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101.
In the context of wave propagation through a three-dimensional acoustic medium, an analytical approach to estimate the boundary effects in the high-frequency (single) diffraction by thin rigid obstacles is developed. Starting from the classical Kirchhoff (approximate) representation, explicit formulas regarding three sample cases are obtained. The improvement with respect to previous approaches, usually based on refinements of the classical Ray Theory, is evaluated by comparison with the results from a direct numerical solution of the main integrals involved. 相似文献
102.
103.
Edoardo Ballico 《Rendiconti del Circolo Matematico di Palermo》2001,50(1):209-212
Let X be a smooth curve of genus g with two pencils, sayg k 1 andg m 1 . Here we show that, in general, dim|rg k 1 +ug m 1 | has the expected dimension and construct examples in which dim |rg k 1 +ug m 1 | has a prescribed value. 相似文献
104.
105.
106.
We report the first harmonic vibrational spectra for each of the lowest lying isomers within the four major families of minima of (H2O)20, namely, the dodecahedron, fused cubes, face-sharing pentagonal prisms, and edge-sharing pentagonal prisms. These were obtained at the second-order Moller-Plesset perturbation level of theory (MP2) with the augmented correlation consistent basis set of double zeta quality (aug-cc-pVDZ) at the corresponding minimum energy geometries. The computed infrared (IR) spectra are the first ones obtained from first principles for these clusters. They were found to contain spectral features, which can be directly mapped onto the distinctive spectroscopic signatures of their constituent tetramer, pentamer, and octamer fragments. The dodecahedron spectra show the richest structure in the OH stretching region and are associated with the most redshifted OH vibrations with respect to the monomer. The lowest lying edge-sharing pentagonal prism isomer displays intense IR active vibrations that are redshifted by approximately 600 cm(-1) with respect to the water monomer. Furthermore the most redshifted, IR-active OH stretching vibrations for all four networks correspond to hydrogen bonded OH groups, which exhibit the following two common characteristics: (i) they belong to fragments which have a "free" OH stretch and (ii) they act as donors to a neighboring water molecule along a "dimerlike" (strong) hydrogen bond. The zero-point energy corrected MP2/CBS (complete basis set) limit binding energies D(0) for the four isomers are -163.1 kcal/mol (edge-sharing pentagonal prism), -160.1 kcal/mol (face-sharing pentagonal prism), -157.5 kcal/mol (fused cubes), and -148.1 kcal/mol (dodecahedron). 相似文献
107.
We consider the Max FS problem: For a given infeasible linear system A
x≤b, determine a feasible subsystem containing as many inequalities as possible. This problem, which is NP-hard and also difficult
to approximate, has a number of interesting applications in a wide range of fields. In this paper we examine structural and
algorithmic properties of Max FS and of Irreducible Infeasible Subsystems (IISs), which are intrinsically related since one must delete at least one constraint from each IIS to attain feasibility.
First we provide a new simplex decomposition characterization of IISs and prove that finding a smallest cardinality IIS is very difficult to approximate. Then we discuss
structural properties of IIS-hypergraphs, i.e., hypergraphs in which each edge corresponds to an IIS, and show that recognizing
IIS-hypergraphs subsumes the Steinitz problem for polytopes and hence is NP-hard. Finally we investigate rank facets of the
Feasible Subsystem polytope whose vertices are incidence vectors of feasible subsystems of a given infeasible system. In particular, using the IIS-hypergraph
structural result, we show that only two very specific types of rank inequalities induced by generalized antiwebs (which generalize
cliques, odd holes and antiholes to general independence systems) can arise as facets.
Received: December 2000 / Accepted: November 2002 Published online: February 14, 2003
RID="⋆"
ID="⋆" Part of this work was done while the first two authors were with the School of OR&IE, Cornell University, USA. A preliminary
version appeared in the Proceedings of the 10th IPCO conference [7], held in Graz, Austria, June 1999. This work was supported
by NSF grant DMS-9527124.
Key words. infeasible linear systems – feasible subsystems – Irreducible Infeasible Subsystem (IIS) – IIS-hypergraphs – independence
systems – feasible subsystem polytope – rank facets 相似文献
108.
[reaction: see text] Ruthenium alkylidene complexes 1-3 mediate hydrosilylation of alkynes with silanes. When triethoxy- or triphenylsilanes are used as silylating agents, the reaction affords alpha-substituted vinylsilanes as major products. 相似文献
109.
Edoardo Roncari Ulderico Mazzi Roberto Rossi Adriano Duatti Luciano Magon 《Transition Metal Chemistry》1981,6(3):169-173
Summary Some isomers of the ReOX2LPPh3 complexes [X = Cl or Br and L =N-methylsalicylideneiminate,N-phenylsalicylideneiminate, halfN,N'-ethylenebis(salicylideneiminate) or 8-hydroxyquinolinate] have been synthesized and characterized. Two different mechanisms for reaction of thetrans-ReOX3(PPh3)2 complexes (X = Cl or Br) with the Schiff bases are supported by qualitative studies on such parameters as the Schiff base ligand form, concentration of free triphenylphosphine ligand, reaction solvent and temperature. 相似文献
110.