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set of k points from n on a circle

Source: Netherlands - Dutch NMO 1972 p4

January 27, 2023
combinatoricscombinatorial geometry

Problem Statement

On a circle with radius 11 the points A1,A2,...,AnA_1, A_2,..., A_n lie such that every arc AiAi+iA_iA_{i+i} has length 2πn=a\frac{2\pi}{n}= a. Given that there exists a set VV consisting of k k of these points (k<nk < n), which has the property that each of the arc lengths aa, 2a2a,...,..., (n1)a(n- 1)a can be obtained in exactly one way be taken as the length of an arc traversed in a positive sense, beginning and ending in a point of VV. Express nn in terms of kk and give the set VV for the case n=7n = 7.