>>182 補足

”Modular function”
いまとなっては、モジュラー群(modular group)と殆ど同義ですが
ご参考まで

https://encyclopediaofmath.org/wiki/Modular_function
Modular function
elliptic modular function, of one complex variable

An automorphic function of a complex variable , associated with the group Γ of all fractional-linear transformations γ of the form
z→γ(z)= {az+b}/{cz+d}　ad − bc = 1 　　　(1)
where are real integers (this group is called the modular group).