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Learning random points from geometric graphs or orderings

Abstract

Suppose that there is a family of nn random points XvX_v for vVv \in V, independently and uniformly distributed in the square [n/2,n/2]2\left[-\sqrt{n}/2,\sqrt{n}/2\right]^2. We do not see these points, but learn about them in one of the following two ways. Suppose first that we are given the corresponding random geometric graph GG, where distinct vertices uu and vv are adjacent when the Euclidean distance dE(Xu,Xv)d_E(X_u,X_v) is at most rr. Assume that the threshold distance rr satisfies n3/14rn1/2n^{3/14} \ll r \ll n^{1/2}. We shall see that the following holds with high probability. Given the graph GG (without any geometric information), in polynomial time we can approximately reconstruct the hidden embedding, in the sense that, `up to symmetries', for each vertex vv we find a point within distance about rr of XvX_v; that is, we find an embedding with `displacement' at most about rr. Now suppose that, instead of being given the graph GG, we are given, for each vertex vv, the ordering of the other vertices by increasing Euclidean distance from vv. Then, with high probability, in polynomial time we can find an embedding with the much smaller displacement error O(logn)O(\sqrt{\log n}).

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