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MathNet 2010 — Part 03 Question 110

Math Olympiad Vault · MathNet 2010 — Part 03

Question 110

Consider that we have omitted some of the points of a 2d-lattice (same as $\mathbb{Z}^2$). Look at these points as a graph, two points (vertices) are connected if they are the same in one of their coordinates and differ 1 in another. Each connected component is called a *cluster*. Suppose that for all $n \in \mathbb{N}$, the number of omitted points inside the horizontal square with side length $2n + 1$ and the origin as its center, be less than $n/2$. Prove that the non-omitted points contain exactly one infinite cluster.