https://math.stackexchange.com/questions/326197/the-equivalence-between-cauchy-integral-and-riemann-integral-for-bounded-functio
https://zhuanlan.zhihu.com/p/59637169
谢惠民柯西积分与黎曼积分之间的等价性
https://www.zhihu.com/question/338413565
哈代不等式