Big-Theta

Big-Theta defines $\boxed{f(n)\text{ is }\Theta (g(n))}$ if there is a constant $0<c^{\prime }<c^{\prime \prime }$ and an integer constant $n_0\ge 1$ such that $f(n)\ge 1$ such that $c^{\prime }\cdot g(n)\le f(n)\le c^{\prime \prime }\cdot g(n)$ for $n\ge n_0$.