全文获取类型
收费全文 | 73955篇 |
免费 | 5317篇 |
国内免费 | 8966篇 |
专业分类
化学 | 53879篇 |
晶体学 | 1823篇 |
力学 | 1489篇 |
综合类 | 378篇 |
数学 | 11625篇 |
物理学 | 19044篇 |
出版年
2023年 | 556篇 |
2022年 | 1067篇 |
2021年 | 1310篇 |
2020年 | 1574篇 |
2019年 | 2794篇 |
2018年 | 1693篇 |
2017年 | 2696篇 |
2016年 | 2086篇 |
2015年 | 1898篇 |
2014年 | 2528篇 |
2013年 | 5406篇 |
2012年 | 3569篇 |
2011年 | 4368篇 |
2010年 | 3205篇 |
2009年 | 3816篇 |
2008年 | 4270篇 |
2007年 | 4293篇 |
2006年 | 4060篇 |
2005年 | 3564篇 |
2004年 | 3771篇 |
2003年 | 3412篇 |
2002年 | 3686篇 |
2001年 | 3404篇 |
2000年 | 3185篇 |
1999年 | 3022篇 |
1998年 | 2608篇 |
1997年 | 1870篇 |
1996年 | 1228篇 |
1995年 | 1023篇 |
1994年 | 836篇 |
1993年 | 645篇 |
1992年 | 562篇 |
1991年 | 401篇 |
1990年 | 367篇 |
1989年 | 343篇 |
1988年 | 267篇 |
1987年 | 184篇 |
1986年 | 221篇 |
1985年 | 282篇 |
1984年 | 252篇 |
1983年 | 115篇 |
1982年 | 208篇 |
1981年 | 306篇 |
1980年 | 281篇 |
1979年 | 183篇 |
1978年 | 211篇 |
1977年 | 138篇 |
1976年 | 97篇 |
1975年 | 89篇 |
1974年 | 83篇 |
排序方式: 共有10000条查询结果,搜索用时 17 毫秒
991.
Gretchen D. Onstad 《Analytica chimica acta》2005,534(2):281-292
A refined method for the sub-nanomolar analysis of 13 halogenated furanones in chlorinated drinking water is proposed which uses liquid-liquid extraction, methylation where necessary, gas chromatographic separation, and either micro-electron capture or ion trap mass spectrometric detection. Liquid-liquid extraction with methyl tert-butyl ether was demonstrated to be effective for recovery of halogenated furanones. Confirmation of the halogenated furanones identity and reduction of natural organic matter interference were achieved by ion trap tandem mass spectrometry. Compound stabilities and procedural efficiencies were evaluated to permit optimization of the method for reasonable sample volumes and a 1000:1 pre-concentration factor that would permit feasible sample collection in the field. Both chlorinated and brominated analogues of MX (3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone) were included in a suite of compounds targeted in a national occurrence study of disinfection by-products. 相似文献
992.
Rüdiger Verfürth 《Numerische Mathematik》1986,50(6):685-695
Summary We consider a mixed finite element approximation of the three dimensional vector potential, which plays an important rôle in the simulation of perfect fluids and in the calculation of rotational corrections to transonic potential flows. The central point of our approach is a saddlepoint formulation of the essential boundary conditions. In particular, this avoids the wellknown Babuka paradox when approximating smooth domains by polyhedrons. Using piecewise linear/piecewise constant elements for the vector potential/the boundary terms, we obtain optimal error estimates under minimal regularity assumptions for the solution of the continuous problem. 相似文献
993.
