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Sets and Relations question

2025 · 7 Apr · Shift 1 · Q47
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Sets and Relations question

2025 · 7 Apr · Shift 1 · Q47

JEE MainMathematicsSets and RelationsNumerical+4 / −1
The number of relations on the set A={1,2,3}A=\{1,2,3\}A={1,2,3}, containing at most 6 elements including (1,2)(1,2)(1,2), which are reflexive and transitive but not symmetric, is ‾\underline{\hspace{2cm}}​.
Numerical answer
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Correct answer: 6

Let A={1,2,3}.A=\{1,2,3\}.A={1,2,3}. A relation on AAA is a subset of A×AA\times AA×A, where ∣A×A∣=9|A\times A|=9∣A×A∣=9.

We need relations RRR such that:

  1. (1,2)∈R(1,2)\in R(1,2)∈R
  2. RRR is reflexive
  3. RRR is transitive
  4. RRR is not symmetric
  5. RRR has at most 6 elements

1. Reflexive condition

Since RRR is reflexive, it must contain (1,1),(2,2),(3,3).(1,1),(2,2),(3,3).(1,1),(2,2),(3,3). So 3 elements are already compulsory.

Also given: (1,2)∈R.(1,2)\in R.(1,2)∈R. Thus at least 4 elements are fixed: {(1,1),(2,2),(3,3),(1,2)}.\{(1,1),(2,2),(3,3),(1,2)\}.{(1,1),(2,2),(3,3),(1,2)}.

Since relation can have at most 6 elements, we may add at most 2 more elements from the remaining 5 pairs: (1,3),(2,1),(2,3),(3,1),(3,2).(1,3),(2,1),(2,3),(3,1),(3,2).(1,3),(2,1),(2,3),(3,1),(3,2).


2. Not symmetric condition

Because (1,2)∈R(1,2)\in R(1,2)∈R, for symmetry we would need (2,1)∈R(2,1)\in R(2,1)∈R. So for the relation to be not symmetric, it is enough that (2,1)∉R.(2,1)\notin R.(2,1)∈/R. If (2,1)(2,1)(2,1) were included, we would still need to check other pairs; but since we want not symmetric and have a small case analysis, we will examine all valid possibilities.


3. Transitivity implications

Start with the compulsory part: R0={(1,1),(2,2),(3,3),(1,2)}.R_0=\{(1,1),(2,2),(3,3),(1,2)\}.R0​={(1,1),(2,2),(3,3),(1,2)}. This is already transitive:

  • (1,1)(1,1)(1,1) with (1,2)(1,2)(1,2) gives (1,2)(1,2)(1,2), already present.
  • (1,2)(1,2)(1,2) with (2,2)(2,2)(2,2) gives (1,2)(1,2)(1,2), already present.
  • loops cause no issue.

So we can add up to two more pairs, but any addition must preserve transitivity.

We now test subsets of S={(1,3),(2,1),(2,3),(3,1),(3,2)}S=\{(1,3),(2,1),(2,3),(3,1),(3,2)\}S={(1,3),(2,1),(2,3),(3,1),(3,2)} of size 0,1,0,1,0,1, or 222.


4. Case-by-case counting

Case A: Add 0 extra pairs

Relation: R=R0.R=R_0.R=R0​. It is reflexive, transitive, and not symmetric since (1,2)∈R(1,2)\in R(1,2)∈R but (2,1)∉R(2,1)\notin R(2,1)∈/R.

So this gives 1 relation.


Case B: Add 1 extra pair

We test each of the 5 possibilities.

(i) Add (1,3)(1,3)(1,3)

Then R=R0∪{(1,3)}R=R_0\cup\{(1,3)\}R=R0​∪{(1,3)}. No new transitivity requirement fails. Valid.

(ii) Add (2,1)(2,1)(2,1)

Now (2,1)(2,1)(2,1) and (1,2)(1,2)(1,2) imply by transitivity that (2,2)(2,2)(2,2) is present, yes. But (1,2)(1,2)(1,2) and (2,1)(2,1)(2,1) imply (1,1)(1,1)(1,1), yes. However (2,1)(2,1)(2,1) and (1,2)(1,2)(1,2) don't force anything new beyond loops. This relation is actually symmetric on pair (1,2)(1,2)(1,2), but symmetry overall also requires reverse of every pair present. Since only non-diagonal pairs are (1,2)(1,2)(1,2) and (2,1)(2,1)(2,1), it becomes symmetric. So not allowed.

(iii) Add (2,3)(2,3)(2,3)

Then (1,2)(1,2)(1,2) and (2,3)(2,3)(2,3) imply (1,3)(1,3)(1,3) must be in RRR, but it is not. So not transitive, not allowed.

