Hi, I was wondering if anyone can help me find a recurrence relation for find the number of full (different) binary trees that use 'n' number of nodes.

For example i know that if n=3, there is 1 possible binary tree, and if n=4 it's 0, and if n = 5 its 2. I think I can get all the answers but I cant find a recurrence relation for it!

any help is good, thanks!