®
Transitive relation R in X is a relation satisfying (a, b) ∈ R and (b, c) ∈ R
implies that (a, c) ∈ R ®
Equivalence relation R in X is a relation which is reflexive, symmetric and
transitive ®
Equivalence class [a] containing a ∈ X for an equivalence relation R in X is
the subset of X containing all elements b related to a ®
A function f : X → Y is one-one (or injective) if
f (x1) = f(x2) ⇒ x1 = x2 ∀  x1, x2 ∈ X