The Easiest Unsolved Problem in Graph Theory

Good Will Hunting character, a janitor and amateur mathematician draws all homomorphically irreducible trees in response to the call by the professor at MIT. © 1997 Miramax Pictures

This post is written together with Ekaterina Vorobyeva.

At ten I once said to my friend how great it would if our teacher presented to us unsolved math problems. It would be much more fun to approach these in class instead of the textbook exercises. It was the time when we had no easily available internet, and the information on unsolved problems was scarce. Today, 20 years after, when…