I don't know if it's a joke or not.
If you have a function f(x) such that [itex]f(x_0) \neq f(x_1)[/itex], for all [itex]x_0 \neq x_1[/itex]. Just in that case, you will have [itex]f(x) = f(y) \Leftrightarrow x = y[/itex]
Some function like :f(x) = 0x, f(x) = 1 ^ x, f(x) = x ^ 0 ([itex]x \in \mathbb{R} - \{ 0 \}[/itex]). You cannot have [itex]f(x) = f(y) \Leftrightarrow x = y[/itex]. Why? Because in that 3 examples:
[itex]\forall x, f(x) = const[/itex]
Viet Dao,