## Connected graph

### Set

 context $V,E$ … set
 definiendum $\langle V,E,\psi\rangle \in \mathrm{it}(E,V)$
 postulate $\langle V,E,\psi\rangle$ … undirected graph
 range $X\cap Y=\emptyset$ range $X\cup Y=V$ range $x\in X$ range $y\in Y$
 postulate $\forall X,Y. \exists x,y.\ \{x,y\}\in\mathrm{im}\ \psi$