Finding the complexity of a recursive function

Questions
,
1

First time user here.

I’m trying to use Wolfram|Alpha to calculate the time complexity of a recursion function.
When using the calculator, the results I get don’t include terms such as bigO Omega etc.

For this function:

I should get θ(n), but I can’t find it.

Your help is much appreciated.

2

Why do you expect W|A to give a big-O form for the result? (It is not obvious to me that this shouldn’t happen, but neither is it obvious that it should.)