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61.
Optimally Cutting a Surface into a Disk 总被引:1,自引:0,他引:1
Jeff?EricksonEmail author Sariel?Har-PeledEmail author 《Discrete and Computational Geometry》2004,31(1):37-59
We consider the problem of cutting a subset of the edges of a polyhedral manifold
surface, possibly with boundary, to obtain a single topological disk, minimizing
either the total number of cut edges or their total length. We show that this
problem is NP-hard in general, even for manifolds without boundary and for punctured
spheres. We also describe an algorithm with running time n
O(g+k), where n is
the combinatorial complexity, g is the genus, and k is the number of boundary
components of the input surface. Finally, we describe a greedy algorithm that
outputs a O(log2
g)-approximation of the minimum cut graph in O(g
2
n log n)
time. 相似文献
62.
Higham LT Kreher UP Mulder RJ Strauss CR Scott JL 《Chemical communications (Cambridge, England)》2004,(20):2264-2265
A member of a new class of novel macrocycles possessing both polyether and phenolic functionalities, forms dimers in both the solid-state and in solution when exposed to chloroform, dichloromethane or toluene, but does not self-associate in the presence of dimethyl sulfoxide. 相似文献
63.
64.
65.
Shultz DA Fico RM Bodnar SH Kumar RK Vostrikova KE Kampf JW Boyle PD 《Journal of the American Chemical Society》2003,125(38):11761-11771
A magnetostructural correlation (conformational electron spin exchange modulation) within an isostructural series of biradical complexes is presented. X-ray crystal structures, variable-temperature electron paramagnetic resonance spectroscopy, zero-field splitting parameters, and variable-temperature magnetic susceptibility measurements were used to evaluate molecular conformation and electron spin exchange coupling in this series of molecules. Our combined results indicate that the ferromagnetic portion of the exchange couplings occurs via the cross-conjugated pi-systems, while the antiferromagnetic portion occurs through space and is equivalent to incipient bond formation. Thus, molecular conformation controls the relative amounts of ferro- and antiferromagnetic contributions to exchange coupling. In fact, the exchange parameter correlates with average semiquinone ring torsion angles via a Karplus-Conroy-type relation. Because of the natural connection between electron spin exchange coupling and electronic coupling related to electron transfer, we also correlate the exchange parameters in the biradical complexes to mixed valency in the corresponding quinone-semiquinone radical anions. Our results suggest that delocalization in the cross-conjugated, mixed-valent radical anions is proportional to the ferromagnetic contribution to the exchange coupling in the biradical oxidation states. 相似文献
66.
Numerical analysis of a quadratic matrix equation 总被引:8,自引:0,他引:8
The quadratic matrix equation AX2+ BX + C = 0in n x nmatricesarises in applications and is of intrinsic interest as oneof the simplest nonlinear matrix equations. We give a completecharacterization of solutions in terms of the generalized Schurdecomposition and describe and compare various numerical solutiontechniques. In particular, we give a thorough treatment offunctional iteration methods based on Bernoullis method.Other methods considered include Newtons method with exact line searches, symbolic solution and continued fractions.We show that functional iteration applied to the quadraticmatrix equation can provide an efficient way to solve the associated quadratic eigenvalue problem (2A + B + C)x = 0. 相似文献
67.
Jeff A. Viaclovsky 《Transactions of the American Mathematical Society》2000,352(9):4371-4379
In this paper we will examine a class of fully nonlinear partial differential equations which are invariant under the conformal group . These equations are elliptic and variational. Using this structure and the conformal invariance, we will prove a global uniqueness theorem for solutions in with a quadratic growth condition at infinity.
68.
Jeff T. Gostick Marios A. Ioannidis Michael W. Fowler Mark D. Pritzker 《Electrochemistry communications》2009,11(3):576-579
The condition of liquid water breakthrough at the cathode of polymer electrolyte fuel cells (PEMFC) is studied experimentally and data on corresponding water saturation and capillary pressure are provided for gas diffusion layers (GDL) with and without a microporous layer (MPL). The data demonstrate that the GDL saturation at water breakthrough is drastically reduced from ca. 25% to ca. 5% in the presence of MPL. This observation is consistent with considerations of invasion percolation in finite-size lattices and suggests an explanation for the role of MPL in improving PEMFC performance at high current densities. 相似文献
69.
Semen B. Kharchenko Rangaramanujam M. Kannan Jeff J. Cernohous Shivshankar Venkataramani Gaddam N. Babu 《Journal of Polymer Science.Polymer Physics》2001,39(21):2562-2571
With the increase in sophisticated synthesis methods, it appears that polymer architecture may be a tunable property. Therefore, the role of architecture in rheological and processing properties has received renewed attention, mainly because of dendrimer synthesis and metallocene‐catalyst technology. Linear polymers and hyperbranched polymers represent two ends of branching complexity. Some previous studies have suggested that hyperbranched polymers may behave like unentangled polymers, whereas others have proposed that they exhibit the properties of soft colloids. In an effort to compare the responses of linear and hyperbranched polymers, we synthesized starlike hyperbranched polystyrenes (HBPSs) of various branch lengths and numbers of branches. The HBPSs used in this study were unentangled or weakly entangled, allowing us to study the effect of branch density more readily. Two linear polystyrene (L‐PS) melts and two HBPSs were studied. Using a custom‐built rheooptical apparatus, we characterized the rheology and flow birefringence of these materials. To our knowledge, these are the first flow birefringence measurements on highly branched polymer melts. Our results suggest that the flow behavior of HBPS is significantly different from that of L‐PS: (1) HBPS shows nonterminal behavior in the low‐frequency rheological response; (2) when the stress‐optical rule (SOR) holds, the stress‐optical coefficient of HBPS is much lower than those of analogous linear polymers; and (3) when the branch density is high and the branch length is sufficiently low, the SOR fails for these homopolymer melts. A significant increase in the birefringence for a given amount of stress in the low‐frequency region suggests that there may be a soft core in these materials due to the strong preferential radial orientation of chain segments near the center of a molecule versus those near the periphery. The predominantly elastic response of the soft structures may be responsible for the enhanced form birefringence. Our preliminary results indicate that these materials may exhibit both polymeric and soft‐colloid natures. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2562–2571, 2001 相似文献
70.