Kira Adaricheva, Madina Bolat, Evan Daisy, Ayush Garg,
Grace Ma, Michelle Olson, Rohit Pai, Catherine Raanes, Sean
Riedel, Joseph Rogge, Raviv S. Sarch, James Thompson,
Fernanda Yepez-Lopez and Stephanie Zhou
A convex geometry is a closure system satisfying the antiexchange property. We
document all convex geometries on 4- and 5-element base sets with respect to their
representation by circles on the plane. All 34 nonisomorphic geometries on a
4-element set can be represented by circles, and of 672 known geometries on a
5-element set, we give representations for 623. Of the 49 remaining geometries on a
5-element set, one was already shown not to be representable due to the weak
carousel property, as articulated by Adaricheva and Bolat (Discrete Math. 342:3
(2019), 726–746). We show that seven more of these convex geometries cannot be
represented by circles on the plane, due to what we term the
triangular implicationsproperty.
Keywords
convex geometry, convex hull operator for circles,
representation by circles