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
Solution
Reflexive (1, 1), (2, 2), (3, 3) R, and (1, 2) R. So at most 2 more pairs from \(2, 1), (1, 3), (3, 1), (2, 3), (3, 2)\.
No extra pair: R = (1, 1), (2, 2), (3, 3), (1, 2) is transitive and not symmetric ✓ → 1 relation
One extra pair:
(2, 1): symmetric ✗; (3, 1): needs (3, 2) ✗; (2, 3): needs (1, 3) ✗
(1, 3) ✓; (3, 2) ✓ → 2 relations
Two extra pairs (transitive closure must not need a fourth pair):
(1, 3), (2, 3) ✓ [(1, 2), (2, 3) (1, 3) present]
(1, 3), (3, 2) ✓ [(1, 3), (3, 2) (1, 2) present]
(3, 1), (3, 2) ✓ [(3, 1), (1, 2) (3, 2) present]
Every other pair fails: any set with (2, 1) forces a third pair; (1, 3), (3, 1) needs (3, 2); (3, 1), (2, 3) and (2, 3), (3, 2) need (1, 3). → 3 relations
All of these are not symmetric (they contain (1, 2) but not (2, 1)).
required number = 1 + 2 + 3 = 6
Answer: 6