A function from \(S\) to \(T\)
A relation \(F \subseteq S \times T\) such that \(\forall s \in S:\ \exists! t \in T:\ (s,t) \in F\)
The preimage of an element \(t \in T\) is \(\{s \in S\ |\ F(s)=t\}\)
\(\hookrightarrow\) Injectivity: \(s\ne s' \implies F(s)\ne F(s')\)
\(\twoheadrightarrow\) Surjectivity: \(\forall t \in T:\ \exists s \in S:\ (s,t) \in F\)
\(\xrightarrow \cong\) Bijectivity: both injectivity and surjectivity.