d is a positive integer not equal to 2,5 or 13. How many values of d are there such that for any pair (a,b) from the set {2,5,13,d}, ab−1 is a perfect square?
This is IMO 1986 -1.
This problem is a part of the set "Olympiads and Contests Around the World - 2". You can see rest of the problems here.