全文获取类型
收费全文 | 2467篇 |
免费 | 126篇 |
国内免费 | 227篇 |
专业分类
林业 | 307篇 |
农学 | 248篇 |
基础科学 | 274篇 |
429篇 | |
综合类 | 1212篇 |
农作物 | 51篇 |
水产渔业 | 50篇 |
畜牧兽医 | 141篇 |
园艺 | 41篇 |
植物保护 | 67篇 |
出版年
2024年 | 16篇 |
2023年 | 32篇 |
2022年 | 63篇 |
2021年 | 65篇 |
2020年 | 69篇 |
2019年 | 124篇 |
2018年 | 62篇 |
2017年 | 80篇 |
2016年 | 92篇 |
2015年 | 99篇 |
2014年 | 129篇 |
2013年 | 112篇 |
2012年 | 189篇 |
2011年 | 167篇 |
2010年 | 114篇 |
2009年 | 139篇 |
2008年 | 119篇 |
2007年 | 147篇 |
2006年 | 120篇 |
2005年 | 108篇 |
2004年 | 76篇 |
2003年 | 62篇 |
2002年 | 49篇 |
2001年 | 45篇 |
2000年 | 50篇 |
1999年 | 58篇 |
1998年 | 45篇 |
1997年 | 45篇 |
1996年 | 53篇 |
1995年 | 51篇 |
1994年 | 47篇 |
1993年 | 43篇 |
1992年 | 33篇 |
1991年 | 31篇 |
1990年 | 25篇 |
1989年 | 29篇 |
1988年 | 22篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1985年 | 1篇 |
排序方式: 共有2820条查询结果,搜索用时 15 毫秒
1.
Establishing an eyeball-weight relationship for Litopenaeus vannamei using machine vision technology
This study aimed to establish a shrimp eyeball-weight relationship model for Litopenaeus vannamei using machine vision technology. A total of 295 shrimp were sampled from a recirculating aquaculture system (RAS). The long-axis length (d), body length (L), and body weight (W) of each individual was measured. The long axis length of the shrimp eyeball was identified and measured using machine vision technology. Continuous fitting and piecewise fitting models were used to construct the eyeball-weight relationship model for L. vannamei. The continuous fitting relationship model was described as: W = 38.865d2.7914, while the piecewise model was described as: d < 2 mm, W = 0.0326d3.7363, R² = 0.9288; 2 mm ≤ d < 3.9 mm, W = 0.0401d3.104, R² = 0.9629; 3.9 mm ≤ d < 5.8 mm, W = 0.0421d3.0311, R² = 0.9216; 5.8 mm < d, W = 0.103d2.6226, R² = 0.9457. The root mean square error (RMSE) of the piecewise fitting model (0.0244, 0.1575, 0.5034, 0.7072) was smaller than the continuous fitting model (0.8229). The correlation coefficient (R2) of the piecewise model (0.9288, 0.9629, 0.9216, and 0.9457) was similar to that of the continuous fitting model (R2 = 0.9621). The results indicated that the piecewise fitting model is suitable for calculating the biomass of L. vannamei in RAS and provides a novel way of estimating the biomass of L. vannamei cultured in RAS. The piecewise fitting model can also provide the foundation of evaluating the production of shrimp using underwater image recognition in intelligent aquaculture systems. 相似文献
2.
本文用约1000块样地资抖,系统地研究了黑龙江省落叶松人工林的地位指数级导向曲线模型。通过林业统计分析软件Statistica6.0对各落叶松人工林数据进行拟合、分析比较,结果为:从拟合精度、曲线走向与散点分布趋势、残差分布等综合考虑,舒马切尔曲线函数比较适合落叶松人工林地位指数级导向曲线模型的拟合。 相似文献
3.
Landscape modeling requires the delineation of system boundaries and interior features. Quite often, these components are
complex and difficult to accurately represent. A rectangular grid is used to represent the study and adjacent non-study areas
in most cases. When the non-study area occupies a large portion of the grid, computer memory is wasted, and computational
time increases. An elliptical grid generator for non-orthogonal curvilinear coordinates is used to generate a boundary-fitted
grid for a landscape model. In a boundary-fitted grid coordinate system, one coordinate axis follows the landscape domain
boundary and is non-orthogonal to the second axis. The boundary-fitted grid uses elliptic partial differential equations to
distribute grid points inside the landscape domain. Although the boundary-fitted grid follows the domain boundary, the grid
pattern and point allocation remain structured. Thus, a landscape model can use a boundary-fitted grid without changing the
model’s data structure or the computational scheme. In this study, a boundary-fitted grid and a raster-based grid were applied
to the Everglades Landscape Fire Model. Use of the boundary-fitted grid decreased model simulation time by about one fifth
and computer storage by 58% relative to the raster-based grid. Also, the linear characteristics of interior geographical features
such as rivers and airboat trails were preserved by the boundary-fitted grid, but not by the raster-based grid. This preservation
provided a more reasonable base map for simulating ecological processes, such as fire across heterogenous landscapes. 相似文献
4.
电导法测定林木种子活力的研究 总被引:4,自引:0,他引:4
本文研究了林木种子电导法测定条件和测定电导率或相对电导率,通过回归分析建立电导率或相对电导率与种子活力指数的相关方程,找出了电导法测定林木种子活力的方法。 相似文献
5.
6.
徐海祥 《湖南农业大学学报(自然科学版)》1989,16(1)
本文对于二阶椭圆型Euler方程组 -∑D_αF_(q_α~i)(x,u,Du)+F_(ui)(x,u,Du)=0得到了一些积分恒等式,利用这些恒等式证明了某些方程组非平凡解的不存在性。 相似文献
7.
8.
河南省杨树材种出材率表的研编 总被引:1,自引:0,他引:1
夏丰昌 《中南林业调查规划》2006,25(1):10-12
用削度方程和单株带皮—去皮胸径的转换关系式两个模型对河南省杨树进行计算机理论造材.经精度检验,满足要求,方法可行,依此编制了河南省杨树材种出材率表。 相似文献
9.
简怀玉 《湖南农业大学学报(自然科学版)》1990,17(3)
本文研究一类2k阶非线性偏微分方程组之解的正则性,没有假定通常的椭圆性条件而只假定所谓"无穷远处"的椭圆性条件,证明了解的k-1阶导数为李普希兹连续的. 相似文献
10.