Problem:

Let A be a subset of . The function defined by

is called the characteristic function of A. Show that exists if and only if x* is either an interior or exterior point of the set A.

=====================

Since A is a subset of , then let x* be a point in .

If x* is an interior point then there is an open ball about x* that is contained in A.

My question is that if x* is an interior point, then the since x* is contained in A therefore ?

Similarly, if x* is an exterior point(provided that there is an open ball about x* that is contained in , then since

Thank you very much for your help.

Never mind. I figured it out.