O. Aichholzer, R. Fabila-Monroy, H. González-Aguilar, T. Hackl,
M. Heredia, C. Huemer, J. Urrutia, and B. Vogtenhuber
We consider a variant of a question of Erdos on the number of empty

-gons (

-holes) in a set of

points in the plane, where we allow the

-gons to be non-convex. We show bounds and structural results on
maximizing and minimizing the number of general 4-holes, and
maximizing the number of non-convex 4-holes.