The Euler characteristic of a graph is V – E + F. Where V is the number of vertices, E is the number of edges, and F is the number of faces.
Given a graph $X$, can we write $X$ as a set and find its Euler characteristic with a formula which only refers to cardinality and union?
I think we found a way, but is this interesting? We would like to find a way to write the Euler characteristic completely set-theoretically. And, we have found a way, but still working on it. Has this already been done?