Definisi
[tex]( ∀x_{1},x_{2} )(x_{1} ≠x_{2} \: \Longrightarrow \: f(x_{1})≠ \: f (x_{2}) \\[/tex]
[tex]( ∀x_{1},x_{2} )( f(x_{1}) = f (x_{2})\: \Longrightarrow \: x_{1} = x_{2})\\[/tex]
[tex]( ∀x_{1},x_{2} )(x_{1} ≠x_{2} \: \Longrightarrow \: f(x_{1})≠ \: f (x_{2}) \\[/tex]
[tex]( ∀x_{1},x_{2} )( f(x_{1}) = f (x_{2})\: \Longrightarrow \: x_{1} = x_{2})\\[/tex]