Question

 Let \[A=\{1,\text{ }2,\text{ }3)\] and \[{{R}_{1}}=\{(1,1),(1,3),(3,1),(2,2),(2,1),(3,3)\}\],
Find whether or not each of the relations \[{{R}_{1}}\] on A is
(i) reflexive (ii) symmetric (iii) transitive.

Answer 1

Hint: We will use the definitions of reflexive, symmetric and transitive relations to solve this question. A relation is a reflexive relation If every element of set A maps to itself. A relation in a set A is a symmetric relation if \[({{a}_{1}},{{a}_{2}})\in R\] implies that \[({{a}_{2}},{{a}_{1}})\in R\], for all \[{{a}_{1}},{{a}_{2}}\in A\]. A relation in a set A is a transitive relation if \[({{a}_{1}},{{a}_{2}})\in R\] and \[({{a}_{2}},{{a}_{1}})\in R\] implies that \[({{a}_{1}},{{a}_{3}})\in R\] for all \[{{a}_{1}},{{a}_{2}},{{a}_{3}}\in A\].

Complete step-by-step answer:
 Before proceeding with the question we should know about the concept of relations and different types of relations that are reflexive, symmetric and transitive relations.
A relation in set A is a subset of \[A\times A\]. Thus, \[A\times A\] is two extreme relations.
A relation in a set A is a reflexive relation if \[(a,a)\in R\], for every \[a\in A\].
A relation in a set A is a symmetric relation if \[({{a}_{1}},{{a}_{2}})\in R\] implies that \[({{a}_{2}},{{a}_{1}})\in R\], for all \[{{a}_{1}},{{a}_{2}}\in A\].
A relation in a set A is a transitive relation if \[({{a}_{1}},{{a}_{2}})\in R\] and \[({{a}_{2}},{{a}_{1}})\in R\] implies that \[({{a}_{1}},{{a}_{3}})\in R\] for all \[{{a}_{1}},{{a}_{2}},{{a}_{3}}\in A\].
A relation in a set A is an equivalence relation if R is reflexive, symmetric and transitive.
It is mentioned in the question that the relation \[{{R}_{1}}=\{(1,1),(1,3),(3,1),(2,2),(2,1),(3,3)\}\] defined on the set A is {1, 2, 3).
So according to the above definition the relation is reflexive as (1, 1), (2, 2) and (3, 3) is present and belongs to R.
Also \[(1,3)\in R\], \[(3,1)\in R\], \[(2,1)\in R\] but \[(1,2)\notin R\] , so the relation is not symmetric.
Now \[(1,3)\in R\] but \[(1,2)\notin R\] but \[(2,3)\notin R\]. Hence the relation is not transitive.
Hence \[{{R}_{1}}=\{(1,1),(1,3),(3,1),(2,2),(2,1),(3,3)\}\] is reflexive, not transitive and not symmetric.
Note: Remembering the definition of relations and the types of relations is the key here. We in a hurry can make a mistake in thinking it as a transitive set but we have to check the definition by taking subsets of the given set A.