Detecting Free Products in the Mapping Class Group of Punctured Disks via Dynnikov Coordinates

Elif Medetoğulları, Elif Dalyan, S. Öykü Yurttaş

Published 2025-01-21, updated 2025-01-22Version 2

We prove that Dehn twists about certain $k$ curves on an $n$-punctured disk generate either a free group of rank $k$ or a free product of Abelian groups, depending on the configuration of the curves. Our approach refines previously known criteria, providing a different method for detecting freeness in the mapping class group of the punctured disk. Using Dynnikov coordinates, we further present an algorithm that checks whether Dehn twists about these $k$ curves generate a free group or a free product of Abelian groups.