Relations and Graphs

The cartesian product of two sets A and B , denoted A X B , is the set of all ordered pairs (a, b) where a &isin A and b &isin B .

A relation between two sets is a subset of their cartesian product.

A graph is a pair (S, &Gamma) where S is a set of nodes and &Gamma &sube S X S .

Properties of relations:

Property: Definition:
Reflexive &forall a (a, a) &isin R
Symmetric &forall a, b (a, b) &isin R &rarr (b, a) &isin R
Transitive &forall a, b, c (a, b) &isin R &and (b, c) &isin R
&rarr (a, c) &isin R
Antisymmetric &forall a, b (a, b) &isin R &and (b, a) &isin R &rarr a = b

A relation that is reflexive, symmetric, and transitive is an equivalence relation, which corresponds to a partition of the set (a set of disjoint subsets whose union is the set).

A relation that is reflexive, antisymmetric, and transitive is a partial order. Example: &le .

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