Topologically Distinct Sets of Non-intersecting Circles in the Plane

Abstract

Nested parentheses are forms in an algebra which define orders of evaluations. A class of well-formed sets of associated opening and closing parentheses is well studied in conjunction with Dyck paths and Catalan numbers. Nested parentheses also represent cuts through circles on a line. These become topologies of non-intersecting circles in the plane if the underlying algebra is commutative. This paper generalizes the concept and answers quantitatively - as recurrences and generating functions of matching rooted forests - the questions: how many different topologies of nested circles exist in the plane if (i) pairs of circles may intersect, or (ii) even triples of circles may intersect. That analysis is driven by examining the symmetry properties of the inner regions of the fundamental type(s) of the intersecting pairs and triples.