# 2. 泊松分布与幂律分布

## 2.1 泊松分布

Poisson distribution is a discrete probability distribution that expresses the probability of a given number of events occurring in a fixed interval of time and/or space if these events occur with a known average rate and independently of the time since the last event. The Poisson distribution can also be used for the number of events in other specified intervals such as distance, area or volume.

PS：概率质量函数（probability mass function，pmf）是离散随机变量在各特定取值上的概率。值得注意的是：概率质量函数是对离散随机变量定义的，本身代表该值的概率；概率密度函数是对连续随机变量定义的，本身不是概率，只有对连续随机变量的概率密度函数在某区间内进行积分后才是概率[6]

## 2.2 幂律分布

a power law is a functional relationship between two quantities, where one quantity varies as a power of another.

[1]维基百科词条：帕雷托法则
[2]维基百科词条：长尾
[3]智库百科词条：长尾理论
[4]维基百科词条：利基市场
[5]维基百科词条：泊松分布
[6]维基百科词条：概率质量函数