Notation for average case complexity of an algorithm I'm just wondering what the correct notation is when referring to an average case complexity of an algorithm that was calculated by doing empirical analysis.

For example, I have tested my algorithm and fitted the results to the curve $f(n)=2.65\times 10^{-15}\cdot(2.17^{n})$ and in my report right now I'm saying something like: 


  the average case complexity was found to be $\approx 2.65\times 10^{-15}\cdot(2.17^{n})$, 


but I would rather say something like 


  the average case complexity is $\in \Theta(2.17^{n})$. 


But I'm not sure if this is technically correct because the result hasn't been theoretically proven, only empirically tested and fitted to the curve.
The notation $\Theta(f(n))$ isn't reserved for worst-case complexity, it's asymptotic notation applicable to functions in general. So you can state that the average case complexity is $\Theta(2.17^n)$, and it means exactly what you think it does. (Regardless of whether this has been proved or not.)