Any open interval centered on a is called the neighborhood of point A, and is denoted as U(a).

Let δ be an arbitrary positive number, then (a-δ, a+δ) in the open interval is a neighborhood of point A, which is called δ neighborhood of point A and denoted as U(a, δ), that is, u (a, δ) = {x | a-δ.

After removing the center A, the δ neighborhood of A is called the centripetal δ neighborhood of point A, sometimes the open interval (a-δ, A) is called the left δ neighborhood of A, and the open interval (a, a+δ) is called the right δ neighborhood of A. ..