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Mongolian Mathematical Olympiad

Mongolia algebra

Problem

Let be a fixed positive integer. Let be a finite set and let and be functions satisfying for all . Here is the unit interval. (1) Prove that . (2) For , find an example of a finite set and functions and satisfying the condition of the problem. (Otgonbayar Uuye)
Solution
(1) We have .

(2) For , let and let and for . Then and it is easy to see that and . Moreover, we have for any .
Final answer
S ≥ n − 1; at equality take X = {1, 2, …, 2(n−1)}, define f_i(x) = 1 if i ≤ x ≤ i + n − 2 and 0 otherwise for 1 ≤ i ≤ n, and set g_i(x) = 1 − f_i(x).

Techniques

Linear and quadratic inequalitiesCounting two ways