단사 함수

Let $f$ be a function whose domain is a set $X$. The function $f$ is said to be injective provided that for all $a$ and $b$ in $X$, if $f(a)=f(b)$, then $a=b$; that is $f(a)=f(b)$ implies $a=b$.

Symbolically, $\mathrm{\forall }a,b\in X,f(a)=f(b)⟹a=b$.