A surjection / onto function is a function $f:X\to Y$ that is surjective, i.e. for all $y\in Y$ there is an $x\in X$ such that $f(x)=y$. In other words, $\ran f=Y$.
The set of all surjections from $X$ onto $Y$ is denoted by $\sur(X,Y)$.
A surjection / onto function is a function $f:X\to Y$ that is surjective, i.e. for all $y\in Y$ there is an $x\in X$ such that $f(x)=y$. In other words, $\ran f=Y$.
The set of all surjections from $X$ onto $Y$ is denoted by $\sur(X,Y)$.