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121.
Abstract Fishing leads to truncation of a population's age and size structure. However, large‐sized fish are usually more valuable per unit weight than small ones. Nevertheless, these size‐related factors have mostly been ignored in bioeconomic modeling. Here, we present a simple extension to the Gordon–Schaefer model that accounts for variations in mean individual catch weight, and derive the feedback rule for optimal harvest in this setting. As the Gordon–Schaefer model has no population structure, size effects have to be accounted for indirectly. Here we assume a simple negative relationship between fishing effort and mean individual weight, and a positive relationship between mean catch weight and price. The aim is to emulate alterations of size structure in fish populations due to fishing and the influence of size on price per weight unit and eventually, net revenues. This demonstrates, on a general level, how such size‐dependent effects change the patterns of optimal harvest paths and sustainable revenue in single fish stocks. The model shows clear shifts toward lower levels of optimal effort and yield compared to classical models without size effects. This suggests that ignoring body size could lead to misleading assumptions and policies, potentially causing rent dissipation and suboptimal utilization of renewable resources. 相似文献
122.
Many biological and medical studies have as a response of interest the time to occurrence of some event, such as the occurrence of a particular symptom or disease, remission, relapse, death due to some specific disease, or simply death. In this paper we study the problem of assessing the effect of potential risk factors on the outcome event of interest through a parametric or semi-parametric frailty model where the lifetimes have a reason to be considered dependent. This dependence may arise because of multiple endpoints within the same individual or because, when studying a single endpoint, there are natural groupings between study subjects. The objective of this paper is to extend both parametric and semi-parametric approaches to regression analysis in which the lifetimes of individuals in a group are effected by the same random frailty which follows a positive stable distribution. Some comparisons of the properties of this frailty distribution with other frailty distributions are made and an example which assesses the effect of a treatment in a litter-matched tumorigenesis study is presented. 相似文献
123.
张福保 《应用数学和力学(英文版)》2000,21(3):363-370
IntroductionConsiderthefollowingperiodicboundaryvalueproblem(PBVP,forshort)forfirst_orderintegro_differentialequationofmixedtypeu′=f(t,u,T1u,T2u) (a.e.t∈I),(1)u(0)=u(2π),(2)whereI=[0,2π],fsatisfiesCaratheodory’sconditions,T1isaVolterraintegraloperator,T2isaFredholminte… 相似文献
124.
Piotr Gwiazda 《Mathematical Methods in the Applied Sciences》2005,28(18):2201-2223
This paper concerns measure‐valued solutions for the two‐dimensional granular avalanche flow model introduced by Savage and Hutter. The system is similar to the isentropic compressible Euler equations, except for a Coulomb–Mohr friction law in the source term. We will partially follow the study of measure‐valued solutions given by DiPerna and Majda. However, due to the multi‐valued nature of the friction law, new more sensitive measures must be introduced. The main idea is to consider the class of x‐dependent maximal monotone graphs of non‐single‐valued operators and their relation with 1‐Lipschitz, Carathéodory functions. This relation allows to introduce generalized Young measures for x‐dependent maximal monotone graph. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
125.
在负象限相依结构下,得到了支撵在(-∞,∞)上的(D)族随机变量非中心化以及中心化部分和的精致大偏差.同时,还在较弱的条件下,得到了相应的中心化随机和的精致大偏差. 相似文献
126.
研究了一个带有HollingⅠ类功能反应和时间延迟的离散半比率依赖捕食-食饵系统,给出了系统保持持久性的充分条件. 相似文献
127.
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129.
Yang Li Zhang Qingling Liu Guoyi 《Annals of Differential Equations》2005,21(3):497-502
We discuss the delay dependent robust dissipative problem for a class of time-delay systems with nonlinear perturbations. And the sufficient conditions for the delay dependent robust dissipation and quadratic stability are given by the linear matrix inequalities (LMIs), then the time-delay bound and the delay dependent robust dissipative state feedback controller are presented. 相似文献
130.
Reza Zanjirani Farahani Zvi Drezner Nasrin Asgari 《Annals of Operations Research》2009,167(1):353-368
In this paper a single facility location problem with multiple relocation opportunities is investigated. The weight associated
with each demand point is a known function of time. We consider either rectilinear, or squared Euclidean, or Euclidean distances.
Relocations can take place at pre-determined times. The objective function is to minimize the total location and relocation
costs. An algorithm which finds the optimal locations, relocation times and the total cost, for all three types of distance
measurements and various weight functions, is developed. Locations are found using constant weights, and relocations times
are the solution to a Dynamic Programming or Binary Integer Programming (BIP) model. The time horizon can be finite or infinite. 相似文献