title: The complexity of n choose 2 is in Theta (n^2)?
body: <p>I'm reading <a href="http://en.tjcities.com/wp-content/uploads/Books/Algorithms_3rd.pdf" rel="noreferrer">Introduction to Algorithms 3rd Edition (Cormen and Rivest)</a> and on page 69 in the "A brute-force solution" they state that n choose 2 = Theta (n^2). I would think it would be in Theta (n!) instead. Why is n choose 2 tightly bound to n squared? Thanks!</p>