全文获取类型
收费全文 | 3628篇 |
免费 | 456篇 |
国内免费 | 162篇 |
专业分类
化学 | 356篇 |
晶体学 | 5篇 |
力学 | 115篇 |
综合类 | 138篇 |
数学 | 1495篇 |
物理学 | 2137篇 |
出版年
2024年 | 8篇 |
2023年 | 30篇 |
2022年 | 268篇 |
2021年 | 249篇 |
2020年 | 129篇 |
2019年 | 96篇 |
2018年 | 77篇 |
2017年 | 119篇 |
2016年 | 159篇 |
2015年 | 114篇 |
2014年 | 210篇 |
2013年 | 230篇 |
2012年 | 158篇 |
2011年 | 203篇 |
2010年 | 197篇 |
2009年 | 214篇 |
2008年 | 216篇 |
2007年 | 238篇 |
2006年 | 184篇 |
2005年 | 163篇 |
2004年 | 133篇 |
2003年 | 121篇 |
2002年 | 95篇 |
2001年 | 72篇 |
2000年 | 61篇 |
1999年 | 45篇 |
1998年 | 47篇 |
1997年 | 42篇 |
1996年 | 33篇 |
1995年 | 48篇 |
1994年 | 47篇 |
1993年 | 21篇 |
1992年 | 19篇 |
1991年 | 20篇 |
1990年 | 20篇 |
1989年 | 23篇 |
1988年 | 18篇 |
1987年 | 13篇 |
1986年 | 11篇 |
1985年 | 18篇 |
1984年 | 10篇 |
1983年 | 10篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 5篇 |
1979年 | 5篇 |
1978年 | 8篇 |
1976年 | 5篇 |
1974年 | 4篇 |
1972年 | 4篇 |
排序方式: 共有4246条查询结果,搜索用时 15 毫秒
181.
用光子学方法研究了叠栅技术中,当试件光栅被拉压和旋转后,叠栅条纹的空间周期和相对于基准光栅的取向,试件光栅被拉压后的节距等相关问题。根据衍射光波的空间周期可能大于试件光栅空间周期的特点,对二维亚波长周期结构衍射成像进行了设计研究。首先,对二维亚波长周期结构衍射物进行编码,以获得包含编码光栅空间信息的均匀波;其次,使编码得到的均匀波通过光学系统,并被放大到CCD相机所能辨识的大小;再次,经解码光栅解码,滤掉编码波,最终获得二维亚波长周期结构物的空间结构信息,达到超分辨的目的。同时,对成像过程进行了较为详尽的分析,对编码器、解码器的位置以及它们相对衍射物的取向进行了设计研究,对滤波器的选择给予了必要的说明,指出了取得超分辨成像的关键。 相似文献
182.
Sándor Bozóki 《Central European Journal of Operations Research》2008,16(4):345-358
The aim of the paper is to present a new global optimization method for determining all the optima of the Least Squares Method
(LSM) problem of pairwise comparison matrices. Such matrices are used, e.g., in the Analytic Hierarchy Process (AHP). Unlike
some other distance minimizing methods, LSM is usually hard to solve because of the corresponding nonlinear and non-convex
objective function. It is found that the optimization problem can be reduced to solve a system of polynomial equations. Homotopy
method is applied which is an efficient technique for solving nonlinear systems. The paper ends by two numerical example having
multiple global and local minima.
This research was supported in part by the Hungarian Scientific Research Fund, Grant No. OTKA K 60480. 相似文献
183.
We consider the realization of a quantum computer in a chain of nuclear spins coupled by an Ising interaction. Quantum algorithms
can be performed with the help of appropriate radio-frequency pulses. In addition to the standard nearest-neighbor Ising coupling,
we also allow for a second neighbor coupling. It is shown, how to apply the 2π
k method in this more general setting, where the additional coupling eventually allows to save a few pulses. We illustrate
our results with two numerical simulations: the Shor prime factorization of the number 4 and the teleportation of a qubit
along a chain of 3 qubits. In both cases, the optimal Rabi frequency (to suppress non-resonant effects) depends primarily
on the strength of the second neighbor interaction. 相似文献
184.
Jeevan Jaisingh Jack Barron Shailendra Mehta Alok Chaturvedi 《European Journal of Operational Research》2008
The issues we address here are – How should a firm (e.g. Internet service provider (ISP)) that is capable of collecting personal information (browsing information, purchase history, etc.) about consumers, price its service, given that consumers vary in their valuation for privacy, and also vary in terms of the value of their personal information to a third party (firms that need consumer information)? Should the firm have a blanket policy of never collecting, or a policy of always collecting and revealing information? Surprisingly we find that in some cases the collector of information may be no worse off in the asymmetric information case than in the full information case. The paper provides a justification for the strategy of some firms such as ISP’s which never collect information and also for the strategy of other firms, like grocery stores that do. We also find that it is non-optimal for the firm to design contracts where the consumer can choose an intermediate level of privacy. 相似文献
185.
有相似产品信息情形下的可靠性评定方法 总被引:1,自引:0,他引:1
两个具有局部相同组成部分的相似设备为模型,建立有相关信息情形下进行可靠性评估的指数模型,并给出了指数模型中的参数估计方法,在此基础上建立了相似设备到目标设备信息的融合算法,进而提出了一种具有相似产品信息情形下的可靠性评定方法. 相似文献
186.
