one of feasible solutions for the relaxed problem, and another of integer points. This short paper extends FP, such that the integer point is obtained by rounding a point on the (feasible) segment between the computed feasible point and the analytic center for the relaxed linear problem. 相似文献
996.
In this paper we introduce the notion of generalized implication for lattices, as a binary function ⇒ that maps every pair of elements of a lattice to an ideal. We prove that a bounded lattice
A is distributive if and only if there exists a generalized implication ⇒ defined in A satisfying certain conditions, and we study the class of bounded distributive lattices A endowed with a generalized implication as a common abstraction of the notions of annihilator (Mandelker, Duke Math J 37:377–386,
1970), Quasi-modal algebras (Celani, Math Bohem 126:721–736, 2001), and weakly Heyting algebras (Celani and Jansana, Math Log Q 51:219–246, 2005). We introduce the suitable notions of morphisms in order to obtain a category, as well as the corresponding notion of congruence.
We develop a Priestley style topological duality for the bounded distributive lattices with a generalized implication. This
duality generalizes the duality given in Celani and Jansana (Math Log Q 51:219–246, 2005) for weakly Heyting algebras and the duality given in Celani (Math Bohem 126:721–736, 2001) for Quasi-modal algebras. 相似文献
997.
We study the collision of two solitons for the nonlinear Schrödinger equation iψt=−ψxx+F(2|ψ|)ψ, F(ξ)=−2ξ+O(ξ2) as ξ→0, in the case where one soliton is small with respect to the other. We show that in general, the two soliton structure is not preserved after the collision: while the large soliton survives, the small one splits into two outgoing waves that for sufficiently long times can be controlled by the cubic NLS: iψt=−ψxx−22|ψ|ψ. 相似文献
998.
999.
Most decision making research in real options focuses on revenue uncertainty assuming discount rates remain constant. However, for many decisions revenue or cost streams are relatively static and investment is driven by interest rate uncertainty, for example the decision to invest in durable machinery and equipment. Using interest rate models from Cox et al. (1985b), we generalize the work of Ingersoll and Ross (1992) in two ways. Firstly, we include real options on perpetuities (in addition to zero coupon cash flows). Secondly, we incorporate abandonment or disinvestment as well as investment options, and thus model interest rate hysteresis (parallel to revenue uncertainty in Dixit (1989a)). Under stochastic interest rates, economic hysteresis is found to be significant, even for small sunk costs. 相似文献
1000.
In a paper from 1954 Marstrand proved that if K⊂R2 has a Hausdorff dimension greater than 1, then its one-dimensional projection has a positive Lebesgue measure for almost all directions. In this article, we give a combinatorial proof of this theorem when K is the product of regular Cantor sets of class C1+α, α>0, for which the sum of their Hausdorff dimension is greater than 1. 相似文献