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Geometry Difficulty 7.8 National olympiad, round 2 Find the answer

Does there exist a set MM in usual Euclidean space such that for every plane λ\lambda the intersection MλM \cap \lambda is finite and nonempty ?

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Solution

To determine if there exists a set M M in usual Euclidean space such that for every plane λ\lambda, the intersection Mλ M \cap \lambda is finite and nonempty, we need to consider a construction that satisfies these conditions.

One possible approach is to construct the set M M using a version of the "space-filling curve" concept, but within certain constraints. However, space-filling curves like the Peano or Hilbert curves fill an entire region and would not make the intersection with a plane finite, thus a different approach is needed.

Instead, consider the following construction:

Construct the set M M by taking a dense set of points on every line parallel to one of the coordinate axes, such that these points are sparse in other coordinate directions. One way to do this is:

- For a subset of lines along the x x-axis, y y-axis, and z z-axis (in R3\mathbb{R}^3), include points spaced in such a way that each point belongs to a single line only. Specifically, for each integer point on the x x-axis of the form (n,0,0)(n, 0, 0), place a point (n,1n,1n)(n, \frac{1}{n}, \frac{1}{n}).

This construction ensures:

1. Non-empty intersection: For any plane λ\lambda in R3\mathbb{R}^3, there will be at least one axis-aligned line passing through or intersecting this plane at some point, and since we have points densely populating these lines, Mλ M \cap \lambda is nonempty.

2. Finite intersection: Given the specific choice of constructing sparse points only on one type of line, the intersection of any plane λ\lambda with these lines would result in a finite number of points on that plane.

Thus, the set M M satisfies the conditions of having finite and nonempty intersections with any plane λ\lambda.

Therefore, it is indeed possible to construct such a set M M .

The existence of such a set M M in usual Euclidean space is conclusively:
yes \boxed{\text{yes}}

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.