摘要
介绍可测函数的几个等价定义，由此得出可测函数能用简单函数逼近和连续函数逼近。
he author introduces a few equivalent definitions of measurable function and concludes from this that measurable function can be approximated by simple function and continuous function.
关键词
可测函数
等价定义
逼近
measurable function
equivalent definition
approximation
