JEE MAIN - Mathematics (2025 - 7th April Morning Shift - No. 22)
The number of relations on the set $A=\{1,2,3\}$, containing at most 6 elements including $(1,2)$, which are reflexive and transitive but not symmetric, is __________.
Answer
6
Explanation
Since relation needs to be reflexive the ordered pairs $(1,1),(2,2),(3,3)$ need to be there and $(1,2)$ is also to be included.
Let's call $R_0=\{(1,1),(2,2),(3,3),(1,2)\}$ the base relation.
$\because A \times A$ contain $3 \times 3=9$ ordered pairs, remaining 5 ordered are
$$2,1),(1,3),(3,1),(2,3),(3,2)$$
We have to add at most two ordered pairs to $R_0$ such that resulting relation is reflexive, transitive but not symmetric.
Following are the only possibilities.
$$R=R_0 U\{(1,3)\}$$
OR $R_0 U\{(3,2)\}$
OR $R_0 U\{(1,3),(3,1)\}$
OR $R_0 U\{(1,3),(3,2)\}$
OR $R_0 U\{(3,1),(3,2)\}$
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