Let A=\1, 4, 7\ and B=\2, 3, 8\. Then the number of elements, in the relation R=\((a1, b1), (a2, b2)) ((A B) (A B)) : a1+b2 divides a2+b1\ is .
Answer
18
Solution
Let u=a1+b2 and v=a2+b1; each of (a1, b2) and (a2, b1) runs over the 9 pairs of A B.
Values of a+b: a=1: 3, 4, 9; a=4: 6, 7, 12; a=7: 9, 10, 15 (9 occurs twice)
Count the values v divisible by each u:
u=3: v=3, 6, 9, 9, 12, 15 6
u=4: v=4, 12 2; u=6: v=6, 12 2; u=7: v=7 1
u=9 (two pairs): v=9, 9 2 2=4
u=10: v=10 1; u=12: v=12 1; u=15: v=15 1
n(R)=6+2+2+1+4+1+1+1=18
Answer: 18