After constrcting the perfective space, prove that this space is just the space of Lebesgue integratiable function, thus explain that Lebesgue integral is the form of the perfective Riemann integral.

• 在构造了完备化空间之后，证明了该空间就是勒贝格可积函数空间，从而说明了黎曼积分的完备化形式是勒贝格积分。