Erich Novak 《Numerische Mathematik》1986,50(2):245-252
Summary The definition of the average error of numerical methods (by example of a quadrature formula
to approximateS(f)= f d on a function classF) is difficult, because on many important setsF there is no natural probability measure in the sense of an equidistribution. We define the average a posteriori error of an approximation
by an averaging process over the set of possible information, which is used by
(in the example of a quadrature formula,N(F)={(f(a
1), ...,f/fF} is the set of posible information). This approach has the practical advantage that the averaging process is related only to finite dimensional sets and uses only the usual Lebesgue measure. As an application of the theory I consider the numerical integration of functions of the classF={f:[0,1]/f(x)–f(y)||x–y|}. For arbitrary (fixed) knotsa
i
we determine the optimal coefficientsc
i
for the approximation
and compute the resulting average error. The latter is minimal for the knots
. (It is well known that the maximal error is minimal for the knotsa
i
.) Then the adaptive methods for the same problem and methods for seeking the maximum of a Lipschitz function are considered. While adaptive methods are not better when considering the maximal error (this is valid for our examples as well as for many others) this is in general not the case with the average error. 相似文献
994.
Summary Under suitable conditions, we prove the convergence of the Bateman method for integral equations defined over bounded domains inR
d
,d1. The proof makes use of Hilbert space methods, and requires the integral operator to be non-negative definite. For one-dimensional integral equations over finite intervals, estimated rates of convergence are obtained which depend on the smoothness of the kernel, but are independent of the inhomogeneous term. In particular, for aC
kernel andn reasonably spaced Bateman points, the convergence is shown to be faster than any power of 1/n. Numerical calculations support this result. 相似文献
995.
Summary A scheme that uses singular perturbation theory to improve the performance of existing finite element methods is presented. The proposed scheme improves the error bounds of the standard Galerkin finite element scheme by a factor of O(n+1) (where is the small parameter andn is the order of the asymptotic approximation). Numerical results for linear second order O.D.E.'s are given and are compared with several other schemes. 相似文献
996.
Rüdiger Verfürth 《Numerische Mathematik》1986,50(6):697-721
Summary We consider a mixed finite element approximation of the stationary, incompressible Navier-Stokes equations with slip boundary condition, which plays an important rôle in the simulation of flows with free surfaces and incompressible viscous flows at high angles of attack and high Reynold's numbers. The central point is a saddle-point formulation of the boundary conditions which avoids the well-known Babuka paradox when approximating smooth domains by polyhedrons. We prove that for the new formulation one can use any stable mixed finite element for the Navier-Stokes equations with no-slip boundary condition provided suitable bubble functions on the boundary are added to the velocity space. We obtain optimal error estimates under minimal regularity assumptions for the solution of the continous problem. The techniques apply as well to the more general Navier boundary condition. 相似文献
997.
Robert Kosler 《Numerische Mathematik》1986,50(1):45-55
Summary Operator equationsTu=f are approximated by Galerkin's method, whereT is a monotone operator in the sense of Browder and Minty, so that existence results are available in a reflexive Banach spaceX. In a normed spaceY error estimates are established, which require a priori bounds for the discrete solutionsu
h in the norm of a suitable space
. Sufficient conditions for the uniform boundedness u
h
Z
=O(1) ash0 are proved. Well-known error estimates in [3] for the special caseX=Y=Z are generalized by this. The theory is applied to quasilinear elliptic boundary value problems of order 2m in a bounded domain
. The approximating subspaces are finite element spaces. Especially the caseX=W
0
m, p
(), 1<p<,Y=W
0
m. 2
(),Z=W
0
m. max (2,p)
()Wm, () is treated. Some examples for 1<p<2 are considered. Forp2 a refined technique is introduced in the author's paper [7]. 相似文献
998.
999.
HUANG Xiao-wen SHI Peng-fei 《高等学校化学研究》2006,22(1):73-75
IntroductionLithium ion batteries are key components of mobiletelephones and portable computers.Among the knownLi-intercalation materials for lithium ion battery cath-odes,LiCoO2,LiNiO2,and LiMn2O4have been stud-ied extensively[1—3].LiCoO2is nowused in c… 相似文献
1000.
首次合成了喹喔啉-2,3-二甲酰胺(Qxda)与3d过渡金属的三种固体配合物:M(Qxda)2cl2(M=Co(I),Ni(I),Cu(Ⅱ)。通过元素分析,摩尔电导,红外光谱,电子光谱,磁化率,热分析等手段对配合物的组成和性质进行了研究。 相似文献