So I have this question to prove a statement:$O(n)\subset\Theta(n)$...I don't need to know how to prove it, just that in my mind this makes no sense and I think it should rather be that $\Theta(n)\subset O(n)$. My understanding is that $O(n)$ is the set of all functions who do no worse than $n$ while $\Theta(n)$ is the set of all functions that do no better and no worse than n. Using this, I can think of the example of a constant function say $g(n)=c$. This function will surely be an element of $O(n)$ as it will do no worse than $n$ as $n$ approaches a sufficiently large number. However, the same function $g$ would not be an element of $\Theta(n)$ as g does do better than $n$ for large $n$... Then since $g  \in  O(n)$ and $g \not\in \Theta(n)$, then $O(n)\not\in\Theta(n)$  So is the question perhaps wrong ? I've learnt it is dangerous to make that assumption and usually I have missed something, I just can't see what it might be in this case. Any thoughts ? Thanks a lot.. 
Is $O$ contained in $\Theta$?
asymptotics;mathematical analysis;landau notation
At the suggestion of Raphael, I have turned a previous comment into this answer.It is not true that $O(f(n)) \subset \Theta(f(n))$. In fact, $\Theta(f(n)) = O(f(n)) \cap \Omega(f(n))$, by definition. So we have $\Theta(f(n)) \subset O(f(n))$.