]
若k个随机变量、……、
是相互独立，符合标准正态分布的随机变量（数学期望为0、方差为1），则随机变量Z的平方和
被称为服从自由度为 k 的卡方分布，记作
-
]
If Z1, ..., Zk are independent, standard normal random variables, then the sum of their squares,
is distributed according to the chi-squared distribution with k degrees of freedom. This is usually denoted as
The chi-squared distribution has one parameter: k — a positive integer that specifies the number of degrees of freedom (i.e. the number of Zi’s)
（应用原理没搞懂）