# Plus Two Maths Notes Chapter 1 Relations and Functions

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## Kerala Plus Two Maths Notes Chapter 1 Relations and Functions

Introduction
A relation from a non-empty set A to a non-empty set B is a subset of A × B. In this chapter we study different types of relations and functions, composition of functions, and binary operations.

Basic Concepts
I. Types of Relations:
Here we study different relations in a set A

Empty Relation:
R : A → A given by R = Φ ⊂ A × A.

Universal Relation:
R : A → A given by R = A × A.

Reflexive Relation:
R : A → A with (a, a) ∈ R, ∀a ∈ A.

Symmetric Relation:
R : A → A with
(a, b) ∈ R ⇒ (b, a) ∈ R, ∀a, b ∈ A.

Transitive Relation:
R : A → A with (a, b) ∈ R and (b, c) ∈ R ⇒ (a, c) ∈ R,
∀a, b, c ∈ A.

Equivalence Relation:
R : A → A which is Reflexive, Symmetric, and T ransitive.

Equivalence Class:
Let R be an Equivalence Relation in a set A. If a ∈ A, then the subset {x ∈ A, (x, a) ∈ R} of A is called the Equivalence Class corresponding to ‘a’ and it is denoted [a].

II. Types of Functions
One-One or Injective function.
A function f : A → B is said to be One-One or Injective, if the image of distinct elements of A under fare distinct.
i.e; f(x1) = f(x2) ⇒ x1 = x2
Otherwise f is a Many-One function.

1. Graphical approach:
If lines parallel to x-axis meet the curve at two or more points, then the function is not one-one.

Onto or Surjective function:
A function f : A → B is said to be Onto or Surjective, if every element of B is some image of some elements of A under f.
ie; If for every element y ∈ Y then there exists an element x in A such that f(x) = y.

Bijective function:
A function f : A → B is said to be Bijectiveit it is both One-One and Onto.

Composition of Functions.
Let f : A → B and g : B → C be two functions. Then the composition of f and g denoted by is gof defined
gof : A → C and gof (x) = g(f(x)).

1. If f : A → B and g : B → C are One-One, then gof : A → C is One-One.
2. If f : A → B and g : B → C are Onto, then gof : A → C is Onto.
3. If f : A → B and g : B → C are Bijective, ⇔ gof : A → C is Bijective.

Inverse Function:
If f : A → B is defined to be invertible, if there exists a function g : B → A such that gof = IA and
fog = IB. The function g is called the inverse of ‘f’ and is denoted by f-1.

1. If function f : A → B is invertible only if f is bijective.
2. (gof)-1 = f-1og-1.

III. Binary Operations
A binary operation ‘*’ on a set A is a function * : A × A → A, defined by a * b, a, b ∈ A.

1. * : A × A → A is commutative if a * b = b * a, ∀a, b ∈ A.
2. * : A × A → A is associative if a * (b * c) = (a * b) * c, ∀a, b, c ∈ A.
3. e ∈ A is the identity element for the binary operation * : A × A → A if a * e = a = e * a, ∀a ∈ A.
4. An elements a ∈ A is invertible for the binary operation * : A × A → A, if there exists an element b ∈ A such that a * b = e = b * a, where ‘e’ is the identity element for the operation ‘*’. Then ‘b’ is denoted by a-1.