全文获取类型
收费全文 | 140333篇 |
免费 | 2121篇 |
国内免费 | 462篇 |
专业分类
化学 | 77909篇 |
晶体学 | 2082篇 |
力学 | 5221篇 |
综合类 | 5篇 |
数学 | 14338篇 |
物理学 | 43361篇 |
出版年
2016年 | 1650篇 |
2015年 | 1338篇 |
2014年 | 1663篇 |
2013年 | 5427篇 |
2012年 | 4005篇 |
2011年 | 5168篇 |
2010年 | 3086篇 |
2009年 | 2856篇 |
2008年 | 4636篇 |
2007年 | 4691篇 |
2006年 | 4720篇 |
2005年 | 4542篇 |
2004年 | 3911篇 |
2003年 | 3586篇 |
2002年 | 3482篇 |
2001年 | 3891篇 |
2000年 | 2948篇 |
1999年 | 2447篇 |
1998年 | 2087篇 |
1997年 | 2071篇 |
1996年 | 2016篇 |
1995年 | 1988篇 |
1994年 | 1743篇 |
1993年 | 1766篇 |
1992年 | 1971篇 |
1991年 | 1899篇 |
1990年 | 1835篇 |
1989年 | 1856篇 |
1988年 | 1826篇 |
1987年 | 1839篇 |
1986年 | 1750篇 |
1985年 | 2351篇 |
1984年 | 2479篇 |
1983年 | 2080篇 |
1982年 | 2406篇 |
1981年 | 2221篇 |
1980年 | 2271篇 |
1979年 | 2255篇 |
1978年 | 2395篇 |
1977年 | 2292篇 |
1976年 | 2294篇 |
1975年 | 2196篇 |
1974年 | 2033篇 |
1973年 | 2232篇 |
1972年 | 1350篇 |
1971年 | 1043篇 |
1970年 | 976篇 |
1969年 | 979篇 |
1968年 | 1117篇 |
1967年 | 1188篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
G. Scamarcio L. Tapfer W. König K. Ploog A. Cingolani 《Applied Physics A: Materials Science & Processing》1990,51(3):252-254
Far infrared reflectivity measurements are performed on a series of GaAs/AlAs multiple quantum well (MQW) heterostructures with systematically varied thicknesses of the constituent layers. In addition to the artificial anisotropy we observe two distinct bulk-like Reststrahlen regions. The widths of the GaAs-like and the AlAs-like Reststrahlen bands strongly depend on the relative thicknesses of the constituent layers of the MQW heterostructures, in excellent agreement with the predictions of the effective-medium theory.Prof. Aldo Cingolani passed away just before the publication of this article. We would like to dedicate this paper to his memory 相似文献
992.
In this paper the existence and uniqueness of solutions of the following initial boundary value problem for non-linear symmetric hyperbolic equations of the first order are shown, where M = I + ? S , has the same from as the Kreiss' condition, but S must be sufficiently small ( I + is the unit matrix in the space generated by eigenvectors of the matrix ? A · n? , corresponding to positive eigenvalues) and n? is a unit outward vector normal to the boundary. The main result of the paper is obtaining an a priori estimate for non-linear equations. This estimate is obtained for sufficiently small time and norms of given data functions. The existence of solutions is proved by the method of successive approximations, which can be used because at each step such properties as symmetry of matrices and the numbers of positive and negative eigenvalues of the matrix ? A · n? are assured. This can be done because we restrict our attention to such systems of equations for which these properties are satisfied for solutions from some neighbourhood of initial data u 0. Therefore, using the fact that solutions in the class of continuous functions are sought, these properties can be satisfied for sufficiently small time. Moreover, some examples of initial boundary value problems for equations of hydrodynamics and magnetohydrodynamics are considered. 相似文献
993.
A global existence theorem for the general coagulation–fragmentation equation with unbounded kernels
In this article an existence theorem is proved for the coagulation–fragmentation equation with unbounded kernel rates. Solutions are shown to be in the space X+ = {c∈L1: ∫ (1 + x)∣c(x)∣dx < ∞} whenever the kernels satisfy certain growth properties and the non-negative initial data belong to X+. The proof is based on weak L1 compactness methods applied to suitably chosen approximating equations. 相似文献
994.
