On the duality between trees and disks

Abstract: A combinatorial category Disks was introduced by Andr\'e Joyal to play a role in his definition of weak omega-category. He defined the category Theta to be dual to Disks. In the ensuing literature, a more concrete description of Theta was provided. In this paper we provide another proof of the dual equivalence and introduce various categories equivalent to Disk or Theta, each providing a helpful viewpoint. In this second version the paper's contents have been reorganized with the goal of a more readable presentation. We define augmented categories and their reduced counterparts (which lack a single trivial object of the augmented category). These augmented categories are more suitable for inductive arguments and their reduced counterparts are equivalent to Disk and Theta. The equivalence between Disk and Theta is demonstrated in Sections 4 and 6 using categories inductively defined (in Section 3) from intervals and ordinals. The last two sections take a more categorical perspective, constructing categories of so-called labeled trees and showing that they are equivalent to their inductively defined counterparts, and so to Disk and Theta. The distinction between augmented and reduced categories corrects an error in the first version where the terminal tree was included in the category Disk.