: A function f is said to be one-to-one, or injective, if
and only if f(x) = f(y) implies x = y for all x, y in the domain of
f. A function is said to be an injection if it is one-to-one.
Alternative: A function is one-to-one if and only if f(x) f(y),
whenever x y. This is the contrapositive of the definition