I'm taking a course 'Introduction to AI' and, in one of the tutorials, it was written that when pruning the game tree using $\alpha$-$\beta$ boundaries, the number of nodes that will be developed, when using random sort function for the children (i.e., in the average case) will be $$ O(b^{3d/4})$$.
Since the proof is not in the scope of the course we weren't given one at the tutorial, so I tried looking for the proof online, but couldn't find anything, and I didn't think of anything myself. I wondered whether someone could give me a lead or refer me to some reading material that shows the full proof?