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51.
Polarization transfer under planar mixing conditions is a widely used tool in modern NMR-experiments. In the case of two coupled spins 1/2 or a chain of three or more spins 1/2 with only nearest neighbor couplings, it is only possible to transfer a single magnetization component (longitudinal magnetization in the principle axis system of the planar coupling tensors). However, if all couplings in a three-spin system are non-zero, it turns out that all magnetization components can be efficiently transferred even under strictly planar mixing conditions. In this article a detailed theoretical analysis is presented based on analytical transverse coherence transfer functions and on the underlying commutator algebra. In addition, transverse magnetization transfer is demonstrated experimentally. The results show that in highly coupled spin systems, as for example in the case of partially aligned samples with many residual dipolar couplings, special care has to be taken to avoid phase distortions if planar mixing steps are used. 相似文献
52.
Shaolin Liao Ronald J. Vernon 《International Journal of Infrared and Millimeter Waves》2007,28(6):479-490
The newly developed Taylor-Interpolation-FFT (TI-FFT) algorithm dramatically increases the computational speeds for millimeter
wave propagation from a planar (cylindrical) surface onto a “quasi-planar” (“quasi-cylindrical”) surface. Two different scenarios
are considered in this article: the planar TI-FFT is for the computation of the wave propagation from a plane onto a “quasi-planar” surface and the cylindrical TI-FFT is for the computation of wave propagation from a cylindrical surface onto a “quasi-cylindrical” surface. Due to the
use of the FFT, the TI-FFT algorithm has a computational complexity of O(N
2 log2
N
2) for an N × N computational grid, instead of N
4 for the direct integration method. The TI-FFT algorithm has a low sampling rate according to the Nyquist sampling theorem.
The algorithm has accuracy down to −80 dB and it works particularly well for narrow-band fields and “quasi-planar” (“quasi-cylindrical”)
surfaces. 相似文献
53.
Ideal Bragg reflection waveguides (BRWs) are assumed to have an infinitely extended periodic cladding whereas in practice, the cladding of BRWs is of a finite extent. Bloch theorem is widely used to analyze the propagation characteristics of the BRWs. Since Bloch theorem is ideally valid only for an infinitely extended periodic medium, its application to study such BRWs is an approximation. We present a matrix method for a more accurate analysis of finite-clad BRWs and estimate the extent of errors involved in the values of the propagation constant obtained by the Bloch wave formalism. The proposed method can be used to obtain the mode effective indices as well as the radiation loss of a finite-clad BRW without resorting to solving any complex transcendental equation. In addition, since the method does not inherently assume a periodic cladding, it can also be used to analyze symmetric multi-channel BRWs, chirped structures and directional couplers. 相似文献
54.
Alejandro R. Diaz Noboru Klkuchl Panos Papalambros John E. Taylor 《基于设计的结构力学与机械力学》2013,41(2):215-230
ABSTRACT Finite element solutions of improved quality are obtained by optimizing the location of nodes of the finite element grid, while keeping the number of degrees of freedom fixed. The formulation of the grid optimization problem is based on the reduction of error associated with interpolation of the exact solution, using functions from the finite element space. Element sizes are selected as design variables: length in R1 and area in R2. Analytically derived optimality conditions are presented and an approximation to these conditions is introduced to obtain a set of operationally useful equations that can be used as guidelines for construction of improved grids. Example problems are given for illustration. 相似文献
55.
Enzo Orsingher 《Stochastics An International Journal of Probability and Stochastic Processes》2013,85(3-4):617-631
The exact distribution of a cyclic planar motion with three directions is explicitly derived in terms of Bessel functions of order three (suitably combined). The absolutely continuous part of the distribution is proved to satisfy suitable boundary conditions and some of its properties are analyzed. The transformations converting the governing equations of order three is presented and its solutions (used here) derived by applying the Frobenius method. 相似文献
56.
We address the problem of online route discovery for a class of graphs that can be embedded either in two or in three-dimensional space. In two dimensions we propose the class of quasi-planar graphs and in three dimensions the class of quasi-polyhedral graphs. In the former case such graphs are geometrically embedded in R2 and have an underlying backbone that is planar with convex faces; however within each face arbitrary edges (with arbitrary crossings) are allowed. In the latter case, these graphs are geometrically embedded in R3 and consist of a backbone of convex polyhedra and arbitrary edges within each polyhedron. In both cases we provide a routing algorithm that guarantees delivery. Our algorithms need only “remember” the source and destination nodes and one (respectively, two) reference nodes used to store information about the underlying face (respectively, polyhedron) currently being traversed. The existence of the backbone is used only in proofs of correctness of the routing algorithm; the particular choice is irrelevant and does not affect the behaviour of the algorithm. 相似文献
57.
58.
This paper presents the derivation of the precise relation between the displacement of a light spot on an object's surface and the displacement of its image on the detector in an optical triangulation device, along with applications of the design of triangulation devices. Based on this relation, improved designs of optical triangulation devices, including devices of adjustable configurations, are proposed and discussed. 相似文献
59.
In 1995, Voigt constructed a planar triangle-free graph that is not 3-list-colorable. It has 166 vertices. Gutner then constructed such a graph with 164 vertices. We present two more graphs with these properties. The first graph has 97 vertices and a failing list assignment using triples from a set of six colors, while the second has 109 vertices and a failing list assignment using triples from a set of five colors. 相似文献
60.
At present, the most successful approach for solving large-scale instances of the Symmetric Traveling Salesman Problem to optimality is branch-and-cut. The success of branch-and-cut is due in large part to the availability of effective separation procedures; that is, routines for identifying violated linear constraints.
For two particular classes of constraints, known as comb and domino-parity constraints, it has been shown that separation becomes easier when the underlying graph is planar. We continue this line of research by showing how to exploit planarity in the separation of three other classes of constraints: subtour elimination, 2-matching and simple domino-parity constraints. 相似文献