Theta notation provides an asymptotically tight bound for f(n). Θ notation is simply written as, f(n) ∈ Θ(g(n)), where n is the problem size and
Θ(g(n)) = {h(n): ∃ positive constants c1 , c2 , and n0 such that 0 ≤ c1 g(n) ≤ h(n) ≤ c2 g(n), ∀ n ≥ n0 }
Hence, we can say that Θ(g(n)) comprises a set of all the functions h(n) that are between c1 g(n) and c2 g(n) for all values of n ≥ n0
If f(n) is between c1 g(n) and c2 g(n), ∀ n ≥ n0 ,then f(n) ∈ Θ(g(n)) and g(n) is an asymptotically tight bound for f(n) and f(n) is amongst h(n) in the set