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Geometry Difficulty 8.1 Shortlist Prove it Bulgaria

A number rX>0r_X > 0 is assigned to any point XX in the plane such that 2rXrYXY2|r_X - r_Y| \le |XY| for any two points XX and YY. A cricket can jump from XX to YY if rX=XYr_X = |XY|. Prove that for any two points XX and YY the cricket can move from XX to YY by finite number of jumps.

Solution

Denote by D(Z,r)D(Z, r) and S(Z,r)S(Z, r) the open disc and the circle with center ZZ and radius rr, respectively.
Let A=AXA = A_X be the set of points that can be reached from XX by finite number of jumps (possibly zero). We have to show that A=R2A = \mathbb{R}^2.

First, we shall prove that D(X,2rX)AD(X, 2r_X) \subset A. We may assume that X=OX = O and r0=1r_0 = 1. Set an=2na_n = 2^{-n}, bn=2anb_n = 2 - a_n and An={XR2:anOXbn}A_n = \{X \in \mathbb{R}^2 : a_n \le |OX| \le b_n\}, nN0n \in \mathbb{N}_0. It is enough to show that AnAA_n \subset A.

We shall proceed by induction. For n=0n=0, this follows from the given condition. Suppose that AkAA_k \subset A. It suffices to prove that YAY \in A for Y[ak+1,ak)(bk,bk+1]Y \in [a_{k+1}, a_k) \cup (b_k, b_{k+1}] and then apply rotation. Let f(Z)=fY(Z)=YZrZf(Z) = f_Y(Z) = |YZ| - r_Z. It follows by the condition of the problem that 1ak+1rak1+ak+11 - a_{k+1} \le r_{a_k} \le 1 + a_{k+1} and ak+1rbkbk+1a_{k+1} \le r_{b_k} \le b_{k+1}. So, if Y[ak+1,ak)Y \in [a_{k+1}, a_k), then f(ak)<ak10<f(bk)f(a_k) < a_k - 1 \le 0 < f(-b_k), and if Y(bk,bk+1]Y \in (b_k, b_{k+1}], then f(bk)<0<1ak+1<f(ak)f(b_k) < 0 < 1 - a_{k+1} < f(-a_k). Since AkA_k is a linearly connected set and ff is a continuous function, we get that f(Z)=0f(Z) = 0 for some ZAkZ \in A_k and hence YAY \in A.

Assume now that AR2A \ne \mathbb{R}^2. Then sup{r:D(X,r)A}=R<\sup\{r : D(X, r) \subset A\} = R < \infty. Let m=inf{rY:YD(X,R)}m = \inf\{r_Y : Y \in D(X, R)\}. Since rr is a continuous function, then m>0m > 0 and, by the proved above, D(Y,2m)AD(Y, 2m) \subset A for YD(X,R)Y \in D(X, R). Hence D(X,R+2m)AD(X, R + 2m) \subset A, a contradiction.

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