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81.
82.
最短时限运输问题及图上求解法 总被引:3,自引:0,他引:3
提出了最短时限运输问题,借助于赋权二分图研究了其解的最优性充要条件,并给出了在赋权二分图上求解的具体步骤,最后给出了一个实例。事实证明,该法是一个有效的算法 相似文献
83.
Well known extensions of the classical transportation problem are obtained by including fixed costs for the production of
goods at the supply points (facility location) and/or by introducing stochastic demand, modeled by convex nonlinear costs,
at the demand points (the stochastic transportation problem, [STP]). However, the simultaneous use of concave and convex costs
is not very well treated in the literature. Economies of scale often yield concave cost functions other than fixed charges,
so in this paper we consider a problem with general concave costs at the supply points, as well as convex costs at the demand
points. The objective function can then be represented as the difference of two convex functions, and is therefore called
a d.c. function. We propose a solution method which reduces the problem to a d.c. optimization problem in a much smaller space,
then solves the latter by a branch and bound procedure in which bounding is based on solving subproblems of the form of [STP].
We prove convergence of the method and report computational tests that indicate that quite large problems can be solved efficiently.
Problems up to the size of 100 supply points and 500 demand points are solved.
Received October 11, 1993 / Revised version received July 31, 1995 Published online November 24, 1998 相似文献
84.
南京关口每年从国外进口铅、锌精矿达几百万吨,随着我国国民经济的发展,作为重要有色金属来源的该矿种的进口量仍将逐年增加。硫化物精矿在运输和储存过程中都有可能发生氧化,由此导致金属品位的波动是常见现象,然而此前在这方面的报导甚少。为此,作者以铅锌硫化物精矿为例对其进行了模拟状态下的氧化实验并采用能谱、光学显微镜、化学物相分析、X射线衍射分析、扫描电镜等手段对氧化前及氧化后的样品进行了鉴定。结果表明,在模拟精矿储运条件下两种精矿的氧化皆甚为明显,铅精矿为1.5%,而锌精矿达到了14.5%,由此引起的品位变化是不可忽视的。鉴于有色金属硫化物精矿在储运过程中呈现了它们的易氧化性并对品位产生影响,所以有关这类矿产品的商贸过程中涉及取样、样品加工和保管、公证等问题,均应在科学实验基础上规范化。 相似文献
85.
86.
87.
在化工原理实验课程中,流体流动综合实验包含了离心泵曲线的测定、流量计的标定等实验项目。通过最新变频技术开展了离心泵流量的测定与频率的关系的实验,有效弥补了目前教材中缺乏有关变频节能内容的不足,具有很高的创新性,对于提高学生主动学习能力具有重要的意义,培养了学生的工程素质,提高了他们的综合能力。 相似文献
88.
89.
D.?Helbing M.?Treiber A.?Kesting M.?Sch?nhof 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,69(4):583-598
Starting from the instability diagram of a traffic flow model, we
derive conditions for the occurrence of congested traffic states, their
appearance, their spreading in space and time, and the related
increase in travel times. We discuss the terminology of traffic phases
and give empirical evidence for the existence of a phase diagram of
traffic states. In contrast to previously presented phase diagrams, it
is shown that “widening synchronized patterns”
are possible, if the maximum flow is located inside of a metastable density
regime. Moreover, for various kinds of traffic models with different
instability diagrams it is discussed, how the related phase diagrams
are expected to approximately look like. Apart from this, it is pointed out that combinations of
on- and off-ramps create different patterns than a
single, isolated on-ramp. 相似文献
90.
D. Helbing 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,69(4):569-570
This contribution compares several different approaches allowing one to derive macroscopic traffic equation directly from
microscopic car-following models. While it is shown that some conventional approaches lead to theoretical problems, it is
proposed to use an approach reminding of smoothed particle hydrodynamics to avoid gradient expansions. The derivation circumvents
approximations and, therefore, demonstrates the large range of validity of macroscopic traffic equations, without the need
of averaging over many vehicles. It also gives an expression for the “traffic pressure”, which generalizes previously used
formulas. Furthermore, the method avoids theoretical inconsistencies of macroscopic traffic models, which have been criticized
in the past by Daganzo and others. 相似文献