I am just doing a research on a project and came across a problem. I would be very grateful if anybody could help me out with this. Consider the figure below:

![enter image description here](https://i.stack.imgur.com/6PU7B.png)

Two dots joined by a line results in only one diagram, three dots joined by single lines also results in one figure no matter how you join the dots, the result is the same. But as we increase the dots there are different possibilities, as seen with four dots.

Is there a formula for counting the number of unlabeled trees that can be formed from a set of nodes?