Are big-theta, Big-O, etc. all representative only of GROWTH of the function? For example, $2^{n-1}$... is that $\Theta(2^n)$? it GROWS the same... but it in actuality will never be greater than or equal to the actual 2^n function, for example. $\log_2(n)$, is that $\Theta(\log_4(n))$? How do we handle growth of logarithm with different bases when describing things in these notations? How about this: $2^{n^2}$, is that $O(2^n)$? How about exponentials on the nth term? Are these ignored like constants? Just trying to understand the semantics of these notations in practicality.

$2^{n-1}$ is that $\Theta(2^n)$?

**hint**
You need to show that (for large enough $n$) there are constants $A,B$ so that $$ A 2^n \le 2^{n-1} \le B 2^n . $$ Can you do that?