为什么需要虚数？
这部分内容来源于可汗学院的在线教程Introduction to complex numbers
The answer is simple. The imaginary unit iii allows us to find solutions to many equations that do not have real number solutions.
This may seem weird, but it is actually very common for equations to be unsolvable in one number system but solvable in another, more general number system.
Here are some examples with which you might be more familiar.
With only the counting numbers, we can't solve $x+8=1$; we need the integers for this!
With only the integers, we can't solve $3x-1=0$; we need the rational numbers for this!
With only the rational numbers, we can't solve $x^2=2$. Enter the irrational numbers and the real number system!
And so, with only the real numbers, we can't solve $x^2=-1$. We need the imaginary numbers for this!...

今天在升级论坛程序的时候发现一直提示无法写入文件，已经把相应的目录权限修改为777，还是不行。在网上搜到一个可能的原因，搞定。
可能是由于selinux的原因
查看SELinux状态：
/usr/sbin/sestatus -v
如果SELinux status参数为enabled即为开启状态
也可以用这个命令检查
getenforce
关闭SELinux：
1、临时关闭（不用重启机器）：
设置SELinux 成为permissive模式
setenforce 0
2、修改配置文件需要重启机器：
修改/etc/selinux/config 文件
将SELINUX=enforcing改为SELINUX=disabled
重启机器即可

Bar-Cohen and Rohsennow Solution
Finned surfaces of various shapes, called heat sinks, are frequently used in the cooling of electronic devices. Energy dissipated by these devices is transferred to the heat sinks by conduction and from the heat sinks to the ambient air by natural or forced convection, depending on the power dissipation requirements. Natural convection is the preferred mode of heat transfer since it involves no moving parts, like the electronic components themselves. However, in the natural convection mode, the components are more likely to run at a higher temperature and thus undermine reliability. A properly selected heat sink may considerably lower the operation temperature of the components and thus reduce the risk of failure.
Natural convection from vertical finned s...

Natural convection flow through a channel formed by two parallel plates as shown in Fig. 20–16 is commonly encountered in practice. When the plates are hot (Ts> Too), the ambient fluid atToo enters the channel from the lower end, rises as it is heated under the effect of buoyancy, and the heated fluid leaves the channel from the upper end. The plates could be the fins of a finned heat sink, or the PCBs (printed circuit boards) of an electronic device. The plates can be approximated as being isothermal (Ts= constant) in the first case, and isoflux ( · = constant) in the second case.
Boundary layers start to develop at the lower ends of opposing surfaces, and eventually merge at the midplane if the plates are vertical and sufficiently long. In this case, we will have fully developed chann...

Introduction
With thermal solutions becoming more challenging, there is a push for novel cooling ideas or materials to further mitigate thermal issues facing today’s electronics. In these design situations, the proven method of analytical calculations, modeling, and laboratory testing is sometimes bypassed in the search for a quick “cure-all” solution. Evolutionary progress is needed in the thermal industry of course. However, in a rush to implement new ideas/materials thorough testing should not be overlooked in determining thermal performance of a solution before implementation.
Table 1. Heatsink GeometryThe stated thermal properties of engineered graphite foams have been a motivator for their consideration as heat sink materials. Yet, the literature is void of true compariso...

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