Binary relation over a set X
≡ Homogeneous relation ≡ endorelation,
An example of a homogeneous relation is the relation of kinship, where the relation is over people.
Teo: A relation over a set may be identified with a directed graph
a relation XRX is identified with the directed graph where
It is called the adjacency relation of the graph.
If the relation is symmetric, with an undirected graph.