I was given the following problem:
Calculate the computational time complexity of the recurrence $$P(n) = \begin{cases} 1 & \text{if } n = 1 \\ \sum_{k=1}^{n-1} P(k) P(n-k) & \text{if } n \geq 2 \end{cases}$$ using the substitution method. Answer: $\Omega(2^n)$.
I have seen there is a closed-form for this recurrence but I am unsure whether it can be used out of the blue or not. I don't know if I have to prove it by deriving it from the recurrence or if there are simpler ways of solving such a problem by mere substitution.
On the other hand, I think substitution is not the right tool to use here. I have tried with both reverse and forward substitution, but in neither approach I could determine the function as with other problems (e.g. QuickSort).
Is it possible to solve this problem with only substitution at all? If so, how can I devise it?