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Combinatorics Difficulty 6.5 National olympiad Find the answer

Example 5 For which nN,n5n \in \mathbf{N}^{\cdot}, n \geqslant 5, can the vertices of a regular nn-gon be colored using no more than 6 colors, such that any 5 consecutive vertices have distinct colors?

A number or a short expression. Spacing and $ signs are ignored.

Solution

Let the colors be a,b,c,d,e,fa, b, c, d, e, f. Define the sequence A:a,b,c,d,eA: a, b, c, d, e and the sequence B:a,b,c,d,e,fB: a, b, c, d, e, f.

If there exist non-negative integers x,yx, y, such that n=5x+6yn=5 x+6 y, then for a regular nn-gon, the vertices can be colored by first coloring yy sequences of BB, followed by xx sequences of AA, ensuring that any 5 consecutive vertices are of different colors.

Using the conclusion from Example 3, we know that when n5×6(5+6)+1=20n \geqslant 5 \times 6-(5+6)+1=20, the equation n=5x+6yn=5 x+6 y always has non-negative integer solutions. For 5n195 \leqslant n \leqslant 19, direct calculation shows that the equation n=5x+6yn=5 x+6 y has no non-negative integer solutions only when n{7,8,9,13,14,19}n \in \{7,8,9,13,14,19\}.

On the other hand, for n{7,8,9,13,14,19}n \in \{7,8,9,13,14,19\}, there exists kNk \in \mathbf{N}^{*} such that 6k<n<6(k+1)6 k<n<6(k+1). Therefore, there must be a color that appears k+1k+1 times. Since these k+1k+1 points of the same color are at least 4 points apart, we have
n5(k+1)n \geqslant 5(k+1)

Now,
when n{7,8,9}n \in \{7,8,9\}, k=1k=1, requiring n10n \geqslant 10, which is a contradiction.
when n{13,14}n \in \{13,14\}, k=2k=2, requiring n15n \geqslant 15, which is a contradiction.
when n=19n=19, k=3k=3, requiring n20n \geqslant 20, which is also a contradiction.
In summary, when n5n \geqslant 5, except for the numbers in the set {7,8,9,13,14,19}\{7,8,9,13,14,19\}, all other positive integers meet the requirement.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.