what is the math branch that research functional space where composition of the functions are operations?
Hilbert space and other linear functional space mainly research on the addition and scalar multiplication of two functions in the space. such as if f1 + f2 , and a*f,
what if compositions of functions are allowed inside the space? such as f1(f2). ?
Is there a particular math branch that research this space? maybe a sub-branch of functional analysis?