A weakly informative prior for Bayesian dynamic model selection with applications in fMRI

In recent years, Bayesian statistics methods in neuroscience have been showing important advances. In particular, detection of brain signals for studying the complexity of the brain is an active area of research. Functional magnetic resonance imagining (fMRI) is an important tool to determine which parts of the brain are activated by different types of physical behavior. According to recent results, there is evidence that the values of the connectivity brain signal parameters are close to zero and due to the nature of time series fMRI data with high-frequency behavior, Bayesian dynamic models for identifying sparsity are indeed far-reaching. We propose a multivariate Bayesian dynamic approach for model selection and shrinkage estimation of the connectivity parameters. We describe the coupling or lead-lag between any pair of regions by using mixture priors for the connectivity parameters and propose a new weakly informative default prior for the state variances. This framework produces one-step-ahead proper posterior predictive results and induces shrinkage and robustness suitable for fMRI data in the presence of sparsity. To explore the performance of the proposed methodology, we present simulation studies and an application to functional magnetic resonance imaging data.     

均匀磁场中粒子的运动方程

本文证明了亥姆霍兹线圈在其中心附近产生均匀磁场;推出了运动电荷受磁场力作用时的运动方程。     

光纤的模理论

光纤的优点是直径小,重量轻,应力强,工作温度范围大及无磁干扰,更重要的是其工作频带宽及损耗低。沿光纤传播的波可以形式上视为一个电磁场的边值问題,其中沿一介质圆柱在折射率较低无限仲延的媒质中的传播问题, 已有Hondros and DeBye于1910年解出。本文介绍最新的研究成果中的多层结构光纤的解,同时,指出了在新近的文献中的一些误解供读者参考。     

推力磁轴承转子系统温度场

以实心铁芯涡流损耗及线圈铜损耗为主要热源,研究了推力磁轴承转子系统的电磁场、温度场及电磁场-温度场耦合效应.给出了基于电磁场-温度场耦合的轴对称温度场有限元求解方法.研究表明,造成系统温升的主要原因是推力磁轴承实心推力盘的涡流损耗.     

分散在油中的磁粉磁性能的测量方法

按确定的重量百分比把磁粉分散在不易挥发的蓖麻油里,使用西德生产的1.033型铁磁仪测量其磁性能.结果表明:该方法的饱和磁化场大大低于直接对干粉测量的饱和磁化场;测量的重复性明显提高;磁粉的矩形比与涂粉磁带的矩形此密切相关,这对磁带及软磁盘的生产具有指导意义. 按照下述的要求就能得到准确的测量结果.当磁粉的矩形比是0.7或0.8时,则所需的饱和磁化场应分别是矫顽力的2.5倍或2.0倍;样品的长度不短于10厘米;重量百分比R选择在R～Hc曲线的水平部分;即选择轻敲密度ρ在0.33g/cm~3～1.45g/cm~3. 本方法特别适用于国内测量设备磁化场低这一实际情况.     

载流螺线管磁场的全空间解

针对普通物理学中螺线管模型不能说明的无限长载流密绕螺线管外的磁感应强度不为零的问题，提出建立与实际更接近的螺线管模型，进而计算出其全空间的磁感应强度。