全文获取类型
收费全文 | 1628篇 |
免费 | 42篇 |
国内免费 | 7篇 |
专业分类
化学 | 793篇 |
晶体学 | 22篇 |
力学 | 69篇 |
数学 | 268篇 |
物理学 | 525篇 |
出版年
2023年 | 15篇 |
2022年 | 25篇 |
2021年 | 35篇 |
2020年 | 44篇 |
2019年 | 43篇 |
2018年 | 45篇 |
2017年 | 37篇 |
2016年 | 63篇 |
2015年 | 42篇 |
2014年 | 70篇 |
2013年 | 91篇 |
2012年 | 97篇 |
2011年 | 121篇 |
2010年 | 97篇 |
2009年 | 78篇 |
2008年 | 81篇 |
2007年 | 88篇 |
2006年 | 64篇 |
2005年 | 71篇 |
2004年 | 41篇 |
2003年 | 40篇 |
2002年 | 35篇 |
2001年 | 16篇 |
2000年 | 22篇 |
1999年 | 19篇 |
1998年 | 11篇 |
1997年 | 15篇 |
1996年 | 27篇 |
1995年 | 17篇 |
1994年 | 12篇 |
1993年 | 19篇 |
1992年 | 10篇 |
1991年 | 11篇 |
1990年 | 5篇 |
1989年 | 9篇 |
1988年 | 12篇 |
1987年 | 8篇 |
1986年 | 12篇 |
1985年 | 15篇 |
1984年 | 12篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 15篇 |
1980年 | 10篇 |
1979年 | 8篇 |
1978年 | 6篇 |
1977年 | 8篇 |
1976年 | 6篇 |
1974年 | 6篇 |
1973年 | 6篇 |
排序方式: 共有1677条查询结果,搜索用时 15 毫秒
61.
Let M be an irreducible projective variety, over an algebraically closed field k of characteristic zero, equipped with an action of a connected algebraic group S over k. Let E
G
be a principal G-bundle over M equipped with a lift of the action of S on M, where G is a connected reductive linear algebraic group. Assume that E
G
admits a reduction of structure group to a maximal torus TG. We give a necessary and sufficient condition for the existence of a T-reduction of E
G
which is left invariant by the action of S on E
G
. 相似文献
62.
I. El-Kady R. Biswas Y. Ye M. F. Su I. Puscasu Martin Pralle E. A. Johnson J. Daly A. Greenwald 《Photonics and Nanostructures》2003,1(1):69-77
In this work, we present both the theoretical basis as well as supporting experimental measurements for development of a novel mid-infrared thermally stimulated narrow band emitter with a spectral bandwidth of less than 10%. To achieve this, we utilize a metallized-surface 2D photonic crystal of air voids in a silicon background with hexagonal structure symmetry. Our results are based on the generation of discrete surface plasmon (SP) modes in the thin metallized layer residing on the top surface. This yields a series of adequately spaced discrete peaks in the reflection spectrum, dominated by a single sharp feature corresponding to the lowest plasmon order, in an otherwise uniform highly reflective spectrum (>90%) over most of the IR spectrum. This, in turn, gives rise to a sharp absorption feature with a correspondingly narrow thermal emission peak in the emission spectrum. Transfer matrix calculations simulate well both the position and strengths of the absorption peaks. By altering the period of the surface photonic lattice, the SP peak and emissive band can be tuned to the desired wavelength. These devices promise a new class of tunable infrared emitters with high power in a narrow spectral bandwidth. Such narrow band sources are critical to achieving high efficiency gas sensors. 相似文献
63.
64.
In the present research, Nd:YAG laser micro-drilling of gamma-titanium aluminide, a new material which has performed well in laboratory tests as well as in different fields of engineering, is studied. The effect of different process parameters in the optimization of the process is investigated. The aspects considered are the hole circularity at exit and the hole taper of the drilled hole. Lamp current, pulse frequency, air pressure and thickness of the job are selected as independent process variables. The central composite design (CCD) technique based on response surface methodology (RSM) is employed to plan the experiments to achieve optimum responses with a reduced number of experiments. 相似文献
65.
