The main features of this chapter are as follows:
Empty relation
Universal relation
Reflexive relation
Symmetric relation
Transitive relation
Equivalence relation
A function f : X → Y is one-one (or
injective) if f (x1) = f(x2) ⇒ x1 = x2 ∀ x1, x2 ∈ X.
A function f : X → Y is onto (or surjective)
if given any y ∈ Y, ∃ x ∈ X such that f(x) = y.
A function f : X → Y is one-one and onto (or
bijective), if f is both one-one and onto.
A function f : X → Y is invertible if and
only if f is one-one and onto.