(iv) Add (3,1)(3,1)(3,1)

Then (3,1)(3,1)(3,1) and (1,2)(1,2)(1,2) imply (3,2)(3,2)(3,2) must be in RRR, but it is not. So not transitive, not allowed.

(v) Add (3,2)(3,2)(3,2)

No transitivity failure occurs:

  • (3,2)(3,2)(3,2) with (2,2)(2,2)(2,2) gives (3,2)(3,2)(3,2).
  • (3,3)(3,3)(3,3) with (3,2)(3,2)(3,2) gives (3,2)(3,2)(3,2).
  • (1,2)(1,2)(1,2) with anything starting at 2 only gives something if we had (2,1)(2,1)(2,1) or (2,3)(2,3)(2,3), which we do not. Valid.

Hence Case B gives 2 valid relations: {(1,3)}, {(3,2)}.\{(1,3)\},\ \{(3,2)\}.{(1,3)}, {(3,2)}.


Case C: Add 2 extra pairs

Choose 2 pairs from SSS. There are (52)=10\binom52=10(25​)=10 possibilities. Check each.

1. (1,3),(2,1)(1,3),(2,1)(1,3),(2,1)

Since (2,1)(2,1)(2,1) and (1,3)(1,3)(1,3) imply (2,3)(2,3)(2,3), which is absent. Not transitive.

2. (1,3),(2,3)(1,3),(2,3)(1,3),(2,3)

Since (1,2)(1,2)(1,2) and (2,3)(2,3)(2,3) imply (1,3)(1,3)(1,3), which is present. No further issue. Not symmetric because (2,1)(2,1)(2,1) absent. Valid.

3. (1,3),(3,1)(1,3),(3,1)(1,3),(3,1)

Then (3,1)(3,1)(3,1) and (1,3)(1,3)(1,3) imply (3,3)(3,3)(3,3), present; (1,3)(1,3)(1,3) and (3,1)(3,1)(3,1) imply (1,1)(1,1)(1,1), present. But (3,1)(3,1)(3,1) and (1,2)(1,2)(1,2) imply (3,2)(3,2)(3,2), absent. So not transitive.

4. (1,3),(3,2)(1,3),(3,2)(1,3),(3,2)

Check:

  • (1,3)(1,3)(1,3) and (3,2)(3,2)(3,2) imply (1,2)(1,2)(1,2), present. No other issue. Valid.

5. (2,1),(2,3)(2,1),(2,3)(2,1),(2,3)

Then (1,2)(1,2)(1,2) and (2,1)(2,1)(2,1) imply (1,1)(1,1)(1,1), present. Also (1,2)(1,2)(1,2) and (2,3)(2,3)(2,3) imply (1,3)(1,3)(1,3), absent. So not transitive.

6. (2,1),(3,1)(2,1),(3,1)(2,1),(3,1)

Then (2,1)(2,1)(2,1) and (1,2)(1,2)(1,2) imply (2,2)(2,2)(2,2), present. (3,1)(3,1)(3,1) and (1,2)(1,2)(1,2) imply (3,2)(3,2)(3,2), absent. So not transitive.

7. (2,1),(3,2)(2,1),(3,2)(2,1),(3,2)

Then (3,2)(3,2)(3,2) and (2,1)(2,1)(2,1) imply (3,1)(3,1)(3,1), absent. So not transitive.

8. (2,3),(3,1)(2,3),(3,1)(2,3),(3,1)

Then (1,2)(1,2)(1,2) and (2,3)(2,3)(2,3) imply (1,3)(1,3)(1,3), absent. So not transitive.

9. (2,3),(3,2)(2,3),(3,2)(2,3),(3,2)

Then (1,2)(1,2)(1,2) and (2,3)(2,3)(2,3) imply (1,3)(1,3)(1,3), absent. Also (3,2)(3,2)(3,2) and (2,3)(2,3)(2,3) imply (3,3)(3,3)(3,3), present. But missing (1,3)(1,3)(1,3) already fails. Not transitive.

10. (3,1),(3,2)(3,1),(3,2)(3,1),(3,2)

(3,1)(3,1)(3,1) and (1,2)(1,2)(1,2) imply (3,2)(3,2)(3,2), present. No further issue. Valid.

Thus Case C gives 3 valid relations.


5. Total count

Total valid relations: 1+2+3=6.1+2+3=6.1+2+3=6.

So the required number of relations is 6.\boxed{6}.6​.


6. Comparison with stored answer

Stored correct answer = 666.

Our derived answer also is 666, so they agree.

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