Standard definition of sequence limit: for sequence {xn}, if there is a constant a, for any ε > 0, there is always a positive integer n, so when n >: | xn-a |.

Definition of function limit standard: Let function f (x) be defined when | x | is greater than a positive number. If there is a constant a, for any ε >; 0, there is always a positive integer x, so when x >; At x, if | f (x)-a | < ε holds, then a is called the limit of the function f (x) at infinity.

Let the function f(x) be defined in the eccentric neighborhood at x0. If there is a constant a, for any ε >; 0, there is always a positive number δ, so when | x-XO |.