A function $f: A \rightarrow B$ is injective (or one-to-one) if $\forall a_1, a_2 \in A$, $a_1 \ne a_2 \implies f(a_1) \ne f(a_2)$, or equivalently, $f(a_1) = f(a_2) \implies a_1 = a_2$
A function $f: A \rightarrow B$ is injective (or one-to-one) if $\forall a_1, a_2 \in A$, $a_1 \ne a_2 \implies f(a_1) \ne f(a_2)$, or equivalently, $f(a_1) = f(a_2) \implies a_1 = a_2$