Classical (or “elliptic”) modular forms are functions in the complex upper half-plane which transform in a certain way under the action of a discrete subgroup of SL(2, R) such as SL(2,Z).
Classical (or “elliptic”) modular forms are functions in the complex upper half-plane which transform in a certain way under the action of a discrete subgroup of $\text{SL}(2, \Bbb R)$ such as $\text{SL}(2, \Bbb Z)$.