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Efficient intramolecular cycloadditions of N-3-alkenyl- and N-4-alkenylnitrones proceed with opposite regioselection which is modified by dipolarophile-substituent effects. Polycyclic isoxazolidines are obtained in a highly stereocontrolled fashion, consistent with an endo-addition of the Z-nitrones. 相似文献
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An overview is given of work we have done in recent years on the semantics of concurrency, concentrating on semantic models built on metric structures. Three contrasting themes are discussed, viz. (i) uniform or schematic versus nonuniform or interpreted languages; (ii) operational versus denotational semantics, and (iii) linear time versus branching time models. The operational models are based on Plotkin's transition systems. Language constructs which receive particular attention are recursion and merge, synchronization and global nondeterminacy, process creation, and communication with value passing. Various semantic equivalence results are established. Both in the definitions and in the derivation of these equivalences, essential use is made of Banach's theorem for contracting functions.Dedicated to Peter Naur on the occasion of his 60th birthday 相似文献
84.
In NMR imaging rho, T1 and T2 images are usually calculated from a set of partial saturation, saturation recovery or inversion recovery experiments with multiple echoes and multiple repetition times. Several methods can be envisaged to extract parameter images from such a set of source images. These methods to a greater or lesser extent take advantage of the fact that a multiple echo/multiple repetition time experiment provides a set of largely independent T1 and T2 measurements. In this study several data analysis methods, including weighted and non-weighted averaging of results of independent T1 and T2 measurements, weighted and non-weighted averaging of source images prior to data reduction and simultaneous three-parameter fitting, were compared against another in terms of precision, computational efficiency and robustness. The predicted performance of the examined methods was verified by stochastic simulation experiments. 相似文献
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H. J. Hilhorst A. F. Bakker C. Bruin A. Compagner A. Hoogland 《Journal of statistical physics》1984,34(5-6):987-1000
This talk describes a new approach for large-scale computational problems which is particularly effective when a relatively simple algorithm is used. We demonstrate that it is possible to design and construct, at modest cost, special purpose computers for various classes of problems. By exploiting the principles of pipelining and parallel processing, and by adapting the hardware design to the specific structure of a particular algorithm, one can obtain a device which is as fast as or faster than general-purpose commercial supercomputers. The user of a such a processor has the double advantage of its speed and of its continuous availability for the particular problem for which it was constructed. In statistical mechanics special purpose computers have been built recently (i) for Monte Carlo simulation of the Ising model, and (ii) for the molecular dynamics of classical many-particle systems with short-range interactions. The design and performance of these machines are discussed and compared to those of commercial computers. 相似文献
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We present a general method for the linear least-squares solutionof overdetermined and underdetermined systems. The method isparticularly efficient when the coefficient matrix is quasi-square,that is when the number of rows and number of columns is almostthe same. The numerical methods for linear least-squares problemsand minimum-norm solutions do not generally take account ofthis special characteristic. The proposed method is based onLU factorization of the original quasi-square matrix A, assumingthat A has full rank. In the overdetermined case, the LU factorsare used to compute a basis for the null space of AT. The right-handside vector b is then projected onto this subspace and the least-squaressolution is obtained from the solution of this reduced problem.In the case of underdetermined systems, the desired solutionis again obtained through the solution of a reduced system.The use of this method may lead to important savings in computationaltime for both dense and sparse matrices. It is also shown inthe paper that, even in cases where the matrices are quite small,sparse solvers perform better than dense solvers. Some practicalexamples that illustrate the use of the method are included. 相似文献
87.
A review is presented of the direct fitting procedures which are used in the analysis of Mössbauer spectra. Direct lineshape fitting with alternative profiles as well as shape-dependent, shape-independent and quasi shape-independent distribution fitting methods all can easily be incorporated in one computer program scheme yielding a large versatility for modification and/or extension of the programs according to specific spectra.Research director at the National Fund for Scientific Research, Belgium. 相似文献
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