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Geometry Difficulty 7.3 National olympiad, round 2 Find the answer

In a fictional world, each resident (viewed as geometric point) is assigned a number: 1,2,1,2, \cdots. In order to fight against some epidemic, the residents take some vaccine and they stay at the vaccination site after taking the shot for observation. Now suppose that the shape of the Observation Room is a circle of radius 14\frac{1}{4}, and one requires that the distance dm,nd_{m, n} between the Resident No. mm and the Resident No. nn must satisfy (m+n)dm,n1(m+n) d_{m, n} \geq 1. Where we consider the distance on the circle, i.e., the length of the minor arc between two points. Proof Question: Give a proof of your answer to Question (i).

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Solution I. We can place the Residents No. 1,2,1,2, \ldots according to the following rule. First, put Resident No. 1 arbitrarily. For n>2n>2, if Residents No. 1,2,,n11,2, \ldots, n-1 have already been placed, we consider the positions where Resident No. n cannot be placed. For 1mn11 \leq m \leq n-1, by dm,n1m+nd_{m, n} \geq \frac{1}{m+n}, we know that the Resident No. nn cannot be placed in the arc that is centered at Resident No. mm, and of the length 2m+n\frac{2}{m+n}. The total length of these arcs is 2n+1+2n+2++22n1<2(lnn+1n+lnn+2n+1++ln2n12n2)=2ln2n1n<2ln2\frac{2}{n+1}+\frac{2}{n+2}+\cdots+\frac{2}{2 n-1}<2\left(\ln \frac{n+1}{n}+\ln \frac{n+2}{n+1}+\cdots+\ln \frac{2 n-1}{2 n-2}\right)=2 \ln \frac{2 n-1}{n}<2 \ln 2. Therefore, the total length of the union of these arcs does not exceed 2ln22 \ln 2, while the perimeter of the circle is 142π=π2\frac{1}{4} \cdot 2 \pi=\frac{\pi}{2}. It is easy to observe that π2>1.5>2ln2\frac{\pi}{2}>1.5>2 \ln 2, so these arcs would not cover the whole circle, hence it is always possible to find a place for Resident No. nn such that its distances to Residents No. 1,2,,n11,2, \ldots, n-1 satisfy the requirement. By induction we conclude that the circle can accommodate any quantity of residents.

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