Finite subgroups of the extended modular group
arXiv:1802.02000
Abstract
We show that in the extended modular group PGL(2,Z) there are exactly seven finite subgroups up to conjugacy; three subgroups of size 2, one subgroup each of size 3, 4, and 6, and the trivial subgroup of size 1.
4 pages; version 2 has a new introduction, a minor change to the statement of Proposition 1, and improved proofs of Proposition 1 and Theorems 2 and 3. Version 3 has updated bibliography and modified introduction