Define Big Theta notation

Big Theta notation (θ): The upper and lower bound for the function 'f' is given by the big oh notation (θ). Considering 'g' to be a function from the non-negative integers to the positive real numbers, we describe θ(g) as the set of function f  such that  for some real constant c1 and c2 >0 and some non negative integers constant n0 , c1g(n)≤f f(n)≤c2g(n) for all n≥n0.  Mathematically, θ(g(n))={f(n): there exists positive constants c1 and c2 like that c1g(n)≤f f(n)≤c2g(n) for all n, n≥n0} , we say "f is oh of g"