Question

If an interval is closed, can it be unbounded?

Answer 1

No.

Explanation:

A closed interval necessarily includes the endpoints, such as `[1, 2]`.

If either of the ends of the interval is infinite, we know that we have to write it as open (or half closed), such as `[1, infty) or (-infty, infty)`.

This means that an unbounded (i.e. including `pm infty`) cannot be closed.