泊松分布:https://baike.baidu.com/item/%E6%B3%8A%E6%9D%BE%E5%88%86%E5%B8%83/1442110
指数分布:https://baike.baidu.com/item/指数分布/776702?fr=aladdin
概率密度函数:https://baike.baidu.com/item/%E6%A6%82%E7%8E%87%E5%AF%86%E5%BA%A6%E5%87%BD%E6%95%B0/5021996?fr=aladdin
http://www.ruanyifeng.com/blog/2015/06/poisson-distribution.html