On Langford’s Problem (I)
C. J Priday · The Mathematical Gazette · 1959
For numbers a ≥ b ≥ l we shall denote by ( a, b ) the set of numbers b, b + 1, …, a . We shall say that a set S of numbers is perfect if there exists a sequence containing just one pair of each of the numbers in S , satisfying the condition: for every number r in the set, the two r’s are separated by exactly r places , and having no gaps (a perfect sequence ).