Let f be a one-to-one correspondence from A to B. The inverse
function of f , denoted by f -1, is the function from B to A that assigns
to every element b of B the (unique) element a of A such that f(a)=b.
Example: What would be the inverse function of f: R ?R
--- It would be defined by f -1(x) = ?x.
Only bijections are invertible!