title: Why we use big O notation for best and average cases also?
body: <p><img src="https://i.stack.imgur.com/GylD4.jpg" alt="here"></p>
  
  <p>As we can see best, worst and average case time complexities for different algorithms, then suppose for merge sort, best case should be Ω(n logn) but instead it's given O(n logn). Similarly, for average case it should have been given  Θ(n logn) but there also big O notation is used. And this big O notation is used everywhere in this table, no matter whether it is best case or average case. Please explain me why.</p>