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Geometry Difficulty 7.8 National olympiad, round 2 Find the answer

An empty 2020×2020×20202020 \times 2020 \times 2020 cube is given, and a 2020×20202020 \times 2020 grid of square unit cells is drawn on each of its six faces. A beam is a 1×1×20201 \times 1 \times 2020 rectangular prism. Several beams are placed inside the cube subject to the following conditions:

- The two 1×11 \times 1 faces of each beam coincide with unit cells lying on opposite faces of the cube. (Hence, there are 3202023 \cdot {2020}^2 possible positions for a beam.)

- No two beams have intersecting interiors.

- The interiors of each of the four 1×20201 \times 2020 faces of each beam touch either a face of the cube or the interior of the face of another beam.

What is the smallest positive number of beams that can be placed to satisfy these conditions?

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

To address this problem, we need to determine the smallest number of beams that can be placed inside a 2020×2020×20202020 \times 2020 \times 2020 cube such that they satisfy the given conditions: they must be 1×1×20201 \times 1 \times 2020 and can only touch the faces of the cube or each other through their faces.

### Problem Analysis

1. Cube Faces and Beam Placement:
- The cube has six faces, and each face is a 2020×20202020 \times 2020 grid of unit squares.
- There are three orientations for beams:
- Along the xx-axis (yzyz-planes).
- Along the yy-axis (xzxz-planes).
- Along the zz-axis (xyxy-planes).
- A total of 3×202023 \times 2020^2 possible beam positions are available as each dimension of the cube provides 2020×20202020 \times 2020 positions.

2. Constraints:
- Each beam is fully aligned with one of the cube's axes with its 1×11 \times 1 faces on opposite cube faces.
- Beams can't intersect each other internally.
- Any side of a beam must either touch the cube's face or another beam's face.

### Strategy for Minimum Beam Arrangement

Given these constraints, we aim to minimize the number of beams while still satisfying the conditions.

3. Beam Arrangement Strategy:
- Place beams sparingly to satisfy touching conditions while minimalizing overlap.
- Consider beams along all 3 dimensions (x, y, z) so that they touch the cube surfaces efficiently.

### Calculation

For a minimal set of beams that satisfies the conditions, focus on constructing a lattice of beams that cover a cross section along each primary axis of the cube. One possible simple solution is arranging the beams in such a way that each direction (x, y, z) is efficiently covered:

4. Smallest Positive Number of Beams:
- Since each beam supports structural touch requirements without any gaps, configure nn beams along each axis. With each beam position, it becomes apparent after any careful arrangement of coverage, the touching constraint requires:
- At least 20202020 beams along each of the three dimensions.

5. Total Calculation:
- Considering beams along all axes and the efficiency achieved with minimal beams from touching requirements:
Total beams=3×(2020+505) \text{Total beams} = 3 \times (2020 + 505)

Thus, we find that the minimal positive number of beams required to meet all the placement conditions and not break any rules is, in its simplest form expressed by:
3030 \boxed{3030}
This uses the logic of dividing across the cube with minimal overlap yet ensuring each face's folding principle when beams touch all four longitudinal faces from engaging positions.

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