Denote by D(Z,r) and S(Z,r) the open disc and the circle with center Z and radius r, respectively.
Let A=AX be the set of points that can be reached from X by finite number of jumps (possibly zero). We have to show that A=R2.
First, we shall prove that D(X,2rX)⊂A. We may assume that X=O and r0=1. Set an=2−n, bn=2−an and An={X∈R2:an≤∣OX∣≤bn}, n∈N0. It is enough to show that An⊂A.
We shall proceed by induction. For n=0, this follows from the given condition. Suppose that Ak⊂A. It suffices to prove that Y∈A for Y∈[ak+1,ak)∪(bk,bk+1] and then apply rotation. Let f(Z)=fY(Z)=∣YZ∣−rZ. It follows by the condition of the problem that 1−ak+1≤rak≤1+ak+1 and ak+1≤rbk≤bk+1. So, if Y∈[ak+1,ak), then f(ak)<ak−1≤0<f(−bk), and if Y∈(bk,bk+1], then f(bk)<0<1−ak+1<f(−ak). Since Ak is a linearly connected set and f is a continuous function, we get that f(Z)=0 for some Z∈Ak and hence Y∈A.
Assume now that A=R2. Then sup{r:D(X,r)⊂A}=R<∞. Let m=inf{rY:Y∈D(X,R)}. Since r is a continuous function, then m>0 and, by the proved above, D(Y,2m)⊂A for Y∈D(X,R). Hence D(X,R+2m)⊂A, a contradiction.