6
RELATIONS AND FUNCTIONS
SOME IMPORTANT RESULTS/CONCEPTS
** Relation : A relation R from a non-empty set A to a non-empty set B is a subset of A × B.
**A relation R in a set A is called
(i) Reflexive, if (a, a) ∈ R, for every a∈ A,
(ii) Symmetric, if (a, b) ∈ R then (b, a)∈ R,
(iii)Transitive, if (a, b) ∈ R and (b, c)∈ R then (a, c)∈ R.
** Equivalence Relation : R is equivalence if it is reflexive, symmetric and transitive.
** Function :A relation f : A B is said to be a function if every element of A is correlated to unique
element in B.
* A is domain
* B is codomain
* For any x element x A, function f correlates it to an element in B, which is denoted by f(x) and
is called image of x under f . Again if y = f(x), then x is called as pre- image of y.
* Range = {f(x) | x A }. Range Codomain
* The largest possible domain of a function is called domain of definition.
**Composite function :Let two functions be defined as f : A B and g : B C. Then we can define
a function gof: A C is called the composite function of f and g.
** Different type of functions : Let f : A B be a function.
*f is one to one (injective) mapping, if any two different elements in A is always correlated to
different elements in B, i.e. x1 x2 f(x1) f(x2) or f(x1) = f(x2) x1 = x2
*f is many one mapping, if at least two elements in A such that their images are same.
*f is onto mapping (subjective), if each element in B is having at least one pre image.
*f is into mapping if range codomain.
* f is bijective mapping if it is both one to one and onto.
SOME ILUSTRATIONS :
(a) Let A ={1, 2, 3}, then
(i) R = {(1 , 1), (2 , 2), (3 , 3), (1 , 2), (2 , 3) } is reflexive but neither symmetric nor transitive.
As (1 , 1), (2 , 2), (3 , 3) R ,(1 , 2) R but (2 , 1) R, and (1 , 2), (2 , 3) R but (1 , 3) R
(ii) R = {(1 , 1), (2 , 2), (1 , 2), (2 , 3)} is neither reflexive nor symmetric nor transitive.
As (3 , 3) R , (1 , 2) R but (2 , 1) R, and (1 , 2), (2 , 3) R but (1 , 3) R
(iii) R = {(1 , 1), (2 , 2), (3 , 3), (1 , 2), (2 , 1), (2 , 3), (3 , 2), (1 , 3), (3 , 1)} is reflexive, symmetric
and transitive ( Equivalence Relation) as (a, a) ∈ R, for every a ∈ A, (a, b) ∈ R then (b , a)∈ R
and (a , b) ∈ R and (b , c)∈ R then (a , c)∈ R.
(b) The relation R in the set Z of integers given by R = {(a , b) : 2 divides a – b} is an equivalence
relation.
Given R = {(a , b) : 2 divides a – b}
Reflexive a – a = 0, divisible by 2, a A
(a , a) A, a A R is reflexive.
Symmetric : Let (a , b) R 2 divides a – b , say a – b = 2m
b – a = –2m
2 divides b – a
(b , a) R R is symmetric.