IMO Shortlist 1984 problem 17


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2. travnja 2012.
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In a permutation (x_1, x_2, \dots , x_n) of the set 1, 2, \dots , n we call a pair (x_i, x_j ) discordant if i < j and x_i > x_j. Let d(n, k) be the number of such permutations with exactly k discordant pairs. Find d(n, 2) and d(n, 3).
Izvor: Međunarodna matematička olimpijada, shortlist 1984