TITLE: Understanding Proof that Interval $[0,1]$ is Compact.
QUESTION [5 upvotes]: On page 44-45 of the notes from http://www.msc.uky.edu/droyster/courses/fall99/math4181/classnotes/notes5.pdf, the author writes in the proof that the  interval $[0,1]$ is compact that supposing the interval isn't compact, that this implies either $[0,1/2]$ or $[1/2,1]$ isn't compact. How do we know that both $[0,1/2]$ and $[1/2,1]$ are not compact?

REPLY [3 votes]: Suppose $[0,\frac{1}{2}]$ and $[\frac{1}{2},1]$ are compacts. Now take any open cover of $[0,1]$. This open cover is an open cover of $[0,\frac{1}{2}]$ and $[\frac{1}{2},1]$. Now you use the compactness of $[0,\frac{1}{2}]$ and $[\frac{1}{2},1]$ and conclude that $[0,1]$ is compact.