The plasma problem studied is: given R+ find (, d, u) R ?R ? H1() such that
Let 1 < 2 be the first two eigenvalues of the associatedlinear eigenvalue problem: find $$\left(\lambda ,\phi \right)\in\mathrm{R;}\times {\hbox{ H }}_{0}^{1}\left(\Omega \right)$$such that
For 0(0,2) it is well known that there exists a unique solution(0, d0, u0) to the above problem. We show that the standard continuous piecewise linear Galerkinfinite-element approximatinon $$\left({\lambda }_{0},{\hbox{d }}_{0}^{k},{u}_{0}^{h}\right)$$, for 0(0,2), converges atthe optimal rate in the H1, L2, and L norms as h, the mesh length,tends to 0. In addition, we show that dist (, h)Ch2 ln 1/h,where $${\Gamma }^{\left(h\right)}=\left\{x\in \Omega :{u}_{0}^{\left(h\right)}\left(x\right)=0\right\}$$.Finally we consider a more practical approximation involvingnumerical integration. 相似文献
995.
The time evolution of the particle number density and spectrum of massive scalar particles, coupled conformally to a classical Friedman-Robertson-Walker gravitational field is followed numerically. Not only for a pure radiation cosmos, but also for one with an inflationary interlude, the density of particles produced remains constant in time. This results in a constant equivalent temperature scale up to the Compton time of the massive particles, and opens the possibility that during a sufficiently long inflation the energy density of the particles produced can exceed that of the radiation background. 相似文献
996.
A variety of dolphin sonar discrimination experiments have been conducted, yet little is known about the cues utilized by dolphins in making fine target discriminations. In order to gain insights on cues available to echolocating dolphins, sonar discrimination experiments were conducted with human subjects using the same targets employed in dolphin experiments. When digital recordings of echoes from targets ensonified with a dolphinlike signal were played back at a slower rate to human subjects, they could also make fine target discriminations under controlled laboratory conditions about as well as dolphins under less controlled conditions. Subjects reported that time-separation-pitch and duration cues were important. They also reported that low-amplitude echo components 32 dB below the maximum echo component were usable. The signal-to-noise ratio had to be greater than 10 dB above the detection threshold for simple discrimination and 30 dB for difficult discrimination. Except for two cases in which spectral cues in the form of "click pitch" were important, subjects indicated that time-domain rather than frequency-domain processing seemed to be more relevant in analyzing the echoes. 相似文献
997.
Optimal birth control of population dynamics 总被引:9,自引:0,他引:9
The authors studied optimal birth control policies for an age-structured population of McKendrick type which is a distributed parameter system involving 1st order partial differential equations with nonlocal bilinear boundary control. The functional analytic approach of Dubovitskii and Milyutin is adopted in the investigation. Maximum principles for problems with a free end condition and fixed final horizon are developed, and the time optimal control problems, the problem with target sets, and infinite planning horizon case are investigated. 相似文献
998.
Karl Heinz Mayer 《manuscripta mathematica》1989,63(1):99-114
If f is a Morse function on a smooth manifold M there exists a homotopy equivalence from M to a CW complex X such that the critical points of f with index are in a one-one correspondence to the -cells of X. In the equivariant case, a similar result holds for a special type of invariant Morse functions. In this paper we prove the existence of such special invariant Morse functions on compact smooth G-manifolds. As a consequence, any compact smooth G-manifold is homotopy equivalent to a G-CW complex. Other applications deal with the Euler number of the fixed point set and Morse inequalities in equivariant homology theory. 相似文献
999.
It is shown that every plane compact convex set K with an interiorpoint admits a covering of the plane with density smaller thanor equal to 8(23 3)/3 = 1.2376043.... For comparison,the thinnest covering of the plane with congruent circles isof density 2 / 27 = 1.209199576.... (see R. Kershner [3]), whichshows that the covering density bound obtained here is closeto the best possible. It is conjectured that the best possibleis 2 / 27. The coverings produced here are of the double-latticekind consisting of translates of K and translates of K. 相似文献
1000.
We construct functions Mα which are piecewise homogeneous polynomials on the (d+1)-dimensional torus Ud+1. These functions possess complete symmetry with respect to the independent variables. The symmetry and homogeneous relations
for these functions are exploited to obtain a recurrence relation and explicit representations. Furthermore, we show that
, where ω=e12x/k, 0≤jt≤k−1, are linearly independent. By restricting Mα to Ud, we obtain the complex analogue of polynomial box splines on a (d+1)-direction mesh on Ud, which is a multivariate analogue of B-splines on the circle studied by I.J. Schoenberg[8]. 相似文献