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IMO 2005 — Complete Shortlist Question 9

IMO Shortlist · IMO 2005 — Complete Shortlist

Question 9

Let $ n \geq 3 $ be a given positive integer. We wish to label each side and each diagonal of a regular $ n $-gon $ P_1 \ldots P_n $ with a positive integer less than or equal to r so that:

(i) every integer between 1 and r occurs as a label;

(ii) in each triangle $P_{i}P_{j}P_{k}$ two of the labels are equal and greater than the third. Given these conditions:

(a) Determine the largest positive integer r for which this can be done.

(b) For that value of r, how many such labellings are there?