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991.
M. Genkin E. Lindroth 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2009,51(2):205-212
Solar magnetic structures are often observed in the form of flux tubes composed of a number of smaller elements called fibres
or threads, although theoretically such concentrations should not appear but should be flattened by magnetic diffusivity into
a uniform, low intensity field. In this paper we describe a mechanism which may be responsible for the fibrillation and also
for the very large diffusivity which dissipates magnetic flux tubes in hours instead of years. Firstly, the electric current
associated with magnetic field gradients usually increases the local electron temperature and reduces the resistivity, so
that the current becomes concentrated into sheets or streamers. Secondly, the magnetic field gradients continue to increase
until the current magnitude reaches its limit, which is determined by the electron-ion streaming instability. Then with appropriate
temperature and number densities, the Larmor radius of the ions overlaps the near discontinuity in Bz and generates a sharply peaked fluid motion at the edge that is close to the thermal speed. Finally, the resulting vorticity
generates an axial magnetic field opposing Bz in the term , and if this is sufficient to change the sign of this term, the very unstable backward heat equation results. This instability
repeatedly switches on and off and maintains the magnetic structure in the fibrillated form. Such structures are eventually
eliminated by magnetic diffusivity in the usual way, but because of the fluctuations in Bz, this occurs at a vastly increased rate. We show that this phenomenon increases the magnetic diffusivity, D, by a factor
~ 108 in agreement with some observations of plasma loops and supergranules. 相似文献
992.
Naveed Ahmad Khan Osamah Ibrahim Khalaf Carlos Andrs Tavera Romero Muhammad Sulaiman Maharani A. Bakar 《Entropy (Basel, Switzerland)》2021,23(8)
In this study, a novel application of neurocomputing technique is presented for solving nonlinear heat transfer and natural convection porous fin problems arising in almost all areas of engineering and technology, especially in mechanical engineering. The mathematical models of the problems are exploited by the intelligent strength of Euler polynomials based Euler neural networks (ENN’s), optimized with a generalized normal distribution optimization (GNDO) algorithm and Interior point algorithm (IPA). In this scheme, ENN’s based differential equation models are constructed in an unsupervised manner, in which the neurons are trained by GNDO as an effective global search technique and IPA, which enhances the local search convergence. Moreover, a temperature distribution of heat transfer and natural convection porous fin are investigated by using an ENN-GNDO-IPA algorithm under the influence of variations in specific heat, thermal conductivity, internal heat generation, and heat transfer rate, respectively. A large number of executions are performed on the proposed technique for different cases to determine the reliability and effectiveness through various performance indicators including Nash–Sutcliffe efficiency (NSE), error in Nash–Sutcliffe efficiency (ENSE), mean absolute error (MAE), and Thiel’s inequality coefficient (TIC). Extensive graphical and statistical analysis shows the dominance of the proposed algorithm with state-of-the-art algorithms and numerical solver RK-4. 相似文献
993.
得到了一个解调和方程在球上的一类Dirichlet问题的简单方法,即不通过Poisson公式而实际上只解一个Euler方程,从而较容易地求出其解. 相似文献
994.
We describe the distribution of the first finite number of eigenvalues in a newly-forming band of the spectrum of the random Hermitian matrix model. The method is rigorously
based on the Riemann–Hilbert analysis of the corresponding orthogonal polynomials. We provide an analysis with an error term
of order N
−2γ
where 1/γ=2ν+2 is the exponent of non-regularity of the effective potential, thus improving even in the usual case the analysis of the
pertinent literature.
The behavior of the first finite number of zeroes (eigenvalues) appearing in the new band is analyzed and connected with the
location of the zeroes of certain Freud polynomials. In general, all these newborn zeroes approach the point of nonregularity
at the rate N
−γ
, whereas one (a stray zero) lags behind at a slower rate of approach. The kernels for the correlator functions in the scaling coordinate near the emerging
band are provided together with the subleading term. In particular, the transition between K and K+1 eigenvalues is analyzed in detail.
相似文献
995.
Noga Alon Robert Berke Kevin Buchin Maike Buchin Péter Csorba Saswata Shannigrahi Bettina Speckmann Philipp Zumstein 《Discrete and Computational Geometry》2009,42(3):421-442
We show that the vertices of any plane graph in which every face is incident to at least g vertices can be colored by ⌊(3g−5)/4⌋ colors so that every color appears in every face. This is nearly tight, as there are plane graphs where all faces are incident
to at least g vertices and that admit no vertex coloring of this type with more than ⌊(3g+1)/4⌋ colors. We further show that the problem of determining whether a plane graph admits a vertex coloring by k colors in which all colors appear in every face is in ℘ for k=2 and is
-complete for k=3,4. We refine this result for polychromatic 3-colorings restricted to 2-connected graphs which have face sizes from a prescribed
(possibly infinite) set of integers. Thereby we find an almost complete characterization of these sets of integers (face sizes)
for which the corresponding decision problem is in ℘, and for the others it is
-complete.
Research of N. Alon was supported in part by the Israel Science Foundation, by a USA–Israeli BSF grant, and by the Hermann
Minkowski Minerva Center for Geometry at Tel Aviv University.
Research of R. Berke was supported in part by JSPS Global COE program “Computationism as a Foundation for the Sciences.”
Research of K. Buchin and M. Buchin was supported by the Netherlands’ Organisation for Scientific Research (NWO) under BRICKS/FOCUS
project no. 642.065.503.
Research of P. Csorba was supported by DIAMANT, an NWO mathematics cluster.
Research of B. Speckmann was supported by the Netherlands’ Organisation for Scientific Research (NWO) under project no. 639.022.707. 相似文献
996.
Thorsten Bernholt Friedrich Eisenbrand Thomas Hofmeister 《Discrete and Computational Geometry》2009,42(1):22-36
In this paper, we introduce the notion of a constrained Minkowski sum: for two (finite) point-sets P,Q⊆ℝ2 and a set of k inequalities Ax≥b, it is defined as the point-set (P
⊕
Q)
Ax≥b
={x=p+q∣p∈P,q∈Q,Ax≥b}. We show that typical interval problems from computational biology can be solved by computing a set containing the vertices
of the convex hull of an appropriately constrained Minkowski sum. We provide an algorithm for computing such a set with running
time O(Nlog N), where N=|P|+|Q| if k is fixed. For the special case
where P and Q consist of points with integer x
1-coordinates whose absolute values are bounded by O(N), we even achieve a linear running time O(N). We thereby obtain a linear running time for many interval problems from the literature and improve upon the best known
running times for some of them. The main advantage of the presented approach is that it provides a general framework within
which a broad variety of interval problems can be modeled and solved.
T. Bernholt gratefully acknowledges the Deutsche Forschungsgemeinschaft for the financial support (SFB 475, “Reduction of
complexity in multivariate data structures”). 相似文献
997.
For certain random variables that arise as limits of functionals of random finite trees, we obtain precise asymptotics for
the logarithm of the right-hand tail. Our results are based on the facts (i) that the random variables we study can be represented
as functionals of a Brownian excursion and (ii) that a large deviation principle with good rate function is known explicitly
for Brownian excursion. Examples include limit distributions of the total path length and of the Wiener index in conditioned
Galton-Watson trees (also known as simply generated trees). In the case of Wiener index (where we recover results proved by
Svante Janson and Philippe Chassaing by a different method) and for some other examples, a key constant is expressed as the
solution to a certain optimization problem, but the constant’s precise value remains unknown.
Research supported by NSF grants DMS-0104167 and DMS-0406104 and by The Johns Hopkins University’s Acheson J. Duncan Fund
for the Advancement of Research in Statistics. 相似文献
998.
M. Fuchs 《Mathematical Methods in the Applied Sciences》2009,32(7):773-782
We investigate the interior regularity of minimizers for an obstacle problem of higher order that can be seen as a model for the behaviour of a plate subject to a rather general constitutive law including nonlinear elastic materials. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
999.
In this work we propose and analyze numerical methods for the approximation of the solution of Helmholtz transmission problems in two or three dimensions. This kind of problems arises in many applications related to scattering of acoustic, thermal and electromagnetic waves. Formulations based on boundary integral methods are powerful tools to deal with transmission problems in unbounded media. Different formulations using boundary integral equations can be found in the literature. We propose here new symmetric formulations based on a paper by Martin Costabel and Ernst P. Stephan (1985), that uses the Calderón projector for the interior and exterior problems to develop closed expressions for the interior and exterior Dirichlet-to-Neumann operators. These operators are then matched to obtain an integral system that is equivalent to the Helmholtz transmission problem and uses Cauchy data on the transmission boundary as unknowns. We show how to simplify the aspect and analysis of the method by employing an additional mortar unknown with respect to the ones used in the original paper, writing it in an appropriate way to devise Krylov type iterations based on the separate Dirichlet-to-Neumann operators. 相似文献
1000.
NP-hardness of the recognition of coordinated graphs 总被引:1,自引:0,他引:1
A graph G is coordinated if the minimum number of colors that can be assigned to the cliques of H in such a way that no two cliques with non-empty intersection receive the same color is equal to the maximum number of cliques
of H with a common vertex, for every induced subgraph H of G. In previous works, polynomial time algorithms were found for recognizing coordinated graphs within some classes of graphs.
In this paper we prove that the recognition problem for coordinated graphs is NP-hard, and it is NP-complete even when restricted
to the class of {gem, C
4, odd hole}-free graphs with maximum degree four, maximum clique size three and at most three cliques sharing a common vertex.
F.J. Soulignac is partially supported by UBACyT Grant X184, Argentina and CNPq under PROSUL project Proc. 490333/2004-4, Brazil. 相似文献