柯西收敛准则

$\lim_{x \to x_0}f(x)=a$的充要条件是：$\forall\epsilon>0$，$\exists \delta>0$，当$x_1，x_2\in D(f)$且$0<|x_1 - x_0|<\delta$，$0<|x_2 - x_0|<\delta$时，有$|f(x_1) - f(x_2)|<\epsilon$