A function $f: X \to Y$ is called linear iff
$f(x_1+x_2) = f(x_1) + f(x_2)$
It is what I learned.
Is there any concept like 'linear' defined on the functions whose input and output are sets.
A function $f: X \to Y$ is called 'ABCD' iff $f(X1 \cup X2) = f(X1) \cup f(X2)$
Is there any concept corresponding to 'ABCD' here?
// Thanks, lulu I clarify here:
f is defined on some fixed set and that X_i denotes subsets of that set.