Maths Olympiad Prep

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, 2014

Combinatorics Difficulty 4.3 AIME Find the answer United States

Problem:

How many ways can the eight vertices of a three-dimensional cube be colored red and blue such that no two points connected by an edge are both red? Rotations and reflections of a given coloring are considered distinct.

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

Solution

Solution:

Answer: 3535

We do casework on RR, the number of red vertices. Let the cube be called ABCDEFGHA B C D E F G H, with opposite faces ABCDA B C D and EFGHE F G H, such that AA is directly above EE.

- R=0\underline{R=0} : There is one such coloring, which has only blue vertices.

- R=1\underline{R=1} : There are 88 ways to choose the red vertex, and all other vertices must be blue. There are 88 colorings in this case.

- R=2\underline{R=2} : Any pair not an edge works, so the answer is (82)12=16\binom{8}{2}-12=16.

- R=3\underline{R}=3 : Each face ABCDA B C D and EFGHE F G H has at most two red spots. Assume WLOG ABCDA B C D has exactly two and EFGHE F G H has exactly one (multiply by 22 at the end). There are two ways to pick those in ABCDA B C D (two opposite corners), and two ways after that to pick EFGHE F G H. Hence the grand total for this subcase is 222=82 \cdot 2 \cdot 2=8.

- R=4\underline{R=4} : There are only two ways to do this.

Hence, the sum is 3535.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.