Theorem
If is Continuous on a Closed Interval Then, there is an Global Extrema on that interval
Alternate Theorem
Suppose is closed and bounded. If is Continuous, then there are points where attains its Global Maxima and Global Minima.
Open Interval No EVT
An open interval does not necessarily have a max/min. Consider the example where the absolute max/min would be at the endpoints. There would always be a max/min bigger than the existing max/min.