The number of relations, defined on the set \a, b, c, d\, which are both reflexive and symmetric, is equal to:
- 16
- 64
- 256
- 1024
Answer
(2) 64
Solution
Let A = \a, b, c, d\; A A has 16 ordered pairs.
Reflexive the 4 pairs (a, a), (b, b), (c, c), (d, d) must all be in the relation (only 1 way).
Symmetric the remaining 12 pairs form 6 couples \(x, y), (y, x)\ with x y; each couple is either taken completely or left out (2 ways each).
Number of relations = 26 = 64
(In general: 2 n(n-1)2 with n = 4.)
Answer: (2)