# Big-Theta notation [[Big-Theta notation]] is the tight bound on the [[Run time complexity|runtime]] of an algorithm. This is only really defined when some function is the following $f = O(g)$ using [[Big-O notation]] and $f = \Omega(g)$ then we have $f=\Theta(g)$ using [[Big-Theta notation]] this is also an equivalence relation, so $g = \Theta(f)$ as well.