Constrained diffusions, with diffusion matrix scaled by small ?>0, in a convex polyhedral cone G⊂Rk, are considered. Under suitable stability assumptions small noise asymptotic properties of invariant measures and exit times from domains are studied. Let B⊂G be a bounded domain. Under conditions, an “exponential leveling” property that says that, as ?→0, the moments of functionals of exit location from B, corresponding to distinct initial conditions, coalesce asymptotically at an exponential rate, is established. It is shown that, with appropriate conditions, difference of moments of a typical exit time functional with a sub-logarithmic growth, for distinct initial conditions in suitable compact subsets of B, is asymptotically bounded. Furthermore, as initial conditions approach 0 at a rate ?2 these moments are shown to asymptotically coalesce at an exponential rate. 相似文献
66.
Differential equations of different types and orders are of utmost importance for mathematical modeling of control system problems. State variable method uses the concept of expressing n number of first order differential equations in vector matrix form to model and analyze/synthesize control systems.The present work proposes a new set of orthogonal hybrid functions (HF) which evolved from synthesis of sample-and-hold functions (SHF) and triangular functions (TF). This HF set is used to approximate a time function in a piecewise linear manner with the mean integral square error (MISE) much less than block pulse function based approximation which always provides staircase solutions.The operational matrices for integration and differentiation in HF domain are also derived and employed for solving non-homogeneous and homogeneous differential equations of the first order as well as state equations. The results are compared with exact solutions, the 4th order Runge-Kutta method and its further improved versions proposed by Simos [6]. The presented HF domain theory is well supported by a few illustrations. 相似文献
67.
Indranil Biswas 《Geometriae Dedicata》2000,80(1-3):65-72
Given a compact connected oriented three manifold, equipped with a codimension one foliation, such that the Bott connection on the normal bundle is flat, a 2-form on the space parametrizing flat partial connections on it has been constructed. This form is closed. In the special case where the foliated three manifold is a surface bundle over the circle, this 2-form is identified with a certain 2-form on the parameter space for a class of paths in the representation space for the surface group. The 2-form, in question, on the parameter space for paths is constructed from the natural symplectic form on the representation space for a surface group. 相似文献
68.
A theoretical model is proposed to study the ballistic electron transport for a quasi-periodic multibarrier structure where two different barrier shapes are arranged according to the Thue–Morse sequence. Important tunneling features are revealed form such arrangements. It is noted that the tunneling band spectrum could be fragmented by tailoring the shape of the barriers in the structure. Results for the transmission coefficients and the current densities are compared with the corresponding periodic and single shape barrier arrangements. The quasi-periodic structure consisting of the rectangular and triangular barrier shapes is suggested to be more suitable for the electronic and opto-electronic devices due to its high negative differential conducting effect. 相似文献
69.
A low dissipative framework is given to construct high order entropy stable flux by addition of suitable numerical diffusion operator into entropy conservative flux. The framework is robust in the sense that it allows the use of high order reconstructions which satisfy the sign property only across the discontinuities. The third order weighted essentially non-oscillatory (WENO) interpolations and high order total variation diminishing (TVD) reconstructions are shown to satisfy the sign property across discontinuities. Third order accurate entropy stable schemes are constructed by using third order WENO and high order TVD reconstructions procedures in the diffusion operator. These schemes are efficient and less diffusive since the diffusion is actuated only in the sign stability region of the used reconstruction which includes discontinuities. Numerical results with constructed schemes for various test problems are given which show the third order accuracy and less dissipative nature of the schemes. 相似文献
70.
We consider the infinite horizon risk-sensitive problem for nondegenerate diffusions with a compact action space, and controlled through the drift. We only impose a structural assumption on the running cost function, namely near-monotonicity, and show that there always exists a solution to the risk-sensitive Hamilton–Jacobi–Bellman (HJB) equation, and that any minimizer in the Hamiltonian is optimal in the class of stationary Markov controls. Under the additional hypothesis that the coefficients of the diffusion are bounded, and satisfy a condition that limits (even though it still allows) transient behavior, we show that any minimizer in the Hamiltonian is optimal in the class of all admissible controls. In addition, we present a sufficient condition, under which the solution of the HJB is unique (up to a multiplicative constant), and establish the usual verification result. We also present some new results concerning the multiplicative Poisson equation for elliptic operators in . 相似文献