Definition : A function \(f: D \to C\) is one-to-one (or injective) means for every \(a,b\) in the domain \(D\), if \(f(a) = f(b)\) then \(a=b\). Formally, \(f: D \to C\) is one-to-one means \(\underline{\phantom{\forall a \in D \forall b \in D ~(f(a) = f(b) \to a = b)}}\).