J. S. Nicolis 《Il Nuovo Cimento D》1987,9(11):1359-1388
Summary A given but otherwise random environmental time series impinging on the input of a certain biological processor passes through
with overwhelming probability practically undetected. A very small percentage of environmental stimuli, though, is ?captured?
by the processor's nonlinear dissipative operator asinitial conditions, that is, assolutions of the processor's dissipative dynamics. The processor, then, is in such cases instrumental in compressing or abstracting
those stimuli, thereby making the external world to collapse from a previous regime of a ?pure state? of suspended animation
onto a set of stable eigenfunctions or ?categories?—chaotic strange attractors. The characteristics of this cognitive set
depend on the operator involved and the hierarchical level where the abstraction takes place. In this paper we model the physics
of such a cognitive process and the role that the thalamo-cortical pacemaker of the (human) brain plays in both stimulating
the individual attractors and permutating them on a time division multiplexing basis. A synthesis of the Markovian processes
taking place within each individual attractor-memory and the Markovian or semi-Markovian process involving the intermittent
jumping among the different attractor-memories are discussed.
Riassunto Una data ma casuale serie temporale ambientale che influisce sull'immissione di un determinato processore biologico passa con schiacciante probabilità praticamente inosservata. Una piccolissima percentuale di stimoli ambientali, tuttavia, è catturata dall'operatore dissipativo non lineare del processore comecondizioni iniziali, cioè comesoluzioni della dinamica dissipativa del processore. Il processore è quindi, in tali casi, strumentale nel comprimere o estrarre quegli stimoli, facendo perciò collassare il mondo esterno da un precedente regime di ?stato puro? ad animazione sospesa ad un gruppo di autofunzioni stabili o ?categorie?—attrattori caotici strani. Le caratteristiche di questo gruppo cognitivo dipendono dall'operatore coinvolto e dal livello gerarchico in cui l'estrazione ha luogo. In questo lavoro si modella la fisica di tale processo cognitivo e il ruolo che lo stimolatore talamo-corticale del cervello (umano) ha sia nello stimolare gli attrattori individuali che nel permutarli su una base che manda segnali multiplex a divisione temporale. Si discute una sintesi dei processi markoviani che hanno luogo all'interno di ogni attrattore-memoria individuale e il processo markoviano o semimarkoviano che interessa il salto intermittente tra i diversi attrattori-memoria.
Резюме Временная последовательность на входе некоторого биологического процессора может иметь практически недетектируемую вероятность. Очень небольшой процент стимулов окружающей среды ?захватывается? нелинейным диссипативным оператором процессора как начальные условия, т.е. представляется как решение диссипативпой динамики процессора. Затем процессор обрабатывает внешние стимулы, вынуждая внешний мир коллапсировать из предыдущего режима ?чистого состояния? приостановленного оживления в систему собственных функций или ?категорий?. Характеристики зтой познавательной системы зависят от используемого оператора и иерархического уровня, на котором происходит абстрагирование. В зтой статье мы моделируем физику такого познавательного процесса. Обсуждается синтез марковских процессов, происходящий внутри отдельного злемента ?атрактор-память?, и марковский или полумарковский процесс, включающий пережежающиеся прыжки между различными злементами ?атракторпамять?.相似文献
187.
We design a novel X-ray image detector by lens coupling a Gd2O2S:Tb intensifying screen with a high performance low-light-level (L3,which often means luminescence less than 10-3 Lux) image intensifier.Different coupling effects on imaging performance between zoom lens and fix-focus lens are analyzed theoretically.In experiment,for designing a detector of 15-inch visual field,the system coupled by zoom lens is of 12.25-1p/cm resolution,while the one with fix-focus lens is 10 lp/cm.The superiority of zoom lens is validated.It is concluded that zoom lens preserves the image information better than fix-focus lens and improves the imaging system's performance in this design,which is referential to the design of other optical imaging systems. 相似文献
188.
Labeling algorithm for the shortest path problem with turn prohibitions with application to large-scale road networks 总被引:1,自引:0,他引:1
In real road networks, the presence of no-left, no-right or no U-turn signs, restricts the movement of vehicles at intersections.
These turn prohibitions must be considered when calculating the shortest path between a starting and an ending point in a
road network. We propose an extension of Dijkstra’s algorithm to solve the shortest path problem with turn prohibitions. The
method uses arc labeling and a network structure with low memory requirements. We compare the proposed method with the dual
graph approach in a set of randomly generated networks and Bogotá’s large-scale road network. Our computational experiments
show that the performance of the proposed method is better than that of the dual graph approach, both in terms of computing
time and memory requirements. We co-developed a Web-based decision support system for computing shortest paths with turn prohibitions
that uses the proposed method as the core engine. 相似文献
189.
In this paper, we propose a new design for the recursive least-squares (RLS) Wiener fixed-lag smoother and filter in linear discrete-time wide-sense stationary stochastic systems. It is assumed that the signal is observed with additive white observation noise. The signal is uncorrelated with the observation noise. The estimators require knowledge of the system matrix, the observation matrix and the variance of the state vector. These quantities can be obtained from the auto-covariance function of the signal. In the estimation algorithms, moreover, the variance of the observation noise is assumed to be known, as a priori information. 相似文献
190.
本文研究了带有Radon-Nikodym导数的算子值自由Fisher信息量.利用模框架理论,得到了一个半圆元和一个子代数之间的合并自由关系,推广了D.Voiculescu等人的工作. 相似文献