1, 
the distribution (chi-square distribution)
If n mutually independent random variables ξ₁, ξ₂, ..., ξn, are subject to the standard normal distribution (also known as independent and identically distributed standard normal distribution), then the n obey the standard normal random variable square configuration and a new random variable, the distribution of which is referred to as a chi-square distribution (chi-square distribution).
Distribution is important in mathematical statistics. Distribution Abbe (Abbe) first proposed in 1863, and later by one of the founders were derived from the Haier Mott (Hermert) and modern statistical Karl Pearson (CK.Pearson) in 1875 and 1900 it is a very useful statistics well-known distribution.
n = 1 corresponds to a single normal distribution, chi-square distributions.
2, Weibull distribution
3, Student distribution (S distribution)
[Reference documentation] https://zhuanlan.zhihu.com/c_158803178
[Original document, references to declare the source, yvivid] https://www.cnblogs.com/yvivid/p/reliability_probability.html