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Relation and the properties of relation | Discrete Mathematics

In this article, we will learn about the relations and the properties of relation in the discrete mathematics.
Submitted by Prerana Jain, on August 17, 2018

Cartesian product (A*B not equal to B*A)

Cartesian product denoted by * is a binary operator which is usually applied between sets. It is a set of ordered pairs where the first member of the pair belongs to the first set and the second member of the pair belongs second sets.

    |A|     = m |B| = n
    |A*B|   = mn 


    A = {1,2}  B = {a, b, c}
    A * B = { (1,a), (1,b), (1,c), (2,a), (2,b), (2,c)}


The word relation suggests some familiar example relations such as the relation of father to son, mother to son, brother to sister etc. Familiar examples in arithmetic are relation such as "greater than", "less than", or that of equality between the two real numbers.

Here, we shall only consider relation called binary relation, between the pairs of objects. Before we give a set-theoretic definition of a relation we note that a relation between two objects can be defined by listing the two objects an ordered pair.


Any set of ordered pairs defines a binary relations. We shall call a binary relation simply a relation. It is sometimes convenient to express the fact that particular ordered pair say (x,y) E R where, R is a relation by writing xRY which may be read as "x is a relation R to y".


The relation of father to his child can be described by a set , say ordered pairs in which the first member is the name of the father and second the name of his child that is:

F = { (x , y) |x is the father of y}


Let, S be a binary relation. The set D(S) of all objects x such that for some y, (x,y) E S is said to be the domain of S.


The set R(S) of all objects y such that for some x, (x,y) E S said to be the range of S.

Let r A B be a relation

    DOM(R) = {a|(a, b)E R for some b E B} 
    Range(R) = {b |(a, b) E R } for some

Properties of binary relation in a set

There are some properties of the binary relation:

  1. A binary relation R is in set X is reflexive if , for every x E X , xRx, that is (x, x) E R or R is reflexive in X <==> (x) (x E X -> xRX).
    The relation =< is reflexive in the set of real number since for nay x we have x<= X similarly the relation of inclusion is reflexive in the family of all subsets of a universal set.
  2. A relation R is in a set X is symmetric if for every x and y in x whenever xRy then yRX that is R is a symmetric in x.
    The relation <= and < are not symmetric i the set of real number while the relation of equality is.
  3. A relation R in a set x is transitive if for every x, y and z in X whenever xRy and yRx then xRz that is R is transitive in X.
    The relation <= < and = are transitive in the set of real numbers. The relations and equality are also transitive in the family of a subset of a universal set.

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