ABSTRACT
Set https://www.w3.org/1998/Math/MathML"> ℌ = { z = x + i y ∈ ℂ : y > 0 } https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780138747022/abb99e87-ffb7-4196-a4c4-acb0033e3d6a/content/eq1214.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> and let Γ be the modular group https://www.w3.org/1998/Math/MathML"> PSL 2 ( ℤ ) = { ( a b c d ) : a , b , c , d ∈ ℤ , a d − b c = 1 } / ± 1. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780138747022/abb99e87-ffb7-4196-a4c4-acb0033e3d6a/content/eq1215.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/>
