A function \(f\colon X\to Y\) is called surjective or onto if for every \(y\in Y\) there is at least one element \(x\in X\) which is mapped onto \(y\).
A function \(f\colon X\to Y\) is called surjective or onto if for every \(y\in Y\) there is at least one element \(x\in X\) which is mapped onto \(y\).