全文获取类型
收费全文 | 2468篇 |
免费 | 218篇 |
国内免费 | 155篇 |
专业分类
化学 | 146篇 |
晶体学 | 3篇 |
力学 | 223篇 |
综合类 | 14篇 |
数学 | 1503篇 |
物理学 | 952篇 |
出版年
2024年 | 3篇 |
2023年 | 25篇 |
2022年 | 33篇 |
2021年 | 37篇 |
2020年 | 74篇 |
2019年 | 73篇 |
2018年 | 73篇 |
2017年 | 67篇 |
2016年 | 62篇 |
2015年 | 60篇 |
2014年 | 110篇 |
2013年 | 194篇 |
2012年 | 111篇 |
2011年 | 148篇 |
2010年 | 115篇 |
2009年 | 173篇 |
2008年 | 193篇 |
2007年 | 170篇 |
2006年 | 134篇 |
2005年 | 114篇 |
2004年 | 133篇 |
2003年 | 117篇 |
2002年 | 103篇 |
2001年 | 71篇 |
2000年 | 59篇 |
1999年 | 61篇 |
1998年 | 51篇 |
1997年 | 49篇 |
1996年 | 39篇 |
1995年 | 27篇 |
1994年 | 27篇 |
1993年 | 22篇 |
1992年 | 26篇 |
1991年 | 8篇 |
1990年 | 5篇 |
1989年 | 9篇 |
1988年 | 7篇 |
1987年 | 4篇 |
1986年 | 8篇 |
1985年 | 13篇 |
1984年 | 9篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 5篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1974年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有2841条查询结果,搜索用时 15 毫秒
41.
For a scalar evolution equation ut = K(t, x, u, ux, . . . , u2m+1) with m ≥ 1, the cohomology space H1,2() is shown to be isomorphic to the space of variational operators and an explicit isomorphism is given. The space of symplectic operators for ut = K for which the equation is Hamiltonian is also shown to be isomorphic to the space H1,2() and subsequently can be naturally identified with the space of variational operators. Third order scalar evolution equations admitting a first order symplectic (or variational) operator are characterized. The variational operator (or symplectic) nature of the potential form of a bi-Hamiltonian evolution equation is also presented in order to generate examples of interest. 相似文献
42.
In this Letter, a generalized Tu formula is firstly presented to construct Hamiltonian structures of fractional soliton equations. The obtained results can be reduced to the classical Hamiltonian hierarchy of AKNS in ordinary calculus. 相似文献
43.
Yuan Shan 《数学学报(英文版)》2015,31(11):1725-1738
In this paper, we study the nonperiodic first-order Hamiltonian system ù = J L(t)u +J H(t, u), where H ∈ C1(R × R2n). With some assumptions on L, the corresponding Hamiltonian operator has only discrete spectrum. By using the index theory for self-adjoint operator equation,we establish the existence of multiple homoclinic orbits for the asymptotically quadratic nonlinearty satisfying some twist conditions between infinity and origin. 相似文献
44.
In this work the millimeter-wave (MMW) and far infrared (FIR) absorption spectrum for the asymmetrically deuterated Methanol (CH2DOH) species measured recently at a temperature of −60 °C with better accuracy and signal/noise (S/N) ratio than known before has been assigned for transitions originating at the lowest lying trans- to gauche-states. The entire spectrum for 50–1200 cm−1 has been re-recorded recently using the Synchrotron Radiation Source coupled with the Bruker Fourier Transform Infrared (FTIR) spectrometer with a resolution of about 0.001 cm−1 or better. Complete spectrum has not been exploited a great deal but it helped to entangle overlapping lines in the present work. This also fills the gap remaining in the usual FTIR in the range 400–500 cm−1. Hence it is expected to help the interacting partner for the Coriolis interaction encountered earlier. The assigned transitions mostly for the axial rotational angular momentum quantum number K + 1 ← K, both for R- and Q-sub bands for wide range of rotational angular momentum quantum number. The MMW spectrum has been recalibrated and assigned for a number of Q-branches. The assignments are confirmed rigorously using closed loop residual technique. The assigned rR and rQ lines have been analyzed in terms of polynomial expansion parameters. The new assignments are presented for about 750 transitions and a grand atlas of more than 1000 lines has been prepared which will be made available through the open source server at “research gate”. The present work should be useful in the area of astrophysical detection and further understanding of the energy relaxation pathways in the molecule. 相似文献
45.
《印度化学会志》2021,98(9):100114
We demonstrate how a back-propagation artificial neural network can be trained to represent a potential energy surface (PES) in a formless manner with limited data points and exploited to predict interaction energies for configurations not included in the training set. A similar exercise is undertaken for predicting the eigenvalues and eigenvectors of a model Hamiltonian matrix that delicately depends on parameters and exhibits crossing of eigen values. 相似文献
46.
On a manifold equipped with a bivector field, we introduce for every Hamiltonian a Lagrangian on paths valued in the cotangent space whose stationary points project onto Hamiltonian vector fields. We show that the remaining components of those stationary points tell whether the bivector field is Poisson or at least defines an integrable distribution—a class of bivector fields generalizing twisted Poisson structures that we study in detail. 相似文献
47.
Stefan Bilbao Olivier Thomas Cyril Touzé Michele Ducceschi 《Numerical Methods for Partial Differential Equations》2015,31(6):1948-1970
This article is concerned with the numerical solution of the full dynamical von Kármán plate equations for geometrically nonlinear (large‐amplitude) vibration in the simple case of a rectangular plate under periodic boundary conditions. This system is composed of three equations describing the time evolution of the transverse displacement field, as well as the two longitudinal displacements. Particular emphasis is put on developing a family of numerical schemes which, when losses are absent, are exactly energy conserving. The methodology thus extends previous work on the simple von Kármán system, for which longitudinal inertia effects are neglected, resulting in a set of two equations for the transverse displacement and an Airy stress function. Both the semidiscrete (in time) and fully discrete schemes are developed. From the numerical energy conservation property, it is possible to arrive at sufficient conditions for numerical stability, under strongly nonlinear conditions. Simulation results are presented, illustrating various features of plate vibration at high amplitudes, as well as the numerical energy conservation property, using both simple finite difference as well as Fourier spectral discretizations. © 2015 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 31: 1948–1970, 2015 相似文献
48.
Instead of the biharmonic type equation, a set of new governing equations and solving method for circular sector plate bending
is presented based on the analogy between plate bending and plane elasticity problems. So the Hamiltonian system can also
be applied to plate bending problems by introducing bending moment functions. The new method presents the analytical solutions
for the circular sector plate. The results show that the new method is effective.
Project supported by National Natural Science Foundation (No. 19732020) and the Doctoral Research Foundation of China. 相似文献
49.
Based on fractional isospectral problems and general bilinear forms, the gener-alized fractional trace identity is presented. Then, a new explicit Lie algebra is introduced for which the new fractional integrable couplings of a fractional soliton hierarchy are derived from a fractional zero-curvature equation. Finally, we obtain the fractional Hamiltonian structures of the fractional integrable couplings of the soliton hierarchy. 相似文献
50.
L2×L2中的一类无穷维Hamilton算子的剩余谱 总被引:3,自引:0,他引:3
该文得到了一类无穷维Hamilton算子的剩余谱和点谱存在的几个判别准则,从而给出 了求其剩余谱和点谱的方法.在此基础上构造了L2×L2中无穷维Hamilton算子的剩余谱 非空的具体例子,从而进一步验证了判别准则的有效性. 